In vitro activation and resultant binding of benzo[a]pyrene to DNA by microsomes from rats fed corn and menhaden oils.

Dietary unsaturated fat is required for maximum induction of the hepatic mixed function oxidases (MFO) responsible for activating carcinogens to forms that may bind covalently to DNA. The aim of this study was to assess the influence of dietary fat type and content on the activities of some enzymes involved in activation and detoxification of the carcinogen benzo[a]pyrene (B[a]P). Modification of these changes by pretreatment with phenobarbital (PB) was also evaluated. Male rats were fed diet devoid of fat or containing 20% corn oil (CO) or 20% menhaden fish oil (MO) for 4 days. PB induced soluble glutathione S-transferase, a detoxifying enzyme, only in rats fed dietary fats. Microsomes from rats fed both types of dietary fat had increased levels of cytochrome P-450 (P-450) and PB induced P-450 only in rats fed these fats. Although ethoxycoumarin O-dealkylase was significantly elevated in the MO group, the induction by PB was not dependent on dietary fat type or level. Dietary fat increased microsome-catalyzed in vitro binding of [3H]-B[a]P to calf thymus DNA, especially in response to PB. Menhaden oil depressed B[a]P hydroxylation and PB treatment depressed this activity to the greatest extent in rats fed this diet. When calculated as B[a]P metabolized per unit of P-450, PB seems to induce a P-450 in fat fed animals having lower affinity and capacity for B[a]P hydroxylation and activation than in rats fed the fat-free diet.

Dietary fat--a requirement for induction of mixed-function oxidase activities in starved-refed rats.

Male rats were starved 0-48 hr, and then refed diets containing 0% (F.F.) to 20% corn oil (C.O.) lab chow or 20% coconut oil (C.C.O.) for 1-4 days. Some received phenobarbital sodium (80 mg/kg, i.p. daily) for 1-3 days prior to decapitation. Five cytochrome P-450-dependent indicators were assayed as measures of altered hepatic microsomal function: ethylmorphine N-demethylase (EMDM), N-nitrosodimethylamine (DMN)-N demethylase, aniline hydroxylase (AH), benzo[a]pyrene hydroxylase (AHH) and CO-difference spectra (P-450). Increasing dietary corn oil (0, 0.5, 10, 20%) in control rats resulted in a progressive increase in the activities of these five enzymes. Dietary fat influenced phenobarbital (Pb) inducibility of all mixed-function oxidase (MFO) enzymes measured except AHH. Pb induced the remaining enzymes only 11-22% in animals fed fat-free diet as compared to 119-246% in animals fed coconut oil and corn oil. Rats fed fat-free diet for 21 days without prior food deprivation and administered Pb had 79% more EMDM, 34% more AH and 120% more P-450 than non-induced controls, whereas rats fed 20% corn oil diet had 227% more EMDM, 143% more AH and 128% more P-450. A requirement of dietary fat for induction of MFO by Pb was demonstrated by these starvation-refeeding experiments. Coupled with data recovered from the 21-day studies, these experiments suggest that a compensatory mechanism may be operative during chronic feeding of the fat-free diet to partially return inducibility to the drug-metabolizing system.