KINETIC-ANALYSIS OF RAT STEROID 5-ALPHA-REDUCTASE ACTIVITY IN PROSTATE AND EPIDIDYMIS HOMOGENATES AT NEUTRAL PH - EVIDENCE FOR TYPE-I ACTIVITY IN EPIDIDYMIS

Immunocytochemical studies and mRNA measurements have shown that the r at epididymis-like the rat prostate-expresses both rat steroid 5 alpha -reductase isozymes, i.e. type I and II. So far, enzyme activity measu rements in rat epididymis homogenates, however, do not support the pre sence of type I 5 alpha-reductase activity. Incubating homogenates of both tissues with a wide range of substrate concentrations, we were ab le to detect activity of both isozymes in rat prostate and epididymis tissues at neutral pH. In rat prostate the amount of type I activity, as measured by the V-max at pH 7.0, exceeds that of type II 5 alpha-re ductase 50-fold. The efficiency ratio, V-max/K-m, of the type I isozym e accounts for 25% of the total in vivo potential activity. A possible anabolic role for the type I isozyme in rat prostate was thus surmise d. In rat epididymis the V-max of type I and type II 5 alpha-reductase at pH 7.0 were similar. Comparison of the efficiency ratio V-max/K-m of either isozyme in the rat epididymis, however, suggested that the t ype II isozyme would play the major role in the 5 alpha-reduction of t estosterone at physiological concentrations and at neutral pH. The spe cific localization of the isozymes should be considered to allow for c orrect quantification of their in vivo contribution to dihydrotestoste rone formation. (C) 1996 Elsevier Science Ltd.