The Amino Acid Sequence around the Reactive Thiol Group of Chymopapain B

The yellow colored sulfhydryl reagent, N-(4-dimethylamino-3,5-dinitrophenyl)maleimide, was used in the isolation of the peptide containing the reactive sulfhydryl group of chymopapain. Titration of cyanide-activated chymopapain B with p-chloromercuribenzoate at pH 4.6 indicated that there is a maximum of 1.4 moles of sulfhydryl per mole of the enzyme. Alkylation of half of the p-chloromercuribenzoate-titratable sulfhydryl groups led to the total inactivation of chymopapain. Pepsin digestion of the N-(4-dimethylamino-3,5-dinitrophenyl)maleimide-treated enzyme and the subsequent isolation of the labeled peptide showed that the label was predominantly in one peptide with the sequence, Lys-Arg-Val-Pro-Asp-Ser-Gly-Glu-Cys-Tyr. This sequence differed from those of the peptides containing the reactive sulfhydryl groups of papain and ficin, although all three enzymes are sulfhydryl proteases.