Action of serine carboxypeptidases on endopeptidase substrates, peptide-4-methyl-coumaryl-7-amides.

Carboxypeptidase Y hydrolyzed N-substituted peptide-4-methylcoumarin-7-amides (peptide-NH-Mec) at pH 7 by releasing 7-amino-4-methylcoumarin (NH2-Mec) which was then followed by carboxypeptidase action. In particular, a chymotrypsin-directed substrate, Suc-Leu-Leu-Val-Tyr-NH-Mec, was hydrolyzed by the enzyme with a second-order rate constant of 7200 M-1 s-1, which is compatible with the rate for an anilide substrate and some N-substituted dipeptides. The activity was completely inhibited by phenylmethylsulfonyl fluoride and competitively depressed by the presence of an N-substituted dipeptide. Dependences of kinetic parameters on pH were different from those of carboxypeptidase, esterase, amidase, and anilidase activities. Carboxypeptidases P from Penicillium janthinellum and W from wheat also hydrolyzed some of these peptide-NH-Mec derivatives in a similar manner but at a rather low rate. Thus, the NH2-Mec-releasing activity may be considered to be intrinsic to serine carboxypeptidases in general. Taking into consideration this endopeptidase-like activity of serine carboxypeptidases, fluorogenic substrates should be used carefully to specify endopeptidases in crude extracts.