MECHANISM-BASED INACTIVATION OF HUMAN LIVER MICROSOMAL CYTOCHROME-P-450-IIIA4 BY GESTODENE

A series of 17α-acetylenic steroids was examined with regard to ability to inactivate human liver microsomal cytochrome P-450 (P-450) IIIA4, an enzyme involved in the oxidation of a number of drugs, carcinogens, and steroids, including estrogens and progestogens. Of the eight compounds tested, gestodene was found to be particularly active as a mechanism-based inactivator of P-450 IIIA4. Inhibition of both microsomal nifedipine oxidation and 17α-ethynylestradiol (EE) 2-hydroxylation was dependent upon NADPH and gestodene concentration. Rates of inactivation were pseudo first order—values of kinactivation = 0.4 min-1 and ki = 46 μM and a partition ratio of 9 were calculated. The kinactivation is ~50-fold greater than estimated for EE and is one of the highest reported for P-450 mechanism-based inactivators. Spectrally detectable P-450 was also destroyed in microsomes, but several experiments indicate that little covalent binding to amino acid residues of P-450 IIIA4 occurs. Microsomal inactivation of P-450 could be blocked by the presence of other P-450 IIIA4 substrates, and several activities catalyzed by other P-450s were not inhibited under conditions in which >90% of P-450 IIIA4 was inactivated. Consideration of structure/activity relationships among the 17α-acetylenic steroids examined indicates that the Δ15 double bond is critical but is not in itself sufficient for the inactivation process, which is postulated to result from attack of P-450 on the substituted acetylenic carbon and lead to porphyrin N-alkylation. The effectiveness of this mechanism-based inactivator may account for reports of increased estrogen and steroid levels in some women using gestodene in oral contraceptives. © 1990, American Chemical Society. All rights reserved.