A novel [125I]iodinated carboxypeptidase A substrate detects a metallopeptidase activity distinct from carboxypeptidase A in brain

We have designed two radioactive substrates, hippuryl-L-[3H]phenylalanine and 3-(p-hydroxy, m-[125I]phenyl)propionic acid ([125I]Bolton reagent) derivative of L-arginyl-L-phenylalanine, i.e. [125I]BRF, for a highly sensitive assay of carboxypeptidase A (CPA) activity. After cleavage of the C-terminal phenylalanine residue by CPA, the radioactive product of the reaction was conveniently separated by polystyrene bead chromatography. Using [125I]BRF, typical CPA activity inhibited by 1 microM 2-benzyl-3-mercaptopropanoic acid was detected in extracts from rat pancreas or intestine. In brain and some other tissues, however, [125I]BRF-hydrolyzing activity was only inhibited by this compound in 1000-fold higher concentration, suggesting the participation of a metallopeptidase distinct from CPA.