Inhibition of Testicular Microsomal Cytochrome P-450(17α-Hydroxylase/C-17,20-Lyase) by Estrogens*

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Abstract
Highly purified cytochrome P-450 from neonatal pig testicular rnicrosomes is capable of catalyzing both 17αhydroxylation and C-17,20-lyase activity. Estradiol was found to inhibit both activities of the purified enzyme with Δ 4 and with Δ 5 substrates (progesterone, pregnenolone, and the corresponding 17α-hydroxysteroids). For the Δ 4 series, inhibition of lyase is competitive and that of 17α-hydroxylase is noncompetitive; for the Δ 5 series, inhibition was noncompetitive for both activities. Ki values for lyase activity were determined from enzyme kinetics (5.0 μM for the Δ 4 substrate and 20 μM for the Δ 55 substrate). Estradiol produces a typical type I spectral shift with the pure enzyme (Ks = 3.0 μ where Ks is the concentration of steroid required to give half maximal spectral shift), so that Ki values were also determined directly from binding studies by using substrate-induced difference spectroscopy. Fifty per cent inhibition of the maximal spectral shift induced by the 17α-hydroxysubstrates (Ki) are 3.8 and 7.6 μM for the Δ 4 and Δ 5 substrates, respectively. Values for Ki are higher with the substrates of 17α-hydroxylase (progesterone and pregnenolone), by either method, than the corresponding Ki values for the lyase substrates. The concentration of estradiol in Leydig cells of neonatal pig testis is approximately 1.5 nmol/g. It is proposed that estradiol may influence testicular steroidogensis under physiological conditions by competitive inhibition of lyase activity.