The aim of this study was to determine if feeding dietary fiber (cellulose or pectin) to male rats could influence hepatic and intestinal mixed-function oxidation. We simultaneously compared hepatic drug-oxidizing activity with the activity of 3-hydroxy-3-methylglutaryl-coenzyme A (HMG-CoA) reductase, the rate-controlling enzyme for cholesterol biosynthesis. Three groups of six animals were fed a purified diet containing by weight either 10.4% cellulose or 10.4% pectin, or a standard cereal-based diet containing 4.5% crude fiber; the caloric contributions by carbohydrate, protein and fat in the three diets were similar. In the cellulose-fed rats, the hepatic microsomal cytochrome P-450 content and the activities of ethylmorphine N-demethylase and aniline hydroxylase were significantly lower when compared with those of rats fed pectin or the cereal-based diet. The hepatic microsomal cytochrome P-450 content and the activities of ethylmorphine N-demethylase and aniline hydroxylase were similar in the pectin-fed and cereal diet-fed rats. Hepatic HMG-CoA reductase activity, hepatic microsomal cytochrome b5 content, and intestinal benzo[a]pyrene hydroxylase activity were comparably lower in rats fed the purified diet with either dietary fiber when compared to those fed the cereal diet. It is concluded that dietary pectin and cellulose exert distinctly different influences on the hepatic microsomal mixed-function oxidase system for drug metabolism, but not on liver cholesterol synthesis or intestinal benzo[a]pyrene hydroxylation, suggesting that different physiological mechanisms control these enzyme systems.
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