Structural requirements for substrates and inhibitors of angiotensin I-converting enzyme in vivo and vitro.

The mechanism of action of peptide inhibitors on angiotensin I-converting enzyme (Al-converting enzyme) was studied in relation to the substrate requirements of the enzyme in vivo in the dog lung and in vitro in plasma. l-</-Asp-8-Ile-AI prepared by the solid-phase technique was compared with the known peptide inhibitors of Al-converting enzyme. BPP5il (Pyr-Lys-Trp-Ala-Pro) and SQ 20881 (Pyr-Trp-Pro-Arg-Pro-Gln-IlePro-Pro)I-l-d-Asp-8-Ile-AI was evaluated for susceptibility to cleavage by the Al-converting enzyme. In vitro and in vivo, I-l-d-Asp-8-Ile-AI or BPP5ain 50,000-fold molar excess produced only a slight delay in conversion of °1-AI to I-angiotension II (All); SQ 20881 blocked conversion completely. In vivo, l-d-Asp-8-Ile-AI or BPP3a injected into the pulmonary circulation in 250-fold molar excess (250 nmole/kg) did not cause a diminution in the pressor response to AI administered 30 seconds later; SQ 20881 blocked the pressor response to AI for 60-90 minutes. l-rf-Asp-8-Ile-AI was a poor substrate for converting enzyme, since I-l-c/-Asp-8-Ile-AI was riot converted to I-l-dAsp-8-Ile-AII, and unlabeled l-d-Asp-8-Ile-AI did not block the pressor response to exogenous AIL I-l-d-Asp-8-Ile-AI was stable in plasma and in the pulmonary circulation The results suggest that since l-rf-Asp-8-Ile-AI is neither a substrate nor a blocker, substitution of the aliphatic residue He in the 8 position of AI may prevent binding to an active site in the Al-converting enzyme.