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.

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