Distribution and characterization of angiotensin-converting enzyme-like activity in tissues of the blue crab Callinectes sapidus.

Angiotensin-converting enzyme-like activity (ACELA) was determined in tissue homogenates of the blue crab, Callinectes sapidus, using the synthetic substrate hippuryl-histidyl-leucine (Hip-His-Leu) and the specific inhibitor, captopril. ACELA was highest in gill homogenates followed by the hepatopancreas and hemolymph with specific activities of 1.69, 0.37 and 0.10 nmol/min/mg protein, respectively. Gill enzyme activity was membrane-associated and no difference in activity was noted between anterior and posterior gills. The enzyme preparation from gill membranes was activated by chloride and had a Km of 4.1 +/- 0.4 mmol and a Vmax of 39.5 +/- 2.0 nmol/min/mg protein. The enzyme was strongly inhibited by captopril and lisinopril with IC50 s of 3.8 x 10(-8) and 2.6 x 10(-8) M. The enzyme was less strongly inhibited by angiotensin II and SQ-20881 with IC50 s of 5.1 x 10(-5) and 2.5 x 10(-6) M, respectively.