Influence of dietary pectin on intestinal microfloral metabolism and toxicity of nitrobenzene.

Intestinal microfloral metabolism of nitrobenzene is essential for the production of methemoglobin. Since dietary pectin alters intestinal microflora, these studies were designed to examine the effects of dietary pectin on nitrobenzene-induced methemoglobinemia. Male Fischer-344 rats were fed either AIN-76A (purified diet containing 5% cellulose), AIN-76A with 5% pectin replacing the cellulose, or NIH-07 (cereal-based diet containing 8.4% pectin) for 28 days. Following this period, nitrobenzene (200 mg/kg) was administered by gastric intubation, and methemoglobin concentrations were determined after 1, 2, 4, 8, and 24 hr. Nitrobenzene-induced methemoglobinemia was evident as early as 1 hr, peaked at 4 hr, and diminished thereafter in rats fed NIH-07 diet. In contrast, nitrobenzene-induced methemoglobinemia was not detectable in rats fed AIN-76A; however, inclusion of 5% pectin in this diet resulted in methemoglobinemia comparable to that of NIH-07-fed animals at 4, 8, and 24 hr. Administration of 400 or 600 mg/kg nitrobenzene resulted in significant diet-related differences in methemoglobinemia. Administration of 600 mg/kg nitrobenzene to animals fed NIH-07 resulted in the highest methemoglobin concentrations (64 +/- 1%); those fed AIN-76A had the lowest (20 +/- 5%), and those fed AIN-76A containing pectin had intermediate methemoglobin concentrations (44 +/- 6%). No diet-related differences in the microbial population of the stomach or small intestine were observed. However, the number of anaerobes present in the ceca of rats fed AIN-76A containing pectin was 2 to 2.5 times greater than that of rats fed AIN-76A. In vitro reductive metabolism of [14C]nitrobenzene was significantly greater in the cecal contents of rats fed NIH-07 than that in the cecal contents of either of the groups fed the AIN-76A-based diets. These studies indicate that intestinal microfloral metabolism and red blood cell toxicity of nitrobenzene is markedly different in animals fed cereal-based versus purified diets. Furthermore, since inclusion of pectin into the purified diet diminishes the magnitude of these effects, differences in dietary composition of fermentable carbohydrates in cereal-based and purified diets may mediate differences in metabolism and toxicity of nitrobenzene.