The dependence on vitamin E and selenium of drug demethylation in rat liver microsomal fractions

1. The effects of vitamin E deficiency, and of vitamin E and selenium deficiency, on rat liver microsomal aminopyrine demethylase activity were investigated. It was found that, over a wide range of substrate concentrations, the enzyme activity in preparations from deficient animals was significantly lower than that in controls. 2. Addition of antioxidants in vitro, either to the homogenization or to the assay media, was without significant effect on the depressed enzyme activity. Castration and alteration in dietary protein concentration were also without effect. The rate of oxidation of NADPH was however, lower in preparations from deficient animals. 3. Lineweaver-Burk plots of the reciprocal of enzyme activity and substrate concentration showed a higher Km value in preparations from vitamin E-deficient animals, irrespective of whether selenium was present; the Vmax. was unaffected. These parameters were unchanged when antioxidants were added in vitro. Induction with phenobarbitone and 3-methylcholanthrene showed large changes in Km value which, for preparations from vitamin E-deficient animals, was higher than that for corresponding controls. 4. Examination of the synergism between NADH and NADPH as donors of reducing equivalents for aminopyrine demethylation showed that vitamin E and selenium were only minimally involved in the phenomenon. However, both the initial rate and the extent of demethylation were significantly lower in vitamin E- and selenium-deficient preparations and both nutrients were required for the restoration of full activity. 5. The significance of these results is discussed in the light of our working hypothesis.