Effect of onion consumption by rats on hepatic drug-metabolizing enzymes

Fruits and vegetables or their natural constituents which increase detoxica tion enzymes and/or reduce activating enzymes are considered as good candid ates to prevent chemically-induced carcinogenesis. In this study, rats were fed a diet supplemented with 20% onion powder for 9 days. Several cytochro me P450 (CYP)s enzymes (CYP 1A, 213, 2E1, 3A), which are involved in carcin ogen activation, were determined by measuring their enzyme activities using specific substrates. In addition, phase II enzymes activities such as UDP- glucuronosyltransferase (UGT) and glutathione S-transferase (GST), involved in detoxication of carcinogens, were measured. Protein levels of CYPs and GST A1/A2, A3/A5, M1, M2 and P1 were measured using antibodies in Western b lots. Consumption of onion induced CYP 1A and CYP 2B activities while it de creased CYP 2E1 activity. This later modification was accompanied by a decr ease of CYP 2E1 levels. The same dietary treatment caused a slight increase of the total GST activity. The relative proportions of GST subunits were m odified. GST A1/A2 subunits were increased while GST A3/A5 and GST M2 subun its were decreased and GST M1 and P1 were not modified. Onion consumption a lso increased p-nitrophenol UGT activity. Taken together, these results sug gest that the decrease of CYP 2E1 and the increase of phase II enzymes by o nion can afford protection against some carcinogens, while the decrease of some GST subunits could increase the genotoxic effects of other chemicals. The modulating effect of onion could be ascribed to alk(en)yl polysulphides and/or glycosides of flavonols, which were identified in the onion powder. (C) 2001 Elsevier Science Ltd. All rights reserved.