In vitro and in vivo studies of the effect of vitamin E on microsomal cytochrome P450 in rat liver.

Administration of a diet supplemented with 0.06% vitamin E acetate to male rats over a 6-week period doubled hepatic microsomal stores of alpha-tocopherol over those in control (vitamin E adequate) rat liver. Total cytochrome P450 content and NADPH-cytochrome P450 reductase activity were significantly elevated in hepatic microsomes from vitamin E-supplemented rats to 111% and 123% of respective control values. Androstenedione 16 alpha-hydroxylase activity was increased in these fractions (2.57 +/- 0.31 nmol product/min/mg protein vs 1.81 +/- 0.38 in controls) whereas activities of the 6 beta-, 7 alpha- and 16 beta-hydroxylase pathways were unchanged. Immunoquantitation of the microsomal 16 alpha-hydroxylase, P450 IIC11, indicated a corresponding increase in the hepatic content of the enzyme. In view of the established antioxidant role of tocopherols, the effects of dietary vitamin E manipulation on the concentration of protein sulphydryl groups and the susceptibility of microsomes to ferric sulphate-ADP-NADPH-mediated lipid peroxidation were also assessed. Dietary supplementation did not influence microsomal protein sulphydryl content (68 +/- 10 nmol glutathione equivalents/mg protein) but decreased the extent of lipid peroxidation produced by the ferric sulphate-ADP-NADPH system in vitro. Further in vitro experiments demonstrated that vitamin E acetate (2 microM) protected protein sulphydryl groups and lipids against peroxidation in control microsomes and partially reduced the associated losses of P450-mediated steroid hydroxylase activities. Western immunoquantitation of P450 IIC11 revealed that exogenous vitamin E acetate protected completely against peroxidation-induced apoprotein loss. These studies establish that the in vitro protective effects of vitamin E acetate against sulphydryl and lipid peroxidation extend to protection of the P450 apoprotein but that enzyme activity is only partially protected. This finding suggests that peroxidation-dependent loss of P450 in vitro is mediated by haem degradation from the P450 holoenzyme and is not directly related to lipid/sulphydryl oxidation. In contrast, the in vivo effects of dietary vitamin E on drug metabolizing enzymes are regulatory in nature and are unrelated to effects on lipid peroxidation.