Porcine Carboxypeptidase B

Porcine carboxypeptidase B is inhibited rapidly by nitration with an eight‐fold molar excess of tetranitromethane. Enzymie activity towards both basic and non‐basic substrates falls to less than 30% of the control. The loss of activity correlates with the nitration of a single tyrosyl residue. The nitrotyrosyl‐containing peptide as isolated had the sequence Thr‐Ile‐Tyr‐Pro‐Ala, homologous to the peptide containing tyrosine‐248 in the active‐center sequence of bovine carboxypeptidase A. The nitrotyrosyl residue has an apparent pK of 7.5 (compared to 6.3 for the corresponding residue in carboxypeptidase A) indicating a difference in the microenvironment of nitrocarboxypeptidase B and nitrocarboxypeptidase A.