title: Product inhibition of benzo[a]pyrene metabolism in uninduced rat liver microsomes: effect of diol epoxide formation.
abstract: Conversion of benzo[a]pyrene (BP) to BP 7,8-dihydrodiol 9,10-oxides (DE) (measured as 7,10/8,9-tetrols) by untreated (UT) rat liver microsomes is over 10 times slower than following 3-methylcholanthrene (MC) induction. Time courses have been subjected to a kinetic analysis analogous to that previously reported for metabolism by MC-induced microsomes (J. Biol. Chem., 259 (1984) 13770-13776). Competition between BP and 7,8-dihydrodiol for P-450 is the major determinant of the rate of DE formation. Glucuronidation of quinones and phenols only increases the isolated BP metabolites including DE by 40%. This indicates far less inhibition by these products than for metabolism in MC-microsomes (4-6-fold). Thus stimulation may result from a decreased quinone-mediated oxidation of metabolites. In the presence of DNA, UT-microsomes metabolize BP to approximately equal amounts of 9-phenol-4,5-oxide (9-PO) and DE/DNA adducts. Addition of uridine diphosphoglucuronic acid (UDPGA) fails to enhance modification of DNA by DE, but formation of the 9-PO adduct is reduced as a result of lower free 9-phenol levels. The kinetic characteristics of BP metabolism by UT-microsomes are highly sensitive to the presence of very small but variable amounts (2-25 pmol/mg) of the very active cytochrome P-450c, which is the predominant form in MC-microsomes. The major effect of elevated levels of P-450c is an 8-fold increase in DE formation at low concentrations of BP due to a lowering of Km (7.9-2.6 microM) and an increase in the regioselectivity for DE formation from 7,8-dihydrodiol (5-15% of total BP metabolites). The formation of DE was directly correlated with the content of P-450c (r = 0.94). The presence of increased levels of P-450c in UT-microsomes is probably due to previous exposure of the animals to environmental inducers and is minimized by controlled housing and feeding.
mesh_terms: D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide / Q000378:metabolism; D000818:Animals; D001564:Benzo(a)pyrene / Q000378:metabolism*; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism; D004247:DNA / Q000378:metabolism / Q000494:pharmacology; D004101:Dihydroxydihydrobenzopyrenes / Q000378:metabolism; D004790:Enzyme Induction / Q000187:drug effects; D007700:Kinetics; D008297:Male; D008748:Methylcholanthrene / Q000494:pharmacology; D008862:Microsomes, Liver / Q000378:metabolism*; D051381:Rats; D011919:Rats, Inbred Strains; D014535:Uridine Diphosphate Glucuronic Acid / Q000494:pharmacology
chemical_list: D004101:Dihydroxydihydrobenzopyrenes; C017228:benzo(a)pyrene 7,8-dihydrodiol; D014535:Uridine Diphosphate Glucuronic Acid; D001564:Benzo(a)pyrene; D015123:7,8-Dihydro-7,8-dihydroxybenzo(a)pyrene 9,10-oxide; D008748:Methylcholanthrene; D004247:DNA; D003577:Cytochrome P-450 Enzyme System