Kininase II of Human Seminal Fluid: Kinetics and Inhibition

Summary

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-glycl-glycine (1 mmol/1). Mean seminal plasma activity was 335±6l U/g protein; the K_m was 0.7 mmol/1; pH optimum was 8.8 in a 50 mmol/1 HEPES buffer and the chloride optimum was 300 mmol/1. This male genital tract enzyme is inhibited by several kininase II inhibitors. Captopril (SQ 14225) showed IC₅₀ = 1.6 × 10⁻⁸ mol/1, with a competitive pattern (K_i = 7.3 × 10⁻⁹). 3-(Mercaptomethy1)-oxo- piperidineacetic acid showed the same kind of inhibition with an IC₅₀ = 1.8 × 10⁻⁶ mol/1 (K_i = 6.8 × 10⁻⁷ mol/1). Enalapril diacid was the most potent inhibitor and had an IC₅₀ of 4.1 × 10⁻⁹ mol/1 and showed a mixed competitive and non-competitive inhibition (K_i = 10⁻⁹ mol/K_i’ = 9.5 × 10⁻¹⁰ mol/1). 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.