ROLE OF CYTOCHROME P-450 IN MECHANISM OF INHIBITION OF STEROID 11BETA-HYDROXYLATION BY DICUMAROL

The mechanism of dicumarol inhibition of the 11β-hydroxylation of deoxycorticosterone has been examined in an 11β-hydroxylase system extracted from an acetone powder of beef adrenal mitochondria. At low concentrations of dicumarol the inhibition was noncompetitive with substrate deoxycorticosterone while inhibition at high concentrations of dicumarol was of a mixed type. Dicumarol inhibition of 11β- hydroxylation could be related to the effects of this compound on cytochrome P-450, the oxygen-activating and substrate-binding component of the 11β-hydroxylase system. Cytochrome P-450 isolated from an acetone powder of beef adrenal mitochrondria was unstable, being rapidly converted into cytochrome P-420 during incubation at 37°. Addition of the steroid substrates deoxycorticosterone or androstenedione to the incubation mixture diminished both the extent and rate of decomposition of cytochrome P-450. The 11-oxygenated steroids, corticosterone and adrenosterone, did not have this effect. Dicumarol, like deoxycorticosterone, at low concentrations stabilized cytochrome P-450. The stabilizing effects of these two compounds were additive, a finding in agreement with the noncompetitive inhibition of 11β-hydroxylation by dicumarol. Dicumarol at high concentrations enhanced the rate of decomposition of cytochrome P-450. However, this effect was diminished by an increase in deoxycorticosterone concentration, suggesting that this inhibitory action of dicumarol was competitive with steroid substrate. © 1969, American Chemical Society. All rights reserved.