Studies of the human testis. XIV. Properties of C17-C20 lyase

Abstract

Abstract. The properties of C17-C20 lyase in the human testis were investigated using a microsome fraction in the presence of NADPH. Michaelis constants (Km) of the enzyme were 1.7 × 10−5 m for 17α-hydroxyprogesterone and 5.9 × 10−7 m for 17α-hydroxypregnenolone. Km of the lyase for NADPH were determined as 6.3 × 10 −6 m in the presence of either 17α-hydroxyprogesterone or 17α-hydroxypregnenolone. In both instances, the maximum velocity (Vmax) of the lyase for 17α-hydroxyprogesterone was approximately two times higher than that for 17α-hydroxypregnenolone. Optimal pH and temperature of the enzyme for both 17α-hydroxyprogesterone and 17α-hydroxypregnenolone were 7.7 and 40°C, respectively. Each steroid substrate inhibited competitively the lyase activity for the other with inhibition constants (Ki) of 1.9 × 10−5 m for 17α-hydroxyprogesterone and 6.0 × 10−7 m for 17α-hydroprogesterone and 20α-dihydropregnenolone showed non-competitive inhibition of the lyase for either 17α-hydroxyprogesterone or 17α-hydroxypregnenolone. In contrast, testosterone inhibited the enzyme activity for 17α-hydroxyprogesterone competitively, but revealed typical uncompetitive inhibition of the lyase for 17α-hydroxypregnenolone. Similarly, the inhibition of the lyase by 5-androstenediol was competitive for 17α-hydroxyprogesterone and of the mixed type for 17α-hydroxypregnenolone. Furthermore, oestradiol-17β was found to be a competitive inhibitor of the lyase for 17α-hydroxyprogesterone, while it showed almost no inhibition of the enzyme for 17α-hydroxypregnenolone. The monosulphate steroids examined showed significant inhibition of the lyase for 17α-hydroxyprogesterone, but not for 17α-hydroxypregnenolone. From these results, it appears most likely that there are two different active sites for lyase, one for 17α-hydroxyprogesterone and the other for 17α-hydroxypregnenolone, although both sites show very similar properties.