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

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Abstract
The properties of C17-C20 lyase in the human testis were investigated using a microsome fraction in the presence of NADPH. Km of the enzyme were 1.7 .times. 10-5 M for 17.alpha.-hydroxyprogesterone and 5.9 .times. 10-7 M for 17.alpha.-hydroxypregnenolone. Km of the lyase for NADPH were determined as 6.3 .times. 10-6 M in the presence of either 17.alpha.-hydroxyprogesterone or 17.alpha.-hydroxypregnenolone. In both instances, the (Vmax) of the lyase for 17.alpha.-hydroxyprogesterone was .apprx. 2 times higher than that for 17.alpha.-hydroxypregnenolone. Optimal pH and temperature of the enzyme for both 17.alpha.-hydroxyprogesterone and 17.alpha.-hydroxypregnenolone were 7.7 and 40.degree. C, respectively. Each steroid substrate inhibited competitively the lyase activity for the other with inhibition constants of 1.9 .times. 10-5 M for 17.alpha.-hydroxyprogesterone and 6.0 .times. 10-7 M for 17.alpha.-hydroprogesterone and 20.alpha.-dihydropregnenolone showed non-competitive inhibition of the lyase for either 17.alpha.-hydroxyprogesterone or 17.alpha.-hydroxypregnenolone. Testosterone inhibited the enzyme activity for 17.alpha.-hydroxyprogesterone competitively, but revealed typical uncompetitive inhibition of the lyase for 17.alpha.-hydroxypregnenolone. Similarly, the inhibition of the lyase by 5-androstenediol was competitive for 17.alpha.-hydroxyprogesterone and of the mixed type for 17.alpha.-hydroxypregnenolone. Estradiol-17.beta. was a competitive inhibitor of the lyase for 17.alpha.-hydroxyprogesterone, while it showed almost no inhibition of the enzyme for 17.alpha.-hydroxypregnenolone. The monosulfate steroids examined showed significant inhibition of the lyase for 17.alpha.-hydroxyprogesterone, but not for 17.alpha.-hydroxypregnenolone. Apparently there are 2 different active sites for lyase, 1 for 17.alpha.-hydroxyprogesterone and the other for 17.alpha.-hydroxypregnenolone, although both sites show very similar properties.