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-17β 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°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.