IMPROVEMENT IN THE SOLUTION STABILITY OF PORCINE SOMATOTROPIN BY CHEMICAL MODIFICATION OF CYSTEINE RESIDUES

A sustained release formulation is essential to commercialize porcine somatotropin (PST). Poor solution thermostability has hindered its development. Concentrated solutions of PST at elevated temperatures produced a physiologically nonreversible thixotropic gel. Disulfide exchange was noted during this aggregation. Two forms of dimeric PST, one of which was reducible to the monomer, were observed. Selective reductive alkylation of the Cys181,189 disulfide suppressed this exchange and improved PST solution stability up to 3.4 times during incubation at ca. 250 mg/mL in phosphate-buffered saline (pH 7.4) for 14 days at 39-degrees-C. Several cysteine derivatives were prepared which were bioactive in hypophysectomized rats, although the solution stability was variable. Carboxymethylation was used to link poly(ethylene glycol) chains. The results suggest that chemical modification of cysteine residues in PST can improve solution thermostability and is a general strategy for protein modification of either natural or mutated proteins.