Does carboxypeptidase Y have intrinsic endopeptidase activity?

Carboxypeptidase Y preparations from baker's yeast have been found to exhibit endopeptidase activity when assayed with oxidized insulin B-chain. Amino acid analysis and peptide isolation studies indicate that a specific internal cleavage occurs between Leu-15 and Tyr-16 in addition to the C-terminal carboxypeptidase activity. Blocking the C-terminal residue of the substrate prevents the exopeptidase activity of the enzyme, but has no effect on the endopeptidase activity. On the other hand, pepstatin A inhibits the endopeptidase but not the exopeptidase activity. These results suggest that the endopeptidase activity is due to the presence of contaminating amounts of yeast proteinase A and indicate that caution should be taken when employing carboxypeptidase Y preparations for sequence studies.