Structural constraints of inhibitors for binding at two active sites on somatic angiotensin converting enzyme

Angiotensin converting enzyme active sites from rat plasma, lung, kidney and testis were assessed by comparative radioligand binding studies under physiological chloride conditions. Displacement of [125I]Ro 31-8472 from somatic and plasma angiotensin converting enzyme by angiotensin converting enzyme inhibitors of different structure indicated two binding sites (perindoprilat: high affinity carboxyl site, KDC 18 +/- 6 pM), and a single high affinity binding site on testis angiotensin converting enzyme (KDC 20 +/- 1 pM). Displacement of [125I]351A from plasma, somatic and testis angiotensin converting enzyme occurred at a single high affinity binding site. Reduction in affinity at the amino binding site of somatic angiotensin converting enzyme was related to an increased side chain size (lung KDA (pM): Ro 31-8472 175 +/- 38, lisinopril 2205 +/- 1832, and 351A 2271 +/- 489), or hydrophobicity of the competing unlabelled angiotensin converting enzyme inhibitor (lung KDA (pM): quinaprilat 1267 +/- 629 and perindoprilat 824 +/- 6). This trend was reversed at the carboxyl binding site of plasma, somatic and testis angiotensin converting enzyme. Bradykinin hydrolysis by lung angiotensin converting enzyme was inhibited in a similar manner by cilazaprilat or quinaprilat (F = 0.64, F-test based on the extra sum-of-squares principle; P > 0.05), indicating the angiotensin converting enzyme carboxyl active site predominates in bradykinin cleavage. The data demonstrate that the two binding sites on native plasma and somatic angiotensin converting enzyme are of potentially different functional and structural nature, suggesting they may have different substrate specificities.