Traditionally, only mast cell CPA and plasma CPU have been considered as regulatory enzymes within the M14 family of metallocarboxypeptidases [33]. More recently it was proposed that CPA6, an extracellular matrix protease secreted in some areas of human and mouse brains, plays a role in the regulation

of neuropeptides [17]; also, we have shown that the major kininase of the rat MAB perfusate is identical with rat CPB1 [23], another prototypical pancreatic metallopeptidase. Vorinostat Thus, our present demonstration that rat MAB CPA1 and CPA2 are capable of processing Ang peptides extends the current evidence of the participation of metallocarboxypeptidases in regulatory pathways. The peculiar proteolytic profiles displayed by CPA1 and CPA2 toward Ang peptides probably reflect the proposed evolutionary events that have allowed these enzymes to diverge from one another with respect to substrate specificity, resulting in overlapping and complementary preferences [10]. Thus, Ang I was efficiently acted upon Selleckchem NVP-BGJ398 by CPA1, forming Ang-(1-7) by a three-step pathway with Ang-(1-9) and Ang II as intermediates (Fig. 5A); confirmatory evidence of this sequential mode of action of CPA1 has been provided by the formation of the end-product Ang-(1-7) in analogous reactions when either Ang-(1-9) or Ang II, the intermediates

in the conversion of Ang I to Ang-(1-7), were used as substrate for the enzyme (Fig. 5B and C). On the other hand, only Ang-(1-9) was released upon incubation of Ang I with CPA2 (Fig. 6A); in agreement with this result, the substrates Ang II and Ang-(1-9) were negligibly hydrolyzed by CPA2 under the conditions described in Fig. 6B and C. Comparison of the catalytic efficiencies of the CPA1- and CPA2-mediated conversions of Ang II to Ang-(1-7) indicates a value approximately 200-fold higher for the former enzyme (Fig. 7), consistent with the results of Ang II cleavage by these enzymes shown in Fig. 5 and Fig.

6B; such a discrepancy between kinetic parameters of CPA1 and CPA2 toward Ang II is significantly larger than those observed for synthetic substrates having carboxyl-terminal Phe residue [10], the same terminal residue CYTH4 of Ang II. It should also be noted here that the Km value for the CPA1-catalyzed conversion of Ang II to Ang-(1-7), as determined in Fig. 7, is of the same order of magnitude as that of the analogous reaction catalyzed by ACE2 [28], a carboxypeptidase for which there is compelling evidence of participation in the RAS [34]; thus, the binding affinity of rat CPA1 for Ang II seems compatible with the participation of this enzyme in the formation of Ang-(1-7) under physiological conditions. However, the contribution of CPA1 to the in vivo generation of Ang-(1-7) in the rat mesenteric vascular bed is likely to depend on factors beyond that of the enzyme affinity for Ang II, among which the actual CPA1 activity and the presence of circulating carboxypeptidase inhibitors.