Peptic peptide of thiolsubtilisin. Analytical evidence for the chemical transformation of the essential serine-221 to cysteine-221.

Direct evidence is offered to show that the active site Ser-221 of subtilisin is transformed into a cysteine residue in the chemical procedure previously elaborated for the preparation of thiolsubtilisin. Thiolsubtilisin was digested with pepsin and the hydrolyzate was applied to an agarose-mercurial column. After elution with 2-mercaptoethanol a single tetrapeptide was obtained. Dansyl-Edman degradation showed that the SH-peptide conformed to the amino acid sequence around the active site Ser-221 of subtilisin. A simple, single-step procedure for isolation of SH-peptides is also described.