Activation of 6-aminochrysene to genotoxic products by different forms of rat liver cytochrome P450 in an O-acetyltransferase-overexpressing Salmonella typhimurium strain (NM2009).
Metabolic activation of a potent mutagen, 6-aminochrysene, to genotoxic products in a newly developed tester strain, Salmonella typhimurium NM2009, was studied in a rat liver microsomal monooxygenase system containing cytochrome P450 (P450). Since the tester strain was constructed by introducing an O-acetyltransferase gene into the original strain S. typhimurium TA1535/pSK1002, it is highly sensitive toward the reactive metabolites of carcinogenic arylamines. DNA-damaging activities of 6-aminochrysene were detected at very low concentrations of substrate (between 0.01 and 0.2 microM) and liver microsomes (from 0.2 to 2 micrograms protein/mL) in the S. typhimurium NM2009 strain. Thus, the potency of genotoxic activities induced by 6-aminochrysene was about 10- to 20-times greater than those induced by the well-known mutagens 2-aminoanthracene and 2-amino-3,5-dimethylimidazo[4,5-f]quinoline. Liver microsomes isolated from rats treated with phenobarbital (PB) and a polychlorinated biphenyl mixture, Kanechlor 500, catalyzed very efficiently the activation of 6-aminochrysene to genotoxic metabolites. Treatment of rats with beta-naphthoflavone (BNF) and with dexamethasone also caused moderate induction of the microsomal activation of 6-aminochrysene. Studies employing immunoinhibition of microsomal catalytic activities and reconstitution with purified P450 enzymes suggested that the most important enzymes involved in the activation of 6-aminochrysene were P450 2B1 and 2B2; other enzymes including P450 1A1 and 1A2 participated to some extent. We also found that the microsomal activation of 6-aminochrysene was catalyzed more effectively in an acetyltransferase-overexpressing strain (NM2009) than in the original TA1535/pSK1002 strain and that these activities could be inhibited by an acetyltransferase inhibitor, pentachlorophenol, in liver microsomes from PB-treated rats, but not in those from BNF-treated rats. These results suggest that the P450/acetyltransferase system is one of the most important catalysts for the activation of 6-aminochrysene in liver microsomes of PB-treated rats, and that activation by BNF-induced P450 enzymes occurs by different mechanisms, probably through the ring oxidation pathway.
Yamazaki
H
H
Osaka Prefectural Institute of Public Health, Japan.
Shimada
T
T
D000123:Acetyltransferases / Q000037:antagonists & inhibitors / Q000235:genetics; D000818:Animals; D001571:Benzoflavones; D001711:Biotransformation; D002911:Chrysenes / Q000378:metabolism* / Q000633:toxicity; D065607:Cytochrome P-450 Enzyme Inhibitors; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D003907:Dexamethasone; D004305:Dose-Response Relationship, Drug; D015870:Gene Expression / Q000187:drug effects; D008297:Male; D008862:Microsomes, Liver / Q000201:enzymology*; D009152:Mutagenicity Tests; D010416:Pentachlorophenol / Q000494:pharmacology; D010634:Phenobarbital; D051381:Rats; D011919:Rats, Inbred Strains; D012486:Salmonella typhimurium / Q000187:drug effects* / Q000235:genetics; D019324:beta-Naphthoflavone
D001571:Benzoflavones; D002911:Chrysenes; D065607:Cytochrome P-450 Enzyme Inhibitors; D019324:beta-Naphthoflavone; D003907:Dexamethasone; D003577:Cytochrome P-450 Enzyme System; D010416:Pentachlorophenol; D000123:Acetyltransferases; C100101:6-chrysenamine; D010634:Phenobarbital