Carboxypeptidase A hydrolyses benzoylglycyl-histidyl-leucine but not furylacryloyl-phenylalanyl-glycyl-glycine, two usual substrates for angiotensin I-converting enzyme.

We compared angiotensin I-converting enzyme (ACE) and carboxypeptidase A (CPA), two zinc metallopeptidases, for the hydrolysis of the usual ACE synthetic substrate benzoylglycyl-histidyl-leucine (HHL) investigating the possible interference by CPA in the determination of ACE activity in biological fluids. Both purified enzymes hydrolyse HHL in a radiochemical assay with the same optimal pH, a characteristic divalent metal requirement, a close similar behavior against inhibitors of other metallopeptidases, such as enkephalinase and kininase I, and the involvement of arginine and lysine residues in their active site. Conversely, CPA does not show the other catalytic properties of ACe, i.e. chloride dependence, low Km for HHL, inhibition by specific synthetic ACE inhibitors and antibody, also hydrolysis of the other ACE substrate furylacryloylphenylalanyl-glycyl-glycine (FAPGG). We advise the use of ACE inhibitors to validate ACE measurement with HHL or, alternatively, FAPGG, which is a more specific substrate for ACE, must be preferred, although the poor sensitivity of the spectrophotometric assay with this substrate limits its use to blood samples.