A series of 21 different 4-substituted 2,6-dimethyl-3-(alkoxycarbonyl)-1,4-dihydropyridines was considered with regard to oxidation to pyridine derivatives by human liver microsomal cytochrome P-450 (P-450). Antibodies raised against P-450 IIIA4 inhibited the microsomal oxidation of nifedipine and felodipine to the same extent, as did cimetidine and the mechanism-based inactivator gestodene. Gestodene was approximately 10(3) times more effective an inhibitor than cimetidine, on a molar basis. When rates of oxidation of the 1,4-dihydropyridines were compared to each other in different human liver microsomal preparations, all were highly correlated with each other with the exceptions of a derivative devoid of a substituent at the 4-position and an N1-CH3 derivative. A P-450 IIIA4 cDNA clone was expressed in yeast and the partially purified protein was used in reconstituted systems containing NADPH-cytochrome P-450 reductase and cytochrome b5. This system catalyzed the oxidation of all of the 1,4-dihydropyridines except the two for which poor correlation was seen in the liver microsomes. Principal component analysis supported the view that most of these reactions were catalyzed by the same enzyme in the yeast P-450 IIIA4 preparation and in the different human liver microsomal preparations, or by a closely related enzyme showing nearly identical properties of catalytic specificity and regulation. The results indicate that the enzyme P-450 IIIA4 is probably the major human catalyst involved in the formal dehydrogenation of most but not all 1,4-dihydropyridine drugs.
The human liver cytochrome P-450 (P-450) proteins responsible for catalyzing the oxidation of mephenytoin, tolbutamide, and hexobarbital are encoded by a multigene family (CYP2C). Although several cDNA clones and proteins related to this "P-450MP" family have been isolated, assignment of specific catalytic activities remains uncertain. Sulfaphenazole was found to inhibit tolbutamide hydroxylation to a greater extent than mephenytoin or hexobarbital hydroxylation. The inhibition by sulfaphenazole was competitive for tolbutamide and hexobarbital hydroxylation but with much different Ki values (5 vs 480 microM, respectively). Inhibition of mephenytoin hydroxylase was not competitive. The results suggest that different P-450 proteins in the P450MP family may be involved in the metabolism of these compounds. A cDNA clone (MP-8) related to the P-450MP family, isolated from a bacteriophage lambda gt11 human liver library, was expressed in Saccharomyces cerevisiae by using the pAAH5 expression vector. Yeast transformed with pAAH5 containing the MP-8 sequence (pAAH5/MP-8) showed a ferrous-CO spectrum typical of the P-450 proteins. Immunoblotting with anti-P450MP revealed that pAAH5/MP-8 microsomes contained a protein with an Mr similar to that of P-450MP-1 (approximately 48,000) that was not present in microsomes from yeast transformed with pAAH5 alone (1.7 X 10(4) molecules of the expressed P-450 per cell). Microsomes from pAAH5/MP-8 contained no detectable mephenytoin 4'-hydroxylase activity but were more active in tolbutamide hydroxylation, on a nanomoles of P-450 basis, than human liver microsomes. The pAAH5/MP-8 microsomes also contained hexobarbital 3'-hydroxylase activity, although the enrichment compared to liver microsomes was not great with respect to the tolbutamide hydroxylase activity.(ABSTRACT TRUNCATED AT 250 WORDS)
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