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Using guinea pig andrenal microsomes, we studied the inhibitory effects of 20α-hydroxyprogesterone on steroid hydroxylation reactions catalyzed by cytochromes P-450. When 17α-hydroxyprogesterone was used as a substrate, 20α-hydroxyprogesterone functioned as a competitive inhibitor on the C_(17)—C_(20) bond cleavage reaction of P-450_(17α lyase). The inhibition constant, K_i was 1.37 μmol/L. 20α-hydroxyprogesterone also competitively inhibited the convertion of 17α-bydroxyprogesterone to 11-deoxycortisol by the action of P-450_(c21). The value of K_i was 1.73μmol/L. When progesterone was used as a substrate, 20α-hydroxyprogesterone inhibited neither the 21-hydroxylation of P-450_(c21). the C_(17)—C_(20) bond cleavage, nor 17α-hydroxylation of P-450_(17α lyase). Based on the seresults, we can deduce that the production of androstenedione from progesterone by the action of P-450_(17α lyase) proceeds through a successive monooxygenase reaction.
Using guinea pig andrenal microsomes, we studied the inhibitory effects of 20α-hydroxyprogesterone on steroid hydroxylation reactions catalyzed by cytochromes P-450. When 17α-hydroxyprogesterone was used as a substrate, 20α-hydroxyprogesterone functioned as a competitive inhibitor on the C_ (17) The inhibition constant, K_i was 1.37 μmol / L. 20α-hydroxyprogesterone also competitively inhibited the convertion of 17α-bydroxyprogesterone to 11-deoxycortisol by the action of P-450 (17α lyase) 450_ (c21). The value of K_i was 1.73 μmol / L. When progesterone was used as a substrate, 20α-hydroxyprogesterone inhibited neither the 21-hydroxylation of P-450_ (c21) bond cleavage, nor 17α-hydroxylation of P-450_ (17α lyase). Based on the seresults, we can deduce that the production of androstenedione from progesterone by the action of P-450_ (17α lyase) through through successive monooxygenase reactions.