Kininase II of human seminal fluid: kinetics and inhibition

The activity of kininase II in crude human sperm was measured continuously by measuring the hydrolysis of a blocked tripeptide 3-(2-furylacryloyl)-L- phenylalanyl-glycyl-glycine (1 mmol/l). Mean seminal plasma activity was 335 +/- 61 U/g protein; the Km was 0.7 mmol/l; pH optimum was 8.8 in a 50 mmol/l HEPES buffer and the chloride optimum was 300 mmol/l. This male genital tract enzyme is inhibited by several kininase II inhibitors. Captopril (SQ 14225) showed IC50 = 1.6 X 10(-8) mol/l, with a competitive pattern (Ki = 7.3 X 10(-9)). 3-(Mercaptomethyl)-oxo-piperidineacetic acid showed the same kind of inhibition with an IC50 = 1.8 X 10(-6) mol/l (Ki = 6.8 X 10(-7) mol/l). Enalapril diacid was the most potent inhibitor and had an IC50 of 4.1 X 10(-9) mol/l and showed a mixed competitive and non-competitive inhibition (Ki = 10(-9) mol/Ki' = 9.5 X 10(-10) mol/l). These in vitro inhibition data suggest that, in vivo, such drugs may effect the function of kininase II in the male reproductive system. The observed 50% inhibition constants are comparable to those observed in lung enzyme suggesting similar kinetic properties.