Angiotensin-converting enzyme in organs of air-breathing fish.

Angiotensin-converting enzyme (ACE) was measured in tissue homogenates from the African lungfish and six species of air-breathing teleosts (Heteropneustes fossilis, Clarias batrachus, Channa gachua, Anabas testudineus, Notopterus chitala, and Monopterus cuchia) using a standard spectrophotometric assay. In most species, the highest levels of ACE activity were found in the respiratory organs (gills and/or accessory respiratory organs). ACE was also found in heart and kidney tissues from most species and occasionally in liver. Converting enzyme was not found in skin or skeletal muscle from any species and only in blood from H. fossilis and brain from C. batrachus. Captopril, a potent inhibitor of mammalian ACE, inhibited enzymatic activity from all tissues except C. gachua heart and liver and A. testudineus heart. As fish make the transition from aquatic to aerial respiration the gill microcirculation is usually reduced in size and the accessory respiratory organs become elaborated and occupy a more central position in the vascular tree. The presence of ACE in accessory respiratory organs of air-breathing fish appears to greatly enhance the metabolic efficiency of this enzyme on circulating substrates.