The mixed disulfide in the zymogen of streptococcal proteinase. Characterization and implication for its biosynthesis.

The identity of the volatile mercaptide and the metabolic pathway by which it becomes combined with the zymogen of streptococcal proteinase in the mixed disulfide were investigated. Mass spectrometric analysis identified the oxidized form of the volatile mercaptan as methanesulfonic acid. The mass spectrum of a peptide isolated from tryptic and subsequent chymotryptic digests of the zymogen was shown to be consistent with the previously reported amino acid sequence for a chymotryptic peptide with the sequence Val-Gly-Gln-Ala-Ala-Thr-Gly-His-Cys(SCH3)-Val. Studies using [35S] cystine, [methyl-35S]methionine and [methyl-14C]methionine in a cell suspension system revealed that the biosynthesis of the mixed disulfide in the zymogen may involve the formation of protein-S-SH followed by transmethylation rather than result from a direct transfer of an intact methanethiol to the cysteinyl residue of the zymogen. It is proposed that the attachment of the CH3-SH group to the protein-SH to form protein-S-S-CH3 is a process that is intimately related to the mechanism of secretion of the proteinase into the culture fluid by streptococci.