Mechanism-based inactivation of human liver microsomal cytochrome P-450 IIIA4 by gestodene.
A series of 17 alpha-acetylenic steroids was examined with regard to ability to inactivate human liver microsomal cytochrome P-450 (P-450) IIA4, 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 17 alpha-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 microM and a partition ratio of 9 were calculated. The kinactivation is approximately 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 greater than 90% of P-450 IIIA4 was inactivated. Consideration of structure/activity relationships among the 17 alpha-acetylenic steroids examined indicates that the delta 15 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.
Guengerich
F P
FP
Department of Biochemistry, Vanderbilt University School of Medicine, Nashville, Tennessee 37232-0146.
D003276:Contraceptives, Oral / Q000494:pharmacology*; D065607:Cytochrome P-450 Enzyme Inhibitors; D004997:Ethinyl Estradiol / Q000378:metabolism; D006801:Humans; D006900:Hydroxylation; D008862:Microsomes, Liver / Q000201:enzymology*; D009543:Nifedipine / Q000378:metabolism; D009652:Norpregnenes / Q000494:pharmacology*; D010084:Oxidation-Reduction; D013329:Structure-Activity Relationship
D003276:Contraceptives, Oral; D065607:Cytochrome P-450 Enzyme Inhibitors; D009652:Norpregnenes; C033273:Gestodene; D004997:Ethinyl Estradiol; D009543:Nifedipine