Characterization of angiotensin-converting enzyme in the gills of rainbow trout,Salmo gairdneri (Richardson).

Several physical and chemical parameters of angiotensin-converting enzyme (ACE) were determined using a spectrophotometric assay of gill tissue homogenates from rainbow trout. This assay is based on the evolution of free hippuric acid via enzymatic cleavage of histidyl-leucine from the synthetic substrate hippuryl-l-histidyl-l-leucine (HHL). Piscine ACE exhibited enzymatic and kinetic properties similar to those reported for the partially purified mammalian enzyme. Proteolytic activity was both temperature and pH dependent and demonstrated hyperbolic kinetics with an apparent Km of 2.5 mM. Hydrolysis of HHL was activated by Cl(-) at concentrations between 20 mM and 100 mM. Captopril (1 × 10(-6) M) and MK-422 (1 × 10(-6) M) blocked trout gill ACE activity, however, EDTA was inhibitory only at high concentrations (1 × 10(-3) M). These results demonstrate that trout ACE is functionally similar to mammalian ACE and that the spectro-photometric assay for ACE developed by Cushman and Cheung can be applied to analysis of converting enzyme activity in fish tissue homogenates.