Comparative activation of 3,3′-dichlorobenzidine and related benzidines to mutagens in the Salmonella typhimurium assay by hepatic S9 and microsomes from rats pretreated with different inducers of cytochrome P-450

Abstract

The metabolic basis of the differential activation of 4 benzidines — 3,3′-dichlorobenzidine (DCB), benzidine (BZ), o-tolidine (TOL) and o-dianisidine (DIN) — to mutagens was examined in the Ames test, using Salmonella typhimurium TA98. For each benzidine congener, the comparative activation by 3 rat liver enzyme systems — (i) postmitochondrial supernatant (S9), (ii) S9 + acetylcoenzyme A (S9-Ac) and (iii) microsomes — and the effect thereon of animal pretreatment with 3 cytochrome P-450 inducers — DCB, 3-methylcholanthrene (MC) and phenobarbital (PB) — were examined. DCB was the most activated of the benzidines, with activation by the 3 systems being in the order: S9 = S9-Ac > microsomes, whereas dianisidine and tolidine were the least activated. Benzidine was activated only in the S9 systems but the S9-catalyzed activation of benzidine, unlike that of DCB, was enhanced by added acetylcoenzyme A. Pretreatment with either DCB, MC or PB enhanced the activation of DCB, decreased that of benzidine, and had no effect on that of tolidine or dianisidine. The enhanced DCB activation was most pronounced with DCB pretreatment and was 2.5-fold, 2-fold, and 3-fold, in S9-Ac, S9, and microsomes, respectively. The microsomal-catalyzed DCB activation was inhibited by the cytochrome P-450 inhibitors 2,4-dichloro- 6-phenylphenoxyethylamine and α-naphthoflavone by 93% and 48%, respectively. DCB, but not its congeners, elicited NADPH-dependent metabolite complex formation with microsomal cytochrome P-450. The results show that DCB is the most mutagenic of the 4 benzidines under conditions of cytochrome-P- 450-catalyzed activation and suggest that the DCB activation may be catalyzed most effectively by cytochrome P-450 species induced specifically by the compound itself.