Methods of making proteins and peptides containing a single free cysteine

1. A method for refolding an insoluble protein or peptide that lacks essential disulfides and that comprises one free cysteine residue, comprising the following steps:

a) causing a host cell to express a protein or peptide that lacks essential disulfides and that comprises at least one free cysteine residue in an insoluble or aggregated form;

b) lysing the host cell;

c) isolating the protein or peptide;

d) denaturing and reducing the protein or peptide in a solution comprising both a denaturing agent and a reducing agent, wherein said reducing agent does not form a mixed disulfide with the free cysteine in the protein or peptide, and wherein said reducing agent does not inactivate a thiol-reactive polyethylene glycol (PEG) or does not interfere with modification of the protein by a thiol-reactive PEG reagent;

e) refolding the protein by reducing the concentrations of the denaturing agent and reducing agents in the solution of (d) to levels sufficient to allow the protein or peptide to renature into a soluble, biologically active form;

f) wherein steps (d) and (e) occur in the absence of a cysteine blocking agent.

2. The method of claim 1 , wherein the step (b) of lysing is performed by chemical, enzymatic or physical means.

3. The method of claim 1 , wherein said step (b) of lysing comprises lysing the host cell in the presence of a denaturing agent.

4. The method of claim 1 , wherein said step (b) of lysing comprises lysing the host cell in the presence of a denaturing agent and the reducing agent used in step (d).

5. The method of claim 1 , wherein said denaturing agent is selected from the group consisting of: urea, guanidine and N-lauroyl sarcosine.

6. The method of claim 1 , wherein the reducing agent in step (d) is a reducing agent that does not contain a thiol moiety.

7. The method of claim 1 , wherein the reducing agent in step (d) is a phosphine reductant.

8. The method of claim 1 , wherein the reducing agent in step (d) is an alkyl phosphine.

9. The method of claim 8 , wherein the alkyl phosphine is selected from the group consisting of a butyl phosphine, a hydroxypropyl phosphine, a cyanoethyl phosphine, and a carboxyethyl phosphine.

10. The method of claim 8 , wherein the alkyl phosphine is selected from the group consisting of: tri-n-butylphosphine (TBP), tris(hydroxypropyl)phosphine (THP), tris(2-cyanoethyl)phosphine (TCNP), and tris(2-carboxyethyl)phosphine (TCEP), or a combination thereof.

11. The method of claim 8 , wherein the alkyl phosphine is an ester of a carboxyethyl phosphine.

12. The method of claim 1 , wherein the reducing agent in step (d) is Tris(2-carboxyethyl)phosphine-HCl (TCEP).

13. The method of claim 1 , wherein said step (e) of refolding occurs in the presence of sufficient reducing agent to prevent the protein or peptide from forming disulfide bonds.

14. The method of claim 1 , wherein said step (e) of refolding comprises refolding the protein or peptide in the presence of glycerol.

15. The method of claim 1 , wherein said step (e) of refolding comprises refolding the protein or peptide in the presence of an oxidizing agent selected from the group consisting of oxygen, iodine, hydrogen peroxide, dihydroascorbic acid, tetrathionate, or O-iodosobenzoate.

16. The method of claim 1 , wherein step (e) of refolding comprises refolding the protein or peptide in the presence of a metal ion.

17. The method of claim 16 , wherein said metal ion is Cu ++ or Co ++ .

18. The method of claim 1 , further comprising isolating the refolded protein or peptide from other proteins and contaminants in the refold mixture.

19. The method of claim 18 , wherein the protein or peptide is isolated from other contaminants in the refold mixture by column chromatography.

20. The method of claim 19 , wherein the column chromatography buffers contain a reducing agent, wherein said reducing agent does not form a mixed disulfide with the free cysteine in the protein or peptide, and wherein said reducing agent does not inactivate a thiol-reactive polyethylene glycol (PEG) or does not interfere with modification of the protein by a thiol-reactive PEG reagent.

21. The method of claim 20 , wherein the reducing agent is the same reducing agent used in step (d).

22. The method of claim 18 , further comprising the step of exposing the protein or peptide to a cysteine-reactive moiety to obtain a cysteine-modified protein or cysteine-modified peptide, wherein the cysteine-reactive moiety is attached to at least one free cysteine in said isolated protein or peptide.

23. The method of claim 22 , wherein the step of exposing is conducted in the presence of a reducing agent.

24. The method of claim 23 , wherein the reducing agent is the same reducing agent used in step (d).

25. The method of claim 22 , wherein the cysteine-reactive moiety is selected from the group consisting of a polyethylene glycol, a polyvinyl pyrolidone, a carbohydrate, a dextran, a peptide, a lipid and a polysaccharide.

26. The method of claim 22 , wherein the cysteine-reactive moiety is a polyethylene glycol.

27. The method of claim 1 , wherein the protein is a cysteine variant of interferon-gamma (IFN-γ).

28. The method of claim 27 , wherein the protein is a cysteine variant of SEQ ID NO:1 and wherein amino acid Q1 is deleted or substituted by a non-glutamine amino acid.

29. The method of claim 27 , wherein the protein is a cysteine variant of SEQ ID NO:1 and wherein amino acid D2 is deleted or substituted by a non-aspartic acid amino acid.

30. The method of claim 27 , wherein the protein is a cysteine variant of SEQ ID NO:1, wherein amino acid Q1 is deleted or substituted by a non-glutamine amino acid and wherein amino acid D2 is deleted or substituted by a non-glutamine amino acid.

31. The method of claim 27 , wherein the protein is a cysteine variant of SEQ ID NO:1, and wherein amino acid S84 is substituted by cysteine.