Characterization of angiotensin converting enzyme from rat tissue by radio-inhibitor binding studies

Angiotensin converting enzyme (ACE) derived from rat lung, aorta, epididymus, brain, kidney and plasma was characterized by radio-inhibitor (125I-MK351A) binding studies. Under optimal binding conditions at equilibrium 125I-MK351A bound to ACE was displaced from ACE in a concentration related manner by unlabelled MK351A. MK351A binding site concentration for each tissue and equilibrium dissociation constant (KD) was estimated by Scatchard analysis of binding data. Binding sites/mg protein was greatest in lung and least in brain. The KD for kidney ACE was significantly higher than that of lung, aorta, epididymus or brain ACE (P less than 0.005; t-test, d.f. = 10). 125I-MK351A bound to ACE prepared from lung and kidney was displaced in a concentration dependent manner by SQ20881, SQ14225, MK422, and Ro31-3113-000. Concentration of ACE inhibitor required to displace 50% of bound 125I-MK351A (DD50) was consistently higher for kidney-derived ACE than lung-derived ACE. The differences in radio-inhibitor binding characteristics of ACE from different rat tissues suggests that the enzyme active site may not be identical in all organs.