WO2007149686A2

WO2007149686A2 - Stabilized proteins

Info

Publication number: WO2007149686A2
Application number: PCT/US2007/069948
Authority: WO
Inventors: Stephen K. Youngster; Amartya Basu
Original assignee: Enzon Pharmaceuticals, Inc.
Priority date: 2006-06-21
Filing date: 2007-05-30
Publication date: 2007-12-27
Also published as: EP2038424A2; TW200817521A; JP2010500963A; WO2007149686A3; CA2653061A1

Abstract

Methods of stabilizing proteins having at least one oxidizable amino acid thereon, such as recombinant human adenosine deaminase (rhADA) are disclosed. The methods include treating a protein containing an oxidizable amino acid with a sufficient amount of a capping agent such as glutathione under reaction conditions sufficient to cap the oxidizable amino acid without substantially inactivating the protein. Stabilized capped proteins, polymer conjugates of capped proteins, and methods of treatment using the same are also disclosed.

Description

STABILIZED PROTEINS

This patent application claims the benefit of U.S. Patent Application Ser. No. 11/738,012, filed April 20, 2007, which in turn claims the benefit of U.S. Provisional Application Ser. No. 60/805,417, filed on June 21, 2006, the contents of each of which are incorporated by reference herein in their entirety.

FIELD OF THE INVENTION The invention provides proteins, such as adenosine deaminase enzymes, that are stabilized by protecting or capping one or more oxidizable cysteine residues that are exposed to the solution environment of the protein.

BACKGROUND OF THE INVENTION Adenosine deaminase (AD A) has been used in the treatment of an enzyme deficiency disorder called severe combined immunodeficiency disease (SCID) or "Bubble boy" disease for some time. For more than 15 years, Enzon Pharmaceuticals has made a PEGylated ADA available for patients using a bovine source of enzyme. Recently, there have been efforts to replace the bovine source enzyme with a recombinant source enzyme (hereinafter "rADA"). Both recombinant human

("rhADA") and recombinant bovine ("rbADA") have been considered as replacements for purified natural bovine ADA. The rbADA and rhADA enzymes are somewhat less stable than the native purified bovine enzyme that is currently employed. Both rhADA and rbADA are believed to degrade in a manner consistent with cysteine degradation: addition of oxygen; formation of dithiols; increasing degradation as pH increases; precipitation, especially as the pH is increased and the samples are concentrated. In the reduced state, cysteine contains a reactive -SH group (sulfhydryl) which is the form responsible for the degradation.

We have developed evidence suggesting that a single exposed cysteine may be responsible for the degradation that is seen for both rbADA and rhADA. ADA purified from a bovine source has a structure very similar to that of rhADA: both bovine ADA and rhADA have the same number of cysteines in the same positions of the primary sequence. Currently available recombinant bovine ADA contains degradants/impurities (dithiols) that are consistent with cysteine reactivity. Native bovine ADA differs structurally from recombinant bovine ADA in that it has a single mole of cysteine bound to each mole of ADA and native bovine ADA is stable to high pH, suggesting that the cysteine bound to the ADA is functioning as a protecting group.

In spite of the foregoing, it would be beneficial to provide rbADA and/or rhADA that are stable, i.e., without significant degradation, at pH levels which are useful for optimum PEGylation of the enzyme when the desired activated PEG's are used. Other proteins and enzymes having free cysteine group(s) that may be involved in instability such as certain interferons, for example, would similarly benefit from stabilization. The present invention addresses this need.

SUMMARY OF THE INVENTION

Accordingly, the invention provides for a capped protein comprising at least one amino acid residue that is capped with a capping agent selected to inhibit the oxidation of the amino acid when the protein is in an aqueous medium, so that the capped protein is substantially more stable in aqueous medium than the equivalent uncapped protein. The amino acid is an oxidizable amino acid, and can be, for example, a cysteine, tryptophan or methionine. The capped protein can be any protein that is of interest, including, for example, an adenosine deaminase protein, whether native or recombinantly expressed, or an interferon, such as, e.g., beta interferon, an alpha interferon and a gamma interferon. Preferably, the adenosine deaminase is a recombinant human adenosine deaminase (e.g., SEQ ID NO: 2) or a recombinant bovine adenosine deaminase (e.g., SEQ ID NO: 4). Optionally, the recombinant adenosine deaminase lacks the six C-terminal residues indicated by the theoretical translation of the corresponding DNA open reading frame. In an optional alternative aspect, the N-terminal Met indicated by the theoretical translation of the corresponding DNA open reading frame is present in the capped protein.

The inventive capped protein is capped with a capping agent such as oxidized glutathione, iodoacetamide, iodoacetic acid, cystine, other dithiols and mixtures thereof. The inventive capped protein is optionally conjugated to a substantially non- antigenic polymer to form a polymer conjugated capped recombinantly expressed protein, e.g, a polyalkylene oxide such as polyethylene glycol, wherein the polyalkylene oxide ranges in size from about 2,000 to about 100,000 Daltons. The polyalkylene oxide is preferably conjugated to the protein via linker chemistry including, e.g., succinimidyl carbonate, thiazolidine thione, urethane, and amide based linkers. The polyalkylene oxide is preferably covalently attached to an epsilon amino group of a Lys on said cysteine-stabilized recombinant human adenosine deaminase, although other sites for covalent attachment are well known to the art. The inventive capped protein preferably comprises at least 5 polyethylene glycol strands attached to epsilon amino groups of Lys on said protein, but alternatively, can comprise about 11- 18 PEG strands attached to epsilon amino groups of Lys on said protein.

The invention further provides a method of stabilizing a protein having an oxidizable amino acid thereon, comprising, treating a protein containing an oxidizable amino acid with a sufficient amount of a capping agent, in aqueous solution, under reaction conditions sufficient to cap the reactive amino acid without substantially inactivating the recombinantly expressed protein. The reactive amino acid can include cysteine or methionine and/or any other art-known reactive protein. The protein can be any protein of interest, as set forth above. The capping agent is, e.g., oxidized glutathione, iodoacetamide, iodoacetic acid, cystine, other dithiols and mixtures thereof, and oxidized glutathione is preferred. Preferably, the oxidized glutathione is reacted with the protein at a concentration of from about 20 to about 100 mM or alternatively, the oxidized glutathione is reacted with the protein at a concentration of from about 22 to about 30 mM. Preferably, the inventive method provides for the reaction conditions to comprise an aqueous solution at a pH of from about 6.5 to about 8.4. More particularly, the reaction conditions include an aqueous solution atpH of from about 7.2 to about 7.8.

In a preferred aspect, the aqueous solution includes a buffer selected from the group consisting of sodium phosphate, potassium phosphate, Tris, and Hepes at concentrations ranging from 20 to 150 mM. The inventive process further includes reacting the capped protein in the aqueous solution with an activated polymer under condition sufficient to form, capped protein-polymer conjugates, e.g., an activated polyethylene glycol, such as, a succinimidyl carbonate-activated polyethylene glycol, a thiazolidine thione-activated polyethylene glycol, and/or a urethane linkage or amide-linkage forming -activated polyethylene glycol, where the polyethylene glycol preferably has a molecular weight of from about 2,000 to about 100,000 Daltons or more particularly, wherein the polyethylene glycol has a molecular weight of from about 4,000 to about 45,000 Daltons. The inventive process further includes a method of stabilizing a recombinantly expressed protein having an oxidizable cysteine thereon, comprising, recombinantly expressing a protein in a suitable prokaryotic expression system and recovering the recombinantly expressed protein from a cell extract media containing a sufficient amount of a capping agent whereby the oxidizable amino acid on the recombinantly expressed protein is stabilized upon secretion from the prokaryotic cells in said suitable prokaryotic cell expression system. Preferably, the prokaryotic expression system is an E. coli expression system capable of producing recombinant adenosine deaminase, and the recombinantly expressed protein is adenosine deaminase and/or interferon.

The inventive process further includes a method for producing a stabilized derivative of an oxidizable protein of interest, comprising the steps of

(a) identifying an oxidizable protein of interest, wherein the protein is oxidizable at one or more amino acid residues when the protein is in aqueous solution,

(b) identifying at least one oxidizable amino acid of the protein of interest, (c) reacting the oxidizable protein of interest with a capping agent selected to prevent the oxidation of the oxidizable amino acid.

The protein is, e.g., an adenosine deaminase or interferon, the oxidizable amino acid is cysteine or methionine and the capping agent is selected from the group consisting of oxidized glutathione, iodoacetamide, iodoacetic acid, cystine, other dithiols and mixtures thereof.

Still further aspects of the invention include cysteine-stabilized recombinant proteins, enzymes or the like, and methods of making the same. A preferred embodiment in this regard is cysteine stabilized rbADA and/or rhADA. Polymer conjugates thereof are also contemplated as embodiments of the invention. The polymer is preferably a polyalkylene oxide, e.g, more preferably a polyethylene glcyol. The conjugates are prepared by reacting the capped protein with an activated polymer, including, e.g., succinimidyl carbonate- activated polyethylene glycol ("SC-PEG"), thiazolidine thione-activated polyethylene glycol ("T-PEG"), and/or a urethane linkage or amide-linkage forming -activated polyethylene glycol. SC-PEG is preferred.

A still further aspect of the invention includes methods of treatment using a stabilized enzyme in polymer conjugated or unconjugated form. Specifically, the inventive methods include administering an effective amount of the cysteine- stabilized rADA to a patient in need thereof in order to treat SCID or other enzyme deficiency- related conditions.

For purposes of the present invention "effective amount" shall be understood to mean an amount which achieves a desired clinical result, i.e. reduction, slowing, remission, etc. or reversal of the enzyme deficiency or SCID condition in the patient, i.e., in a mammal or a human.

Further, the use of singular terms for convenience in description is in no way intended to be so limiting. Thus, for example, reference to a composition comprising a cell includes reference to one or more of such cells.

DETAILED DESCRIPTION OF THE INVENTION A. Overview

Broadly, the invention provides capped or protected proteins with enhanced stability. In particular, there are provided proteins expressed with one or more oxidizable or reactive cysteine residues that are exposed to the solution environment of the protein so that the protein exhibits instability in solution, e.g., during storage. The protein is preferably a recombinantly produced ADA enzyme.

The number of oxidizable amino acid residues that can be capped according to the invention will depend on the number of such amino acid residues that are present in the protein of interest, and on their location in the tertiary structure of the protein.

Without meaning to be bound by any theory or hypothesis, it is thought that oxidizable amino acids that are exposed to solution when the protein is dissolved or suspended in an aqueous medium, are those most likely to cause oxidation-mediated degradation of the protein. Identifying and capping such amino acids thus inhibits oxidation of such amino acid residues, and thereby provides capped proteins with enhanced stability relative to uncapped proteins. Thus, for example, the residues to be capped can be determined by identifying breakdown products of a protein of interest that are unstable in an aqueous medium, and determining the location of fragment breaks relative to the presence of oxidizable amino acids. The number of residues to be capped can be as few as one, up to as many as necessary to stabilize a protein of interest, e.g., up to as many as 50, or more. More particularly, the number of residues to be capped more typically ranges from 1 to about 20, and even more particularly, from 1 to about 10 amino acid residues.

The invention also provides a method of stabilizing a recombinantly expressed protein having an oxidizable cysteine thereon. The method includes treating a recombinantly expressed protein containing an oxidizable cysteine with a sufficient amount of a capping agent under reaction conditions sufficient to cap the oxidizable cysteine without substantially inactivating the recombinantly expressed protein.

As will be appreciated by those of ordinary skill, "treating" in this aspect, includes allowing the recombinantly expressed protein to react with the capping agent by contacting of the two principal reactants in a suitable buffered solution which will protect the biological activity of the protein while allowing the selective oxidation of the reactive cysteine to occur. The "treating" can occur at various stages of the recombinant production after expression but preferably for this aspect of the invention, during one or more of the post expression purification steps. For example, initial purification of ADA proteins from cell extract can be achieved through an anion exchange chromatography in the presence of reducing agent like dithiothreitol (DTT). Subsequently, upon removal of DTT, the semi-purified ADA can be treated with a sufficient amount of capping agent to block the reactive cysteine. The same procedure can be followed either at any step of purification or with the purified protein at the end of purification and prior to PEGylation. Once capped, no reducing agent is required to maintain stability of the modified ADA. B. ADA Proteins

In certain preferred aspects of the invention, the recombinant^ expressed protein is rhADA or rbAD A. More preferably, the translated rhADA has the amino acid sequence of SEQ ID NO. 2, prior to capping, and the translated rbADA has the amino acid sequence of SEQ ID NO: 4, prior to capping. It should be noted that SEQ ID NO: s 2 and 4 are the predicted mature translation sequences, and reflect the post translational cleavage of the N-termmal Met residue. Thus, in the translated respective mature ADA proteins, the Cys residue is at position 74. In addition, it should be noted that the natural bovine ADA as isolated from bovine intestine also has six residues posttranslationally removed from the C-terminal. It is an optional feature of the present invention that the rbADA is either expressed without the six C-termύial residues (as a mutein) or is posttranslationally modified to remove the same C-terminal residues lacking in the purified natural bovine ADA.

It should further be noted that natural bovine ADA as isolated from bovine intestine has polymorphisms. Thus, with reference to SEQ ID NO: 4, bovine ADA polymorphisms include, e.g., glutamine at position 198 in place of lysine, alanine at position 245 in place of threonine; arginine at position 351 instead of glycine. It is therefore contemplated that recombinant position 74 mutein bovine ADA according to the invention, can also have additional substitutions at one or more of the noted positions or analogs of those positions: GIn in place of Lysi₉₈; Ala in place of Thr245; Arg in place of GIy₃₅₁.

In addition, the rhADA is expressed by a DNA with codons optimized for E. coli expression, according to SEQ ID NO: 1 and the rbADA is expressed by a DNA with codons optimized for E. coli expression according to SEQ ID NO: 3. A suitable expression vector can be prepared from genomic or cDNA encoding rhADA or rbADA, respectively, that is optionally under the control of a suitable operably connected inducible promoter.

As exemplified hereinbelow, the vector (pET9d) containing the basic rhADA nucleotide sequence was obtained from Mike Hershfield of Duke Medical Center, North Carolina (as the HADA1092 clone). The HADA1092 clone was then optimized for bacterial expression, and inserted into a suitable expression vector under the control of an inducible promoter and transformed into the BLR(λDE3) strain. This recombinant clone was designated as EN760.

Similarly, a recombinant rbADA expression clone was constructed by transforming BioCatalytics plasmid (p2TrcGro-RoADA2-23) into fresh E. coli BL21 chemically competent cells (Novagen, catalog number 69449-4, lot number N60830, Genotype F^" ompT ύsdSβ (I_B me ) gal dcm). This clone contains the plasmid p2TrcGro-RoADA2-23 that has the bovine ADA gene provided by Roche and the E. coli GroEL/GroES chaperone operon (U.S. 6,366,860). This new transformed clone was cultured on a plate containing CFB medium (10 g/L soy peptone, 5 g/L yeast extract, 10 g/L sodium chloride), 2% (w/v) agar, and 25 μg/mL kanamycin at 3O⁰C. The expression of rbADA was induced by IPTG.

C. Capping Agents and Reaction Conditions

The capping agents included in the methods of the invention include, without limitation, oxidized glutathione (preferred), iodoacetamide, iodoacetic acid, cystine, and other dithiols known to those of ordinary skill, and mixtures thereof. The amount and concentration of the capping agent included during the reacting phase of the methods described herein will depend somewhat upon the specific capping agent used and the needs of the artisan but will not be subject to undue experimentation. Using oxidized glutathione as a prototype, the concentration of used when reacted with the recombinant protein such as rhADA can range from about 25 μM to about 100 mM. Preferably, the oxidized glutathione is reacted with the recombinant protein at a concentration of from about 5 mM to about 25 mM.

The reaction conditions employed during the reacting of the capping agent and the recombinant protein further include the use of an aqueous solution having a pH of from about 6.5 to about 8.4, preferably from about 7.2 to about 7.8. to. addition, the aqueous solution preferably includes a suitable buffer such as sodium phosphate, potassium phosphate, Tris, and Hepes and mixtures thereof at concentrations ranging from 10 to 150 mM (Comments: capping can take place out of this buffer range, lower than 10 or high than 150 mM). The reaction conditions further include allowing the reaction to proceed at temperatures which will not contribute to degradation of the protein, i.e. from about 4 -37° C. Optionally, capping can take place outside of this temperature range, e.g., at a temperature range lower than 0-4 ° or higher than 37°C. The reaction is conducted for a time sufficient to achieve the desired stabilization of the reactive cysteine. Simply by way of example, the reaction is conducted for a time ranging from about 5 seconds to about 8 hours (e.g., overnight). While the invention is being described in terms of a preferred recombinant protein, i.e. rhADA, or rbADA, it will be understood that the processes described herein can be carried out using any number of recombinantly prepared proteins, enzymes, etc. having free cysteine group(s) that may be involved in instability, such as certain interferons. For example, α, β, or γ interferon can stabilized by the inventive methods. Without limitation, other suitable proteins, polypeptides and peptides of interest include, but are not limited to, hemoglobin, serum proteins such as blood factors including Factors VII, VIII, and IX; immunoglobulins, cytokines such as interleukins, i.e. IL-I through IL-13, etc., colony stimulating factors including granulocyte colony stimulating factors, platelet derived growth factors, etc. Other proteins of biological or therapeutic interest include insulin, plant proteins such as lectins and ricins, tumor necrosis factors and related proteins, growth factors such as transforming growth factors, such as TGFGC'S or β's and epidermal growth factors, hormones, somatomedins, erythropoietin, pigmentary hormones, hypothalamic releasing factors, antidiuretic hormones, prolactin, chorionic gonadotropin, follicle- stimulating hormone, thyroid-stimulating hormone, tissue plasminogen activator, and the like. Immunoglobulins of interest include IgG, IgE, Igm, IgA, IgD and fragments and single chain constructs thereof.

Enzymes of interest include carbohydrate-specific enzymes, proteolytic enzymes, oxidoreductases, transferases, hydrolases, lyases, isomerases and ligases. Without being limited to particular enzymes, examples of enzymes of interest include L-asparaginase, arginase, arginine deiminase, superoxide dismutase, endotoxinases, catalases, chymotrypsin, lipases, uncases, adenosine diphosphatase, tyrosinases and bilirubin oxidase. Carbohydrate-specific enzymes of interest include glucose oxidases, glucodases, galactosidases, glucocerebrosidases, glucuronidases, etc. The foregoing list is simply illustrative of the proteins which are contemplated to be stabilized using the methods of the invention. It is to be understood that those proteins, enzymes, etc. as defined herein, not specifically mentioned but having reactive cysteines are also intended to be embraced by the inventive method.

As a result of the methods of the present invention, the stabilized protein has a capping group attached thereto at a reactive cysteine. For example, the glutathione or other capping group is conjugated to the rhADA via a disulfide bond as such:

ADA - S -S - glutathione where one cysteine in the primary sequence of ADA is bound to a molecule of the glutathione, shown below with the -SH of the ADA Cys reacting with the -SH of the glutathione shown below:

Such disulfide bonds are stable towards the oxidative degradation pathways and thus render rhADA more stable, especially at a pH above 7.4. In addition, the MW of glutathione- adduct of the stabilized protein such as rhADA will be higher than the pre- stabilized form by an amount about equal to the MW of the capping agent used. This will allow separation of the desired adduct, if desired. The stabilized adduct will also have different physical and chemical properties which will allow chromatographic separation and isolation as well.

D. Polymer Conjugates

In another aspect of the invention, the cysteine capped or stabilized recombinant protein is conjugated to a suitable polymer in order to make polymer conjugates. The polymer conjugation is preferably a PEGylation reaction as such reactions are known to those of ordinary skill. Briefly stated, rhADA or rbADA, for example, is reacted with an activated polymer in an aqueous solution under condition sufficient to form cysteine capped or stabilized polymer conjugates. In this regard, a wide variety of activated polyethylene glycols can be used, including those described, for example in commonly assigned US Patent Nos. 5,122,614, 5,324,844, 5,612,460 and 5,808,096 (succinimidyl carbonate-activated polyethylene glycol (SC-PEG) and related activated PEG's), 5,349,001 (cyclic imide thione activated PEG's), 5,650,234, and others known to those of ordinary skill. The disclosure of each of the foregoing is incorporated herein by reference. See also activated polymers available from Nektar / Shearwater Polymers. The activated PEG's can be linear, branched or U-PEG derivatives such as those described in U.S. Patents Nos. 5,605,976, 5,643,575, 5,919,455 and 6,113,906 (also incorporated herein by reference) or branched derivatives. It will be further understood that in addition to the PEG-based polymers, a number of other polyalkylene oxides can also be used. For example, the conjugates of the present invention can be made by methods which include converting the multi-arm PEG-OH or "star-PEG" products such as those described in NOF Corp. Drug Delivery System catalog, Ver. 8, April 2006, the disclosure of which is incorporated herein by reference, into a suitably activated polymer, using the activation techniques described in the aforementioned '614 or O96 patents. Specifically, the PEG can be of the formula:

or

wherein: u' is an integer from about 4 to about 455, to preferably provide polymers having a total molecular weight of from about 5,000 to about 40,000; and up to 3 terminal portions of the residue is/are capped with a methyl or other lower alkyl.

In some preferred embodiments, all 4 of the PEG arms are converted to suitable leaving groups, i.e. SC, etc., for facilitating attachment to the recombinant protein. Such compounds prior to conversion include:

and

In most preferred aspects of the invention, the activated polyethylene glycol is one which provides a urethane linkage or amide-linkage with the protein.

In yet alternative aspects, the activated polymers can employ a hindered ester- based linker. See US Provisional Application No. 60/844,942 entitled "Polyalkylene Oxides Having Hindered Ester-Based Biodegradable Linkers", the contents of which are incorporated by reference. For example, a non-limiting list of such compounds - include: and

wherein "u" is an integer ranging from 10 to about 455.

Suitable polymers will vary substantially by weight. Polymers having molecular number average weights ranging from about 2,000 to about 100,000 are usually selected for the purposes of the present invention. Molecular weights of from about 4,000 to about 45,000 are preferred and 5,000 to about 12,000 are particularly preferred.

Methods of preparing polymers having terminal carboxylic acids hi high purity are described in US Patent Application No, 11/328,662, the contents of which are incorporated herein by reference. The methods include first preparing a tertiary alkyl ester of a polyalkylene oxide followed by conversion to the carboxylic acid derivative thereof. The first step of the preparation of the PAO carboxylic acids of the process includes forming an intermediate such as /-butyl ester of polyalkylene oxide carboxylic acid. This intermediate is formed by reacting a PAO with a /-butyl haloacetate in the presence of a base such as potassium /-butoxide. Once the /-butyl ester intermediate has been formed, the carboxylic acid derivative of the polyalkylene oxide can be readily provided in purities exceeding 92%, preferably exceeding 97%, more preferably exceeding 99% and most preferably exceeding 99.5% purity. hi yet alternative aspects, polymers having terminal amine groups can be employed to make the ADA conjugates. The methods of preparing polymers containing terminal amines in high purity are described in US Patent Application Nos. 11/508,507 and 11/537,172, the contents of each of which are incorporated by reference. For example, polymers having azides react with phosphine-based reducing agent such as triphenylphosphine or an alkali metal borohydride reducing agent such as NaBH₄. Alternatively, polymers including leaving groups react with protected amine salts such as potassium salt of methyl-tert-butyl iniidodicarbonate (KNMeBoc) or the potassium salt of di-tert-butyl imidodicarbonate (KNBoC₂) followed by deprotecting the protected amine group. The purity of the polymers containing the terminal amines formed by these processes is greater than about 95% and preferably greater than 99%. The branching afforded by the polymers of the 5,643,575 patent, cited above, allows secondary of tertiary branching as a way of increasing polymer loading on a biologically active molecule from a single point of attachment. It will be understood that the water-soluble polymer can be functionalized for attachment to the bifunctional linkage groups if required without undue experimentation.

The polymeric substances included herein are preferably water-soluble at room temperature. A non-limiting list of such polymers include polyalkylene oxide homopolymers such as polyethylene glycol (PEG) or polypropylene glycols, polyoxyethylenated polyols, copolymers thereof and block copolymers thereof, provided that the water solubility of the block copolymers is maintained. In addition to rnPEG, Cj_4 alkyl-terminated polymers are also useful.

As an alternative to PAO-based polymers, effectively non-antigenic materials such as dextran, polyvinyl pyrrolidones, polyacrylamides such as HPMA's (hydroxypropylmethacrylamides), polyvinyl alcohols, carbohydrate-based polymers, copolymers of the foregoing, and the like can be used. Those of ordinary skill in the art will realize that the foregoing list is merely illustrative and that all polymer materials having the qualities described herein are contemplated. For purposes of the present invention, "substantially or effectively non-antigenic" means all materials understood in the art as being nontoxic and not eliciting an appreciable immunogenic response in mammals.

E. Cell Extract Media Containing Capping Agent

In an alternative aspect of the invention, there is provided another method of stabilizing a recombinantly expressed protein having a reactive cysteine thereon. In this process, the steps include a recombinantly expressed protein in a suitable prokaryotic expression system and recovering the recombinantly expressed protein from a cell extract media containing a sufficient amount of a capping agent whereby the reactive cysteine on the recombinantly expressed protein is stabilized substantially upon secretion from the prokaryotic cells in the suitable prokaryotic cell expression system. As was the case in the first aspect of the invention, the preferred protein being expressed is rhAD A or rbAD A. A suitable prokaryotic expression system is an E. coli expression system capable of producing the rhAD A or rbAD A.

The rhADA cDNA containing expression vector (pET9d) was introduced into an E.coli expression strain, BL21(λDE3) or an equivalent expression vector and bacterial strain combination and the recombinant protein was allowed to synthesize in the bacteria by IPTG induction. Cells were then lysed in Tris buffer at pH 7.4 supplemented with EDTA and desired concentration of capping agent, 5- 50 mM herein, and an extract was prepared that contained expressed rhADA.

EXAMPLES

The following examples serve to provide further appreciation of the invention but are not meant in any way to restrict the effective scope of the invention. EXAMPLE 1

GENERATION OF RECOMBINANT BACTERIAL CLONE CARRYING SYNTHETIC rhADA cDNA The vector (pET9d-HCADA 1092) containing the basic human adenosine deaminase nucleotide (rhADA) sequence was obtained from Mike Hershfield of Duke Medical Center, North Carolina (as the HADAl 092 clone). The cDNA was codon optimized for bacterial expression by following the standard bacterial codon usage for Escherichia coli K12, using the codon data described by Grantham R. et al.; 1981 ; "Codon catalogue usage in genome strategy modulated for gene expressivity," Nucleic Acid Res. 9:r43-r47, and Lathe, R. 1985; "Synthetic oligonucleotide probes deduced from amino acid sequence data, Theoretical and Practical considerations." J. MoI Biol; 183:1-12, both of which are incorporated by reference herein in their entireties. The corresponding RNA sequence was then analyzed for the formation of hairpin structure or loop formation and was subjected to minimum free energy calculations, and Ncol and HindIII flanking sites were synthetically inserted to allow for cloning into expression vector pET28a (commercially available from Novagen). The resulting recombinant plasmid containing human ADA cDNA, was designated as pHADApET28a, and was transformed into the BLR(λDE3) strain of E.coli (Novagen) to create a viable expression clone for rhADA. This recombinant clone, with the rhADA gene under control of an isopropyl-beta-D-thiogalactopyranoside (IPTG) IPTG- inducible promoter, was designated as the EN760. The codon optimized DNA molecule encoding the rhADA protein is shown as SEQ ID NO: 1.

EXAMPLE 2

FERMENTATION Of EN760 Clone Expressing rhADA

A single colony from a BLR (λDE3) plate was inoculated in a 50ml tube in 5ml LB media containing kanamycin lOμg/ml and tetracycline 12.5μg/ml. The tube was incubated in a 37°C/250 rpm shaker for 7 hours and then, 250μl of the culture was used to inoculate a IL flask containing 250ml of fresh complete LB media. This flask was incubated in a 37°C/250 rpm shaker overnight. The overnight culture (OD_6O0 value 4.401) was used to inoculate a BioFlo 3000 fermenter (5L; North Brunswick Scientific Company, Edison, NJ) containing 4.5L complete SuperBroth (complete: kanamycin lOμg/ml, tetracycline 12.5μg/ml, 5ml/L glycerol, and 1.68ml/L MgSO₄JH₂O) resulting in an initial OD_60O of 0.2445. The culture was grown to an OD₆O₀ value of 22.5 (EFT 6 hours) and induced with 5 mM IPTG to begin the production phase. The culture was harvested 2 h post induction (ODeoo value 44) using a Beckman Coulter Centrifuge (7000rpm/4°C/20min) yielding a wet cell weight of 282 g. Medium composition

Composition of SuperBroth (Difco™SelectAPS™ SuperBroth from BD BioSciences, Inc.) Autoclave components:

Soy Hydrolysate 12.Og/L

Yeast Extract 24.0g/L

Di-potassium phosphate 11 AgJh

Mono-potassium phosphate l-7g/L

Filtered components (supplements):

Kanamycin lOμg/ml

Tetracyclin 12.5μg/ml Fermentation Parameters:

Agitation: lOOO rpm Temperature: 37°C pH: 7.0 Air: 1.5/2.0 (induction) wm

Kanamycin: lOμg/ml Tetracycline: 12.5μg/ml

MgSO₄.7H₂0: 1.68ml/L

EXAMPLE 3

GENERATION OF RECOMBINANT BACTERIAL CLONE CARRYING SYNTHETIC rbADA cDNA The purified mature ADA protein derived from bovine intestinal preparations is a 356 amino acid protein lacking the N-terminal methionine and final six C-terminal residues predicted from the cDNA sequence (GenBankNP_776312, incorporated by reference herein), hi brief, the defined polypeptide sequence was provided to BioCatalytics Inc. for whole gene synthesis of a new gene having codons optimized for expression in E. coli, using their methods that include chemical synthesis of overlapping oligonucleotide segments. The BioCatalytics methods are described in greater detail by U.S. Patent No. 6,366,860, the contents of which are incorporated by reference herein in their entirety.

Bovine ADA expression was investigated in several expression systems. For example, the flanking restriction sites, Ndel and BamHIweτQ included at the termini of the gene. Following digestion of the synthetic DNA with the restriction enzymes Ndel and BamHI, the 1.1 kilobase gene was ligated via T4 DNA ligase into the plasmid vector pET-9d (Novagen Corporation), which had also been digested with these two enzymes. The recombinant plasmid was introduced into E. coH strain BLR (DE3) or HMS 174 (DE3 ) by electroporation using a BTX Electro Cell Manipulator 600 according to the manufacturer's instructions. The transformation mixture was plated on LB agar plates containing kanamycin (15 μg/ml). This allowed for selection of colonies containing the plasmid pET-9d/bADA (designated bADA/pET9d: BLR(DE3) or bADA/pET9d:HMS174(DE3)). The ADA variant gene nucleotide sequence was verified by DNA sequence analysis with ABI Prism 310 Genetic Analyzer using Big Dye Terminators. Isolated colonies were further purified by plating and analyzed for IPTG inducible gene expression in LB medium by standard methods such as those described in Novagen pET System Manual Ninth Edition. Several induction parameters were examined including time, temperature and inducer concentration. A preferred condition was induction with 0.3 % lactose for 12 hrs at 37 ⁰C, which allowed high level production of ADA within the cytoplasm of the host bacteria at about 20% of total cell protein. The ADA product was confirmed on SDS PAGE analysis to exhibit

EXAMPLE 4 PURIFICATION Of RECOMBINANT ADA PROTEIN PRODUCED BY THE

CLONE EXPRESSING rbADA OF EXAMPLE 3

The purification of rbADA was carried out in a 3 chromatographic protocol developed by Enzon. Briefly, thawed cell paste (obtained from Biocatalytics, respectively) of 200 g which was stored at -80°C was re-suspended in 1800 ml buffer of 20 mM Bis-Tris, ImM EDTA, pH 7.4, and homogenized at 1200 RPM for 10 seconds with Tempest Virtis (Sentry™, Microprocessor, Boston, MA). This suspension was passed through a stainless steel mesh (Opening micrometer 250μ, No.60, W.S Tyler) to removed big particles. The homogenous cell suspension was microfluidized for 3 cycles at 15,000 psi (unit was ice-bathed) (Micro Fluidizer, Microfluidics Corp., Model# 11OY, Boston, MA). At the end of micro fluidization, 200 ml of the same buffer as above was used to rinse the unit and this solution was combined with the above suspension. The soluble protein from cell lysates was extracted by centrifugation at 7100 rpm (12000 x g) for 60 minutes at 4° C (Avanti J-201, Beckman Coulter; Rotor# JLA8.1000). The supernatant was collected carefully to avoid unwanted mixing. The resulting volume of supernatant was 2100 ml, total protein concentration by BCA method was 13.2 mg/ml. To remove nucleotides in this cell extract, 31 ml of 10% PEI solution (final PEI

0.15%) was added to the above supernatant and mixed thoroughly by stirring for 10 min. The cell extract was stored at 4° C over night. The precipitant from this over night sample was removed by a centrifugation at 7100 rpm (12000 x g), for 60 minutes at 4° C (Avanti J-201, Beckman Coulter; Rotor# JLA8.1000). Similarly, the supernatant was collected carefully to avoid any unwanted mixing. Now the volume of supernatant was 1940 ml, total protein concentration by BCA method was 3.53 mg/ml. Protection of rbADA or rhADA from oxidation of cysteine was carried out by adding 5-50 mM oxidized glutathione at pH 2-10. To help ADA bind to the first column, 10% PEG4600 was added to this cell extract slowly and the pH of this cell extract was adjusted to 6.5 slowly with 1 N NaOH and IN HCl. This supernatant was centrifuged again at 7100 rpm (12000 x g), for 60 minutes at 4° C (Avanti J-201, Beckman Coulter; Rotor# JLA8.1000) before loading to the next column.

The cell extract was loaded to a pre-equilibrated Capto Q column (Cat# 17- 5316-01, GE Healthcare, Piscataway, NJ. Bed volume 350 ml pre packed in a XK-50 column) with a buffer of 20 mM Tris-Bis, 1 mM EDTA, pH 6.5. Before ADA was eluted off from the column at 80 mM NaCl in the equilibration buffer, elutions at 60 mM and 70 mM NaCl were first performed to remove impurities. The elution profile was analysed by ADA activity, SDS-PAGE analysis, Western Blots, and RP-HPLC.

After Capto Q column, 2 hydrophobic interaction chromatographic purifications were used one by one to further polish the purity of the protein. The first HIC was

Octyl Sepharose 4FF (Cat# 17-0946-02, GE Healthcare, Piscataway, NJ). The pool of ADA fractions from Capto Q column was adjusted to 1.5 M (NHt)₂SO₄ with ammonium sulfate powder directly and the pH was adjusted to 6.5. The filtered sample (Nalgene Nunc, CAT #540887, MEMB 0.2 PES, Rochester, NY) was loaded to the 1^st HIC column which was pre-equilibrated with 1.5 M (NH₄)₂SO₄, 20 mM potassium phosphate, 1 mM EDTA, pH 6.5 (Bed volume 150 ml, in XK-50, GE Healthcare, Piscataway, NJ). The ADA protein was eluted with an ammonium sulfate gradient and the purity profile of this ehαtion was determined by SDS-PAGE and RP-HPLC. The ADA protein in the fractions of first HIC column was pooled and adjusted to 1 M (NHU)₂SO₄ and loaded directly to the second HIC column (Bed volume 150ml, XK-50, HIC Phenyl HP, Cat# 17-1082-01, Piscataway, NJ) which was pre-equilibrated with 1 M (NH₄)₂SO₄, 20 mM KH₂PO₄-K₂HPO₄, 1 mM EDTA, pH 6.5. ADA was eluted with an ammonium sulfate gradient from 1 M to 300 mM in the 20 mM KH₂PO^K₂HPO₄₅ 1 mM EDTA, pH 6.5. ADA purity of these fractions was analyzed by SDS-PAGE and RP-HPLC. The purified rbADA or rhADA was further desalted and concentrated in a LabScale™ TFF systems (Membrane BioMax 5, Bedford, MA).

EXAMPLE 5

PREPARATION OF rhADA CELL EXTRACT TREATED WITH OXIDIZED GLUTATHIONE Frozen cells prepared according to Example 2, were thawed and resuspended in resuspension buffer containing 20 mM Bis-Tris ImM EDTA at pH 7.4. The thawed, resuspended cells were homogenized, filtered through a 250μ stainless steel mesh and microfluidized at 15000 psi for 3 cycles to ensure complete lysis. The resultant cell extract was clarified by centrifugation and half of the supernant was added to 25 mM oxidized glutathione ("GSSG") (final concentration. GSSG was made in 250 mM stock in 200 mM Bis-Tris, pH 7.4). The other half of the extract was treated with an additional 25 mM GSSG (therefore, total concentration of GSSG was 50 mM) before further processing. Both samples were then treated overnight with 0.1% polyethyleneimine ("PEI", pH 7.4) at 4⁰C and centrifuged to remove nucleic acid and protein precipitates and subsequently filtered. The clarified sample was then processed further through anion exchange chromatography. The details of the cell extract preparation are compiled in Table 1, as follows.

TABLE 1

Processing of Cell Paste

The clarification of cell extracts by PEI treatment was confirmed by SDS-PAGE. The protection of ADA protein by GSSG was confirmed by reverse-phase HPLC.

EXAMPLE 6 PURIFICATION OF rhADA USING

ANION EXCHANGE CHROMATOGRAPHY IN THE PRESENCE OF OXIDIZED GLUTATHIONE

The cell extract (100 ml) obtained above was subjected to an anion ion-exchange chromatography column (can be Q Sepharose fast flow, DEAE Sepharose fast flow, Capto Q Sepharose fast flow, etc, GE Healthcare, Piscataway, NJ). Two independent chromatographies were conducted to purify the rhADA from 25 mM- and 50 mM GSSG- treated cell extracts. Briefly, the pH and conductivity of the cell extract were adjusted to 6.5 and 1.5 mS/cm, respectively with addition of IN NaOH, followed by dilution with distilled water. Approximately 100 ml of adjusted cell extract (treated with 25 mM GSSG) was passed through a 27ml DEAE column pre-equilibrated with equilibration buffer containing 20 mM Bis-Tris and 1 mM EDTA at pH 6.5. Non-specifically bound proteins were removed from the column with 2 cv (ie., column volumes) of equilibration buffer, and human ADA was eluted from the column with a 20 cv linear gradient of 20 mM KCl prepared in the above buffer. Fractions corresponding to the peak were collected, analyzed by 4-20% reducing gel and, fractions containing human ADA were pooled. Subsequent to protein elution, column was sanitized with NaOH and acetic acid and was equilibrated with equilibration buffer prior to process and purify human ADA from 50 mM GSSG- treated cell extract following an identical protocol. In order to retain integrity of the glutathione-modified species, no reducing agent was used during purification steps.

The separation of glutathione capped rhADA (GS -rhADA) from other impurities by this chromatographic method was confirmed by running peak fractions on standard SDS-PAGE. The ADA activity of GS-rhADA was confirmed by standard enzymatic assay. EXAMPLE 7

PURIFICATION OF rbADA USING CHROMATOGRAPHIC METHOD IN THE PRESENCE OF OXIDIZED GLUTATHIONE

The purification of rbADA was carried out in a 3 chromatographic protocol. Briefly, thawed cell paste of 200 g (as obtained from Example 4, supra) which was stored at -8O⁰C, was re-suspended in 1800 ml buffer of 20 roM Bis-Tris, ImM EDTA, pH 7.4, and homogenized at 1200 RPM for 10 seconds with Tempest Virtis (Sentry™,

Microprocessor, Boston, MA). This suspension was passed through a stainless steel mesh (Opening micrometer 25 Oμ, No.60, W. S Tyler) to removed big particles. The homogenous cell suspension was microfluidized for 3 cycles at 15,000 psi (unit was ice-bathed) (Micro Fluidizer, Microfluidics Corp., Model# HOY, Boston, MA). At the end of micro fluidization, 200 ml of the same buffer as above was used to rinse the unit and this solution was combined with the above suspension. The soluble protein from cell Iy sates was extracted by centrifugation at 7100 rpm (12000 x g) for 60 minutes at 4° C (Avanti J-201, Beckman Coulter; Rotor# JLA8.1000). The supernatant was collected carefully to avoid unwanted mixing. Now the volume of supernatant was 2100 ml, total protein concentration by BCA method was 13.2 mg/ml.

To remove nucleotides in this cell extract, a 31 ml of 10% PEI solution (final PEI 0.15%) was added to the above supernatant and mixed thoroughly by stirring for 10 min. This cell extract was then stored overnight at 4° C. The precipitant from this overnight sample was removed by a centrifugation at 7100 rpm (12000*g), for 60 minutes at 4° C (Avanti J-201 , Beckman Coulter; Rotor# JLA8.1000). Similarly, the supernatant was collected carefully to avoid any unwanted mixing. Now the volume of supernatant was 1940 ml, total protein concentration by BCA method was 3.53 mg/ml.

Protection of cysteine 74 (ie., Cys75 if the N-terminal Met is present) was performed by capping this amino acid with 25 mM oxidized glutathione at pH 6.5. To help ADA bind to the first column, 10% PEG4600 was added to this cell extract slowly and the pH of this cell extract was adjusted to 6.5 slowly with 1 N NaOH. This supernatant was centrifuged again at 7100 rpm (12000 x g), for 60 minutes at 4° C (Avanti J-201, Beckman Coulter; Rotor# JLA8.1000) before loaded to the next column.

The cell extract was loaded to a pre-equilibrated Capto Q column (Cat# 17-5316- 01, GE Healthcare, Piscataway, NJ. Bed volume 350 ml pre packed in a XK-50 column) with a buffer of 20 niM Tris-Bϊs, 1 mM EDTA, pH 6.5. Before ADA was eluted off from the column at 80 mM NaCl in the equilibration buffer, elutions at 60 mM and 70 mM NaCl were first performed to remove impurities. The elution profile was analysed by ADA activity, SDS-PAGE analysis, Western Blots, and RP-HPLC.

After the Capto Q column, two hydrophobic interaction chromatographic purifications were used one by one to further polish the purity of the protein. The first HIC was Octyl Sepharose 4FF (Cat# 17-0946-02, GE Healthcare, Piscataway, NJ). The pool of ADA fractions from Capto Q column was adjusted to 1.5 M (NHi)₂SO₄ with ammonium sulfate powder directly and the pH was adjusted to 6.5. The filtered sample (Nalgene Nunc, CAT #540887, MEMB 0.2 PES, Rochester, NY) was loaded to the 1^st HIC column which was pre-equilibrated with 1.5 M (NHi)₂SO₄, 20 mM potassium phosphate, 1 mM EDTA, pH 6.5 (Bed volume 150 ml, in XK-50, GE Healthcare, Piscataway, NJ). The ADA protein was eluted with an ammonium sulfate gradient and the purity profile of this elution was determined by SDS-PAGE and RP-HPLC. The ADA protein in the fractions of first HIC column was pooled and adjusted to 1 M (NHi)₂SO₄ and loaded directly to the second HIC column (Bed volume 150ml, XK-50, HIC Phenyl HP, Cat# 17-1082-01, Piscataway, NJ) which was pre-equilibrated with 1 M (NH^SO₄, 20 mM KH₂PO₄- K₂HPO₄, 1 mM EDTA, pH 6.5. ADA was eluted off with an ammonium sulfate gradient from 1 M to 300 mM in the 20 mM KH₂PO₄-K₂HPO₄, 1 mM EDTA, pH 6.5. ADA purity of these fractions was analyzed by SDS-PAGE and RP-HPLC. The purified rbADA was further desalted and concentrated in a LabScale™ TFF systems (Membrane BioMax 5, Bedford, MA). EXAMPLE 8

CHARACTERIZATION OF PARTIALLY PURIFIED rhADA BY MASS SPECTROMETRY AND REVERSE PHASE HPLC (RP-HPLC) A glutathione-modified recombinant human ADA cell extract was partially purified by anion exchange chromatography (Capto Q column, GE Healthcare, Piscataway, NJ) using 80 mM NaCl in 20 mM Tris-Bis, 1 mM EDTA (pH 6.5) for elution. The major product-containing fraction was subjected to liquid chromatography / mass spectrometry ("LC/MS") and analyzed for the presence of the glutathione adduct of ADA. The analysis indicated that the major peak, comprising 85% of the total area of the chromatogram, had a mass of 40,940 Da, which corresponds to the theoretical mass of glutathione-modified rhADA. Several other impurities were present, including a minor component of non- modified rhADA, and accounted for the remaining 15% of the chromatogram.

In addition, an experiment was performed to determine the effect of glutathione concentration on the degree of rhADA modification. The crude cell extract was divided into two portions which were subsequently treated with 25 mM and 50 mM glutathione, respectively. Each preparation was then partially purified by anion exchange chromatography and analyzed by LC/MS. Results of LC/MS analysis indicated that the major peak from each preparation had a mass corresponding to glutathione-modified rhADA, with peak area comprising 83-85% of the total area of the chromatogram. Several other impurities were present, including minor amounts of non-modified rhADA, accounting for the remaining 15-17% of the chromatograms. The results indicate that both 25 mM and 50 mM glutathione treatments efficiently convert rhADA to glutathione- modified rhADA.

EXAMPLE 9

CONFIRMATION OF PROTECTIVE EFFECT FOR rhADA OF CAPPED

CYSTEINE WITH IAA The following experiment was done to confirm the protective effect of capping rhADA with IAA. rhADA, at a concentration of approximately 0.6 mg/mL, was reacted with 125 mM iodoacetamide (IAA) in sodium phosphate buffer at pH 7.4 for 16 hours at 37°C. Within several minutes of beginning the reaction, analysis of the sample by RP- HPLC with UV and mass spectrometric detection showed that approximately 70.9% of the rhADA was monoderϊvatized with IAA and 17.2% was derivatized at two sites. After 2 and 16 hours incubation at 37⁰C, the chromatographic profile was not significantly changed, with no indication that oxidized species were formed, indicating that the IAA derivative was stable towards the oxidative degradation pathways typical of rhADA.

When a sample of rhADA that was not derivatized with IAA was incubated under the same conditions (16 hours at 37⁰C and pH 7.4) and analyzed similarly, the rhADA protein degraded to oxidized species to an extent of 30%.

Thus, capping with IAA protected rhADA from the oxidative degradation that occurs at 37⁰C and pH 7.4.

EXAMPLE 10

CONFIRMATION OF PROTECTIVE EFFECT FOR rhADA OF CAPPED CYSTEINE WITH GLUTATHIONE

As an alternative to the IAA, it was decided to attempt derivatization of the reactive cysteine with oxidized glutathione, a normal biological constituent. rhADA, at a concentration of 0.6 mg/mL, was reacted with 25 mM oxidized glutathione in sodium phosphate buffer at pH 7.4. After incubation for 4, 8, and 12 hours, the sample was analyzed by RP-HPLC with UV and mass spectrometric detection. The monoderivatized form of GS-rhADA was formed exclusively and the derivative was stable after 12 hours incubation at 25⁰C. Additionally, the activity of the mono-glutathionylated rhADA compound was not found to be significantly different from the activity of the non- derivatized rhADA. Thus, capping the reactive cysteine in rhADA with oxidized glutathione yields a predominantly mono-derivatized species that is stable under conditions where the non-derivatized rhADA is prone to oxidative degradation and the derivative has activity comparable to that of the non-derivatized form of rhADA. EXAMPLE 11 CONFIRMATION OF PROTECTIVE EFFECT FOR GS-rbADA

Samples of mature rbADA, capped at cys74 with glutathione (GS-rbADA) at concentrations of approximately 0.5 mg/mL in sodium phosphate buffer (pH 7.8) were used for the stability study. Stability was monitored by reversed-phase HPLC (RP-HPLC) using both UV detection at 220 nm and mass spectrometric detection (Micromass Q-TOF electrospray mass spectrometer). The HPLC conditions were as follows:

Column: Zorbax 300 SB-C8 (Agilent, 250 x 4.6 mm, 300 angstrom pore size, 5 micron particle size

Mobile Phase A: 0.1% trifluoroacetic acid in water

Mobile Phase B: 0.1% trifluoroacetic acid in acetonitrile/water (80/20; v/v)

Gradient: Time % Mobile Phase B

0 20

5 20

45 80 46 20

60 20

Column temperature: 4O⁰C Flow rate : 1.0 mL/min Injection volume: 50 μL

Purity of the compounds was determined by RP-HPLC analysis at the initial time the stability study was started and at various timepoints, including 4, 8, and 17 days, after initiation of the study. It should be noted that the rbADA and GS-rbADA samples were approximately two months old at the start of this study and had already suffered some degradation. However, for the purpose of the present study, the difference in purity between the initial timepoint and after 17 days incubation at 25⁰C is the relevant parameter to examine. As shown in Table 1, the purity of rbADA was 83.7% at the initial timepoint and decreased to 66.1% after 17 days, indicating that 17.6% of rbADA has degraded over this time period. Mass spectrometric analysis of the peaks separated chromatographically indicated that the major degradant eluting at 31.851 min, accounting for 30.5% of the area of the chromatogram, had a mass 32 Da higher than that of rbADA. This mass change is consistent with the addition of 2 oxygens to rbADA to form the sulfinic acid degradant of the free cysteine at position 74 of rbADA. The smaller degradant peak, eluting at 32.538 min, had a mass consistent with the addition of 1 oxygen to rbADA to form the sulfenic acid degradant of the free cysteine at position 74 of rbADA. In contrast to the extensive degradation found for rbADA, GS-rbADA did not exhibit significant degradation when stored under the same conditions, with the purity at 81.9% at the initial timepoint and purity at 87.4% after 17 days at 25⁰C. The apparent increase in purity is likely due to chromatographic variations occurring as is more evident when the % purities at each of the 4 successive timepoints are examined. There is no evidence of the extensive oxidative degradation seen in rbADA demonstrating that capping the reactive cysteine in rbADA protects the enzyme from this degradation pathway. This proves that cysteine74 is indeed the source of the oxidative degradation that occurs in ADA.

Table 2. Stability of rbADA, GS-rbADA, in sodium phosphate buffer (pH 7.8) at

25⁰C

Claims

What is claimed is:

1. A capped protein comprising at least one amino acid residue that is linked to a capping agent, wherein the capped protein is substantially more stable in aqueous medium than the equivalent uncapped protein, wherein the linked amino acid residue is an amino acid that is oxidizable, and wherein the capping agent is selected to inhibit the oxidation of the amino acid.

2. The capped protein of claim 1 wherein the oxidizable amino acid is selected from the group consisting of a cysteine residue, a methionine residue, a tryptophan residue, and combinations thereof.

3. The capped protein of claim 1 that is an adenosine deaminase protein or an interferon.

4. The capped protein of claim 3 wherein the adenosine deaminase is selected from the group consisting of recombinant human adenosine deaminase and recombinant bovine adenosine deaminase.

5. The capped protein of claim 4 wherein the recombinant human adenosine deaminase comprises SEQ ED NO: 2, prior to capping; and the recombinant bovine adenosine deaminase comprises SEQ ID NO: 4, prior to capping.

6. The capped protein of claim 5 wherein the six C-terminal residues of the bovine adenosine deaminase are not present.

7. The capped protein of claim 1 wherein the capping agent is selected from the group consisting of oxidized glutathione, iodoacetamide, iodoacetic acid, cystine, other dithiols and mixtures thereof.

8. The capped protein of claim 4 that is conjugated to a substantially non-antigenic polymer to form a polymer conjugated capped protein.

9. The polymer conjugated capped protein of claim 8 wherein the polymer is a polyalkylene oxide.

10. The polymer conjugated capped protein of claim 9 wherein the polyalkylene oxide ranges in size from about 2,000 to about 100,000 Daltons.

11. The polymer conjugated capped protein of claim 9 wherein the polyalkylene oxide is a polyethylene glycol.

12. The polymer conjugated capped protein of claim 11 wherein the polyethylene glycol is conjugated to the protein via a linker chemistry selected from the group consisting of succinimidyl carbonate, thiazolidine thione, urethane, and amide based linkers.

13. The polymer conjugated capped protein of claim 11 wherein the polyethylene glycol is covalently attached to an epsilon amino group of a Lys on said capped protein.

14. The polymer conjugated capped recombinantly expressed protein of claim 11, wherein protein has at least 5 polyethylene glycol strands attached to epsilon amino groups of Lys on said capped protein.

15. The polymer conjugated capped recombinantly expressed protein of claim 11 , wherein the protein has about 11-18 PEG strands attached to epsilon amino groups of Lys on said protein.

16. A method of capping a protein having at least one oxidizable amino acid thereon, comprising, treating the protein containing an oxidizable amino acid with a sufficient amount of a capping agent under reaction conditions sufficient to cap the oxidizable amino acid without substantially inactivating the protein, to provide a stabilized protein.

17. The method of claim 16, wherein the protein is a recombinant adenosine deaminase selected from the group consisting of recombinant human adenosine deaminase and recombinant bovine adenosine deaminase.

18. The method of claim 17, wherein the recombinant human adenosine deaminase comprises SEQ ID NO: 2, prior to capping; and the recombinant bovine adenosine deaminase comprises SEQ ID NO: 4, prior to capping.

19. The method of claim 16, wherein the capping agent is selected from the group consisting of oxidized glutathione, iodoacetamide, iodoacetic acid, cystine, other dithiols and mixtures thereof.

20. The method of claim 19, wherein the capping agent is oxidized glutathione.

21. The method of claim 20, wherein the oxidized glutathione is reacted with the protein at a concentration of from about 20 to about 100 mM.

22. The method of claim 20, wherein the oxidized glutathione is reacted with the protein at a concentration of from about 22 to about 30 mM.

23. The method of claim 16, wherein the reaction conditions include an aqueous solution at pH of from about 6.5 to about 8.4.

24. The method of claim 23, wherein the reaction conditions include an aqueous solution at pH of from about 7.2 to about 7.8.

25. The method of claim 23, wherein the aqueous solution includes a buffer selected from the group consisting of sodium phosphate, potassium phosphate, Tris, and Hepes at concentrations ranging from 20 to 150 mM.

26. The method of claim 16, further comprising reacting the capped protein in said aqueous solution with an activated polymer under condition sufficient to form capped protein-polymer conjugates.

27. The method of claim 26, wherein the activated polymer is an activated polyethylene glycol.

28. The method of claim 27, wherein the activated polyethylene glycol is selected from the group consisting of a succinimidyl carbonate-activated polyethylene glycol, a thiazolidine thione-activated polyethylene glycol, and a urethane linkage or amide-linkage forming -activated polyethylene glycol.

29. The method of claim 27, wherein the polyethylene glycol has a molecular weight of from about 2,000 to about 100,000 Daltons.

30. The method of claim 29, wherein the polyethylene glycol has a molecular weight of from about 4,000 to about 45,000 Daltons.

31. The method of claim 16, wherein the protein is an interferon.

32. The method of claim 31, wherein the interferon is selected from the group consisting of a beta interferon, an alpha interferon and a gamma interferon.

33. A method of stabilizing a recombϊnantly expressed protein having a reactive cysteine thereon, comprising, recombinantly expressing a protein in a suitable prokaryotic expression system and recovering the recombinantly expressed protein from a cell extract media containing a sufficient amount of a capping agent whereby the reactive cysteine on the recombinantly expressed protein is stabilized upon secretion from the prokaryotic cells in said suitable prokaryotic cell expression system.

34. The method of claim 35, wherein the prokaryotic expression system is an E, coli expression system capable of producing recombinant adenosine deaminase.

35. The method of claim 33, wherein the recombinantly expressed protein is selected from the group consisting of adenosine deaminase and interferon.

36. The method of claim 35 , wherein the recombinantly expressed adenosine deaminase has an amino acid sequence of SEQ ID NO: 2 or SEQ ID NO: 4, prior to capping.

37. The method of claim 33, wherein the capping agent is selected from the group consisting of oxidized glutathione, iodoacetamide, iodoacetic acid, cystine, other dithiols and mixtures thereof.

38. The method of claim 37, wherein the capping agent is oxidized glutathione.

39. The method of claim 38, wherein the glutathione in said cell extract media is present in a concentration of from about 20 to about 100 mM.

40. The method of claim 39, wherein the glutathione in said cell extract media is present in a concentration of from about 20 to about 50 mM.

41. The method of claim 32, wherein the cell extract media includes 20 mM Bis-Tris and 1.0 mM EDTA at pH 7.4.

42. A method of treating an adenosine deaminase -mediated condition in mammals, comprising administering an effective amount of a polymer conjugated capped recombinantly expressed adenosine deaminase of claim 15.

43. The method of claim 42, wherein the adenosine deaminase-mediated condition is severe combined immune deficiency.

44. A method for producing a stabilized derivative of an oxidizable protein of interest, comprising the steps of (a) identifying an oxidizable protein of interest, wherein the protein is oxidizable at one or more amino acid residues when the protein is in aqueous solution,

(b) identifying at least one oxidizable amino acid of the protein of interest,

(c) reacting the oxidizable protein of interest with a capping agent selected to prevent the oxidation of the oxidizable amino acid.

45. The method of claim 44 wherein the protein is adenosine deaminase or interferon.

46. The method of claim 44 wherein the oxidizable amino acid is cysteine.

47. The method of claim 44 wherein the capping agent is selected from the group consisting of oxidized glutathione, iodoacetamide, iodoacetic acid, cystine, other dithiols and mixtures thereof.

48. The method of claim 23, wherein the reaction conditions include an aqueous solution at pH of about 6.5.