1. Angiotensin-converting enzyme (ACE) activity in blood vessels of different species was determined.
2. ACE was solubilized by Nonidet P-40, and assayed by reversible phase high performance liquid chromatography. Approximately 98% ACE was recovered in the liquid phase by the use of the detergent.
3. The ACE activity varied with chloride ion (Cl−) concentrations; the maximum activities in dog, human, monkey and rabbit tissues were obtained at the concentrations of 800, 600, 600 and 300 mmol/l respectively. The optimal Cl− concentration was quite similar in different tissues and plasma obtained from the same species.
4. The ACE activity in the cerebral, mesenteric, pulmonary and renal arteries was in a range between 1.01 and 1.60 m-units/mg of protein in dogs and between 0.43 and 0.94 m-unit/mg of protein in monkeys. The activity in dog aortae was 0.20 ± 0.02 m-unit/mg of protein, and the activity in aortic endothelial cells was 2.61 ± 0.65 m-units/mg of protein. ACE activities in the dog lung, kidney cortex and cerebral cortex were 28.6 ± 2.6, 15.7 ± 3.0 and 3.5 ± 0.6m-units/mg of protein respectively. SA-446, a captopril-like ACE inhibitor, reduced the ACE activity in arteries in a dose-dependent manner.
5. Vascular ACE appears to be concentrated in the endothelium and may contribute to regulate vascular muscle tone and local blood flow by a conversion of angiotensin I into II.