Ascorbic acid deficiency reduces the level of mRNA for cytochrome P-450 on the induction by polychlorinated biphenyls

Ascorbic acid (AsA) deficiency causes a decrease in hepatic concentration of cytochrome P-450 and a decrease in hepatic activity of drug-metabolizing enzymes in rats unable to synthesize AsA (ODS rats). To study the mechanism of the decrease in hepatic concentration of cytochrome P-450 isozymes by AsA deficiency, we chose the xenobiotics-inducible cytochrome P-450 and performed the experiments indicated below. AsA-deficient rats were fed polychlorinated biphenyls (PCB) which markedly induce both CYP1A subfamily and several isozymes in CYP2B subfamily. First, we assayed the activities of two drug-metabolizing enzymes so that one could be functionally distinguished from another. AsA deficiency significantly reduced the hepatic activity of aminopyrine-N-demethylase in ODS rats with and without dietary PCB, but had no effect on benzo(a)pyrene hydroxylase activity. Secondly, quantitative immunoblot analyses demonstrated that the levels of CYP2B1/2B2 and CYP1A1 in the AsA-deficiency rats fed PCB were approximately 60 and 80% lower than those found in rats fed AsA-supplemented diet. The degree of reduction in CYP2B1/2B2 was greater than CYP1A1. Thirdly, AsA deficiency caused a decrease in hepatic abundance of CYP2B1/2B2 mRNA, whereas it had no effect on the levels of CYP1A1 and 1A2 mRNA. These results indicated that dietary AsA selectively affects the levels of CYP2B1/2B2 mRNA among cytochrome P-450 induced by PCB and plays important roles for optimum induction of drug-inducible cytochrome P-450. We concluded that AsA deficiency decreases specific froms of drug-inducible cytochrome P-450, especially CYP2B1/2B2 and that the reduction of CYP2B1/2B2 mRNA level in AsA-deficient rats caused a decrease in cytochrome P-450 concentration and hepatic activity of drug-metabolizing enzymes.