Binding of Peptide Substrates and Inhibitors of Angiotensin - converting Enzyme

Among three biologically active substrates of angiotensin-converting enzyme, enzyme-binding affinities increase and chloride ion rate enhancements decrease in the following order: angiotensin I (Asp-Arg-Val-TyrIle-His-Pro-Phe-His-Leu), bradykinin (Arg-Pro-ProGly-Phe-Ser-Pro-Phe-Arg), bradykinin-potentiating peptide 5, (<Glu-Lys-Trp-Ala-Pro). Qualitatively simil a r properties are observed among the much s impler tripeptide substrates, Hip-His-Leu, Hip-Phe-Arg, and Hip-Ala-Fro, that have the same COOH-terminal dipeptides as the larger substrates. The same order of enzyme-binding affinities is also found with competitive dipeptide inhibitors, His-Leu, Phe-Arg, and Ala-Pro. Such results suggest that selective binding of the COOH-terminal dipeptide residue is an impor tan t dete rminant of both the substrate specificity of angiotensin-converting enzyme and the degree of rate stimulation by chloride ion, and that the nature of this selective binding can be further clarified by studying competitive inhibition b y dipeptides of vary ing structure. NH2-terminal glycyl dipeptides v a r y 300-fold in inhibitory potency, those with tryptophan, tyrosine, or proline being the most inhibitory. COOH-terminal glycyl dipeptides vary only 16-fold in inhibitory activity, with valine, isoleucine, and arginine as the most effective NHz-terminal residues. The most potent dipeptide tested, Val-Trp (& = 0.3 pM), was 10,000 t imes more inhibitory than the weakest inhibitor Pro-Gly, suggesting that angiotensin-converting enzyme is highly specific with regard to the terminal dipeptide residues of peptide substrates or competitive inhibitors. Inhibitory potencies of dipeptides also correlate with the inhibitory potencies of structurally analogous, but nonpeptidic, mercaptopropanoyl amino acids related to the antihypertensive drug captopril (SQ 14,225), as predicted from a hypothetical model of the active site of angiotensin-converting enzyme.