Alterations of the hepatic xenobiotic-metabolizing enzymes by a glucosinolate-rich diet in germ-free rats: influence of a pre-induction with phenobarbital

Abstract

Germ-free growing rats were fed on a glucosinolate-rich diet (rapeseed-meal-based) and compared with counterparts fed on a glucosinolate-free diet (soya-bean-meal-based), both diets being isonitrogenous and isoenergetic. For each diet half the animals received phenobarbital in drinking water as an inducer of xenobiotic-metabolizing enzymes. Some of the usual deleterious glucosinolate-linked effects, i.e. kidney hypertrophy and reduction in growth and feed intake, were followed and three of the major hepatic xenobiotic-metabolizing enzymes were investigated. Growth rate, dietary intake and kidney weight were not altered by glucosinolates in the absence of intestinal microflora, whether the animals were treated with phenobarbital or not. As far as the hepatic xenobiotic-metabolizing enzymes are concerned, the specific level of cytochrome P450 and the specific activities of glutathione-S-transferase (EC2.5.1.18) and UDPglucuronosyltransferase (EC2.4.1.17) remained unchanged in rats receiving the glucosinolate-rich diet compared with the control animals. Despite the low dose given, phenobarbital displayed its usual inducing effect on all three enzymes, similar whatever the diet. A previous counterpart experiment performed with conventional animals had shown that glucosinolate feeding led to large alterations of the variables herein studied, some of these modifications being hugely enhanced by a phenobarbital treatment. Therefore, the present results obtained on germ-free animals prove that alterations of the xenobiotic-metabolizing enzymes induced by glucosinolates are somehow mediated by the intestinal microflora. Furthermore, the involvement of those enzymes in glucosinolate toxicity definitely requires the presence of the intestinal microflora.