Abstract

Angiotensin I-converting enzyme (ACE) is a zinc-dipeptidyl carboxypeptidase, which contains two similar domains, each possessing a functional active site. Respective involvement of each active site in the degradation of the circulating peptide N-acetyl-seryl-aspartyl-lysyl-proline (AcSDKP), a negative regulator of hematopoietic stem cell proliferation, was studied by using wild-type recombinant ACE and two full-length mutants containing a single functional site. Both the N- and C-active sites of ACE exhibit dipeptidyl activity toward AcSDKP, with Km values of 31 and 39 microM, respectively. However, the N-active site hydrolyzes the peptide 50 times faster compared with the C-active site, with kcat/Km values of 0.5 and 0.01 microM-1.s-1, respectively. The predominant role of the N-active site in AcSDKP hydrolysis was confirmed by the inhibition of hydrolysis using a monoclonal antibody specifically directed against the N-active site. The N-domain specificity for AcSDKP will aid the identification of specific inhibitors for this domain. This is the first report of a highly specific substrate for the N-active site of ACE, with kinetic constants in the range of physiological substrates, suggesting that ACE might be involved via its N-terminal active site in the in vivo regulation of the local concentration of this hemoregulatory peptide.