Substrate-induced difference spectra and cholesterol to pregnenolone conversion with adrenal heme protein P-450

Changes in cytochrome P-450 iron spin state as a function of castration, testosterone or estrogen treatment, vigorous exercise, or starvation

3-Methylcholanthrene treatment induced large increases in high-spin P-450 form(s) in intact microsomal membranes from New Zealand White rabbit or C57BL/6N mouse liver but not in rabbit lung, kidney or intestine. Testosterone enhanced the high-spin/low-spin ratio of P-450 in female C57BL/6N liver to the same degree as 3-methylcholanthrene treatment; there was no additive increase in the ratio with treatment of testosterone plus 3-methylcholanthrene. The same effect was seen in ovariectomized females. In the liver of normal or castrated C57BL/6N mice, estradiol-17 beta pretreatment caused no significant change in the high-spin/low-spin ratio. Vigorous homogenization of the microsomes can decrease the high-spin/low-spin ratio, more so in control than in 3-methylcholanthrene-induced C57BL/6N microsomes. Vigorous exercise (20-min swim) and fasting for 2 days significantly enhanced the high-spin/low-spin ratio in C57BL/6N liver. Temperature studies showed that the low-spin electron paramagnetic resonance signal intensity, as a function of absolute temperature in the range of 16 to 70 degrees K, was consistent with a simple Boltzmann dependence within the Kramers doublet giving rise to the microwave transition. We conclude that, within the limits of measurement, the low-spin state acts like an isolated state and is not in thermal equilibrium with the high-spin form of P-450.

Inhibition of the conversion of cholesterol to pregnenolone in bovine adrenocortical preparations

The inhibition of the conversion of [4-14C] cholesterol to [4-14C] pregnenolone by a number of steroids has been studied in bovine adrenocortical mitochondrial acetonedried preparations. At equimolar substrate and inhibitor concentrations (3.3 muM) the most potent inhibitors were cholesterol derivatives containing a nitrogen function at c-22, followed by derivatives containing oxygen functions at c-22 or c-20 or both. The presence of a hydroxyl group at c-17 or the replacement of the 3beta-hydroxyl group by fluorine reduced the inhibitory efficacy. In the presence of inhibitors that were also relatively good substrates of the cholesterol side-chain cleavage system, such as some cholesterol derivatives hydroxylated in the side-chain,the rate of [4-14C] pregnenolone formation increased with time as the inhibitor was consumed. (20S)-20,21-Dihydroxycholesterol exerted such an effect on the kinetics of [4-14C]pregnenolone formation, and yielded 21-hydroxypregnenolone which was identified by gas chromatography-mass spectrometry. The synthesis of (20R)-22-ketocholesterol, of (20R,22R)-22hydroxycholesterol, (20R,22S)-hydroxycholesterol, and of (20S)-desmosterol is described.