3-Aminobenzamide: Effects on Cytochrome P450-Dependent Metabolism of Chemicals and on the Toxicity of Dichlobenil in the Olfactory Mucosa

Abstract

Treatment with 3-aminobenzamide, known as an inhibitor of poly(ADP-ribose)polymerase, decreased the toxicity and covalent binding of the herbicide dichlobenil (2,6-dichlorobenzonitrile; 12 mg/kg; ip) in the mouse olfactory mucosa.In vitrostudies showed that 3-aminobenzamide markedly reduced the NADPH-dependent covalent binding of [14C]dichlobenil and the hydroxylation ofp-nitrophenol which have previously been suggested to be mediated by a common form of cytochrome P450 (P450) in rat olfactory microsomes (Eriksson and Brittebo,Chem.-Biol. Interact.94,183–196, 1995). Furthermore, 3-aminobenzamide markedly reduced the P450-dependent metabolic activation of [3H]NNK (4-(N-methyl-N-nitrosamino)-1-(3-pyridyl)-1-butanone) in rat olfactory microsomes and slightly decreased the P450 2B1-dependent pentoxyresorufindealkylase activity in liver microsomes of phenobarbital-treated rats. The present results suggest that 3-aminobenzamide is also an inhibitor of P450 and that the lack of toxicity of dichlobenil in the olfactory mucosa of 3-aminobenzamide-treated mice is related to a decreased metabolic activation of dichlobenil at this site. Further experiments showed that there was no evidence for a binding of [14C]dichlobenil metabolites to calf thymus DNA or a formation of mutagenic dichlobenil metabolites in Ames’Salmonellaassay when dichlobenil was incubated in the presence of homogenates of the olfactory mucosa. Finally, analysis of proteins from olfactory microsomes incubated with [14C]dichlobenil using SDS–PAGE/fluorography revealed a binding of metabolites to all major proteins. Addition of glutathione or the P450-inhibitor metyrapone prevented the binding, suggesting the formation of relatively stable electrophilic products which can leave the activating enzyme and then unselectively bind to the major olfactory microsomal proteins.