Variation in angiotensin-converting enzyme (ACE) inhibitor affinity at two binding sites on rat pulmonary ACE: influence on bradykinin hydrolysis.

1. ACE from rat lung and testis was characterized by radioligand binding studies using [125I]-Ro 31-8472, the radioiodinated hydroxy derivative of the potent ACE inhibitor cilazaprilat. 2. Analysis of the displacement of [125I]-Ro 31-8472 from ACE by ACE inhibitors of different structure by the LIGAND program was best fitted by a two binding site model for lung ACE and a one binding site model for testis ACE. 3. There was marked variation in ACE inhibitor binding affinity at the two binding sites of lung ACE across the panel of ACE inhibitors studied (equilibrium dissociation constant; Kd; pmol/L) for site one vs site two: cilazaprilat 40 +/- 3 vs 430 +/- 92*; lisinopril 25 +/- 1 vs 848 +/- 107**; and quinaprilat 4 +/- 1 vs 1869 +/- 720; *P less than 0.05; **P less than 0.005, t-test, n = 3). Reduction in binding affinity at site two of lung ACE was related to an increase in ACE inhibitor side chain length or complexity of carboxyl terminal moiety. ACE inhibitor binding affinity at the testis ACE binding site resembled site one of lung ACE. 4. Inhibition of bradykinin hydrolysis by lung ACE in the presence of increasing concentrations of cilazaprilat or quinaprilat was similar (F = 0.64; P greater than 0.05), suggesting that bradykinin cleavage predominates at ACE active site one. 5. The differences in ACE inhibitor affinity at the two ACE active sites has implications in physiological substrate selectivity, and may influence the pharmacodynamic effects of different ACE inhibitors.