The occurrence of angiotensin converting enzyme (EC 3.4.15.1; ACE) was demonstrated for the first time in serum of newt (Triturus carnifex) and frog (Rana esculenta). The enzymatic activity was evidenced following hydrolysis of N-[3-(2-furyl) acryloyl]L-phenylalanyl glycyl glycine (FAPGG), a synthetic substrate of ACE. The serum enzyme liberated N-[3-(2-furyl) acryloyl]L-phenylalanine (FAP) from FAPGG. The properties of the amphibian serum enzymes were compared with those of swine. The amphibian serum FAPGG hydrolysing activities were inhibited by typical ACE inhibitors, captopril and lisinopril. The optimum of pH was 8.3 at 10 and 37 degrees C and the temperature optimum was 45 degrees C. The values were similar to those of swine serum. The FAPGG Michaelis-Menten constants (K(m)) at 37 degrees C of amphibian serum enzymes (0.337 mM and 0.282 mM for frog and newt, respectively) were lower than that of swine (1.305 mM), but close to human serum enzyme. The K(m) values obtained at 10 degrees C were lower than those at 37 degrees C (0.152, 0.086, and 1.029 mM for frog, newt, and swine serum, respectively). Amphibian sera hydrolysed bullfrog synthetic angiotensin I to produce angiotensin II. Captopril (50 microM) inhibited the production of angiotensin II.