INCREASED EXPRESSION OF CYTOCHROME-P450-IIIA2 IN MALE-RAT LIVER AFTER DIETARY VITAMIN-A SUPPLEMENTATION

In this study dietary vitamin A supplementation (25 IU/g diet) was assessed for its effect on hepatic microsomal P450 content and on P450 enzyme-specific drug oxidase activities in rats. Intake of the supplemented diet by male rats over a 15-week period resulted in a fivefold increase in hepatic vitamin A stores over those measured in control liver from rats that received a balanced diet without vitamin A supplementation. Serum retinol was unchanged and there was no evidence of hepatocellular injury in any of the animals. There was a 26% increase in P450 content in vitamin A-supplemented rat liver and regioselective androst-4-ene-3,17-dione (androstenedione) and progesterone hydroxylation revealed changes in several P450 pathways. Thus, androstenedione 16α-hydroxylation (P450 IIC11-mediated) and progesterone 21-hydroxylation (P450 IIC6-mediated) were decreased slightly to 80 and 74% of respective control activities while P450 IIA1/2-dependent androstenedione 7α-hydroxylation was slightly increased. In contrast, the 6β-hydroxylations of androstenedione and progesterone were increased to 169 and 152% of control following dietary supplementation. Kinetic analysis of androstenedione 6β-hydroxylation revealed an increase in maximal reaction velocity (Vmax 4.00 ± 0.47 vs 2.20 ± 0.10 nmol/ min/mg protein) but the Km was unchanged, suggesting an increase in enzyme concentration. Consistent with this assertion, immunoquantitation of the steroid 6β-hydroxylase, P450 IIIA2, revealed a 158% increase in the microsomal expression of this enzyme (9.8 ± 2.7 vs 6.2 ± 1.3 ng/μg microsomal protein). From these studies it now seems clear that vitamin A, as a dietary additive in nontoxic doses, has the capacity to alter the activity of hepatic microsomal drug oxidases by modulating the expression of P450 enzymes. © 1991.