Cytochrome P-450-dependent oxidation of the 17 alpha-ethynyl group of synthetic steroids. D-homoannulation or enzyme inactivation.

Metabolism of the 17 alpha-ethynyl group of the synthetic estrogen, 17 alpha-ethynylestradiol, by hepatic microsomes from female rhesus monkeys was studied. Incubation of 17 alpha-ethynylestradiol, labeled with 14C in both ethynyl group carbons, resulted in the formation of D-14C-homoestrone (identified by high performance liquid chromatography and gas chromatography-mass spectrometry) and 14CO2. The D-homoestrone appears to be formed by a rearrangement in which one ethynyl group carbon is incorporated into the D ring of the steroid (D-homoannulation) and the second is subsequently removed as CO2. Cofactor, inhibition, and subcellular fractionation studies on the rate of 14CO2 formation showed that the ethynyl group is oxidized by cytochrome P-450. Hepatic microsomes isolated from rhesus monkeys treated with phenobarbital, but not 3-methylcholanthrene or pregnenelone-16 alpha-carbonitrile, resulted in increased rates of 14CO2 formation. Oxidation of the 17 alpha-ethynyl group is postulated to produce a high energy intermediate which is rearranged by D-homoannulation. However, as an alternative reaction, this unstable intermediate might covalently bind and inactivate the cytochrome P-450 enzymes catalyzing its formation. In support of this hypothesis, both cytochrome P-450 concentrations and the rate of catalysis of the D-homoannulation pathway by this enzyme were significantly decreased in hepatic microsomes from rhesus monkeys which had been administered mestranol and/or ethynerone (17 alpha-ethynylated steroids) at dosage levels mimicking human exposure to oral contraceptive agents.