Product inhibition of carboxypeptidase H

Carboxypeptidase H is one of several enzymes required for the processing of peptide hormone precursors. In this study, inhibition of carboxypeptidase H by its peptide products was investigated. Carboxypeptidase H activity in bovine adrenal medulla chromaffin granules and rat adrenal medulla homogenate was inhibited by the peptides Met- and Leu-enkephalin, vasopressin, oxytocin, luteinizing hormone-releasing hormone, substance P, and thyrotropin-releasing hormone, with oxytocin and ACTH 1-14 having the least effect, at concentrations of 2-20 mM. Inhibition by amidated peptide products (vasopressin, oxytocin, luteinizing hormone-releasing hormone, substance P, and thyrotropin-releasing hormone) show that the final products of the precursor processing pathway can regulate carboxypeptidase H. These levels of peptides are similar to known intragranular peptide concentrations indicating that product and feedback inhibition of carboxypeptidase H may play a role in the control of neuropeptide synthesis. The proenkephalin-derived peptides Met-enkephalin, Leu-enkephalin, Met-enkephalin-Arg6-Gly7-Leu8, and Met-enkephalin-Arg6-Phe7 competitively inhibited bovine and rat carboxypeptidase H with Ki values of 12.0, 6.5, 7.0, and 5.5 mM, respectively. The significantly greater Ki for Met-enkephalin may reflect the effects of higher intragranular concentration of Met-enkephalin, since one proenkephalin molecule contains four copies of Met-enkephalin and only one copy of each of the other enkephalin peptides. Thus, the products from one multivalent precursor molecule may equivalently inhibit carboxypeptidase H activity. Product inhibition of carboxypeptidase H and perhaps other processing enzymes may serve to limit the maximum peptide concentration within the secretory vesicle.