KINETIC-ANALYSIS OF DUODENAL AND TESTICULAR CYTOCHROME P450C17 IN THERAT

In the adult rat, the duodenal tissue of both sexes can convert proges terone to 17 alpha-hydroxyprogesterone, androstenedione and testostero ne. The transition from C-21 to C-19 steroids is apparently controlled by the same cytochrome P450c17 expressed in the testis, which catalyz es both 17 alpha-hydroxylation and C-17,20 bond scission at a single b ifunctional active site. The kinetic parameters of this enzyme were me asured at the steady state for both reactions using [1,2-H-3]progester one and [1,2-H-3]17 alpha-hydroxyprogesterone as substrates. In the te stis and male and female duodena, the K-m values for progesterone one 17 alpha-hydroxylation were 14.2, 23.8 and 23.2 nM, whereas the V-max values were 105, 3.5 and 3.1 pmol/mg protein/min, respectively. With r espect to C-17,20 lyase activity, the K-m values for exogenous 17 alph a-hydroxyprogesterone were 525, 675 and 637 nM, whereas the V-max valu es were 283, 7.8 and 7.8 pmol/mg protein/min, respectively. However, w hen the K-m values were calculated with respect to intermediate 17 alp ha-hydroxyprogesterone formed from progesterone, they were similar to the K-m values for 17 alpha-hydroxylase, being 15, 31.4 and 24.8 nM, w hereas the V-max values were 26.3, 2 and 1.8 pmol/mg protein/min, resp ectively. The similarity of K-m values is due to the fact that the rel ative androgen formation efficiency (bond scission events/total 17 alp ha-hydroxylation events ratio) was remarkably constant in both testicu lar and duodenal incubates, irrespective of progesterone concentration . Efficiency values were 2-fold higher in duodenal tissue (0.54) than in testis (0.25). Estradiol-17 beta inhibited 17 alpha-hydroxylation b ut not bond scission on intermediate 17 alpha-hydroxyprogesterone, bec ause it did not affect the efficiency value. Rat duodenal P450c17 has the same substrate affinity, a lower specific activity and a higher an drogen formation efficiency than testicular P450c17. Copyright (C) 199 6 Elsevier Science Ltd.