Phenobarbital Depression of Hepatic Microsomal Benzo[a]pyrene Hydroxylation in Rats Starved and Refed a Diet Containing Menhaden Fish Oil: Substrate and Fat Level Dependency

Abstract

The influence of phenobarbital on the activity of hepatic mixed function oxidases responsible for benzo[a]pyrene hydroxylation was studied in rats fed diets containing menhaden fish oil (rich in n-3 fatty acids). Male rats were starved for 2 days and refed diet devoid of fat or containing 0.5, 10, or 20% menhaden oil for 4 days. Phenobarbital increased the apparent K_m value as well as V_max for benzo[a]pyrene hydroxylase in microsomes from rats fed the 20% menhaden oil diet. The increased K_m was due to a progressive decrease in benzo[a]pyrene metabolism at the lower substrate concentrations, even in the presence of increased cytochrome P-450 content. The phenobarbital-induced increase in K_m and the decreases in benzo[a]pyrene hydroxylation were not observed in rats fed 0.5% menhaden oil or a diet devoid of fat.