In vivo and in vitro studies of the hepatotoxic effects of 4-chlorophenol in mice

Abstract

4-Chlorophenol (4-CP) was studied for its toxicological effect on liver by using both in vivo and in vitro approaches. Male mice were administered 4-CP, 1.5 mmol/kg body weight, i.p., and were killed at 10, 20, 30 and 50 min after drug injection. Either i.p. or oral 4-CP administration significantly lowered total liver thiol levels by 20-30% after 30 min and 3 hr respectively. This time-dependent effect of 4-CP after i.p. treatment was enhanced when mice were pretreated with hepatic microsomal enzyme inducers (phenobarbital, 40 mg/kg body weight, b.i.d., 7 days; and beta-naphthoflavone, 80 mg/kg body weight once daily, 4 days). Further, the microsomal cytochrome P-450 inhibitor, SKF 525-A, 75 mg/kg body weight injected i.p. to mice 30 min prior to 4-CP administration, blocked the reduction of liver thiol content produced by 4-CP. The results suggest that a chemically reactive intermediate of 4-CP may be formed in liver which is responsible for the observed decrease in liver thiol content. Other investigations were done to characterize the in vitro irreversible binding of [14C]4-CP. [14C]4-CP was bound irreversibly to mouse liver microsomal proteins in a concentration-dependent manner. Binding was NADPH dependent and gave a maximal binding of 12.0 nmol/mg protein/20 min and an apparent binding constant of 0.222 mM. [14C]-Binding of 4-CP was increased by 155 and 127% in liver microsomes of phenobarbital- and beta-naphthoflavone. SKF 525-A, and CO:O2 (4:1, v/v)] and selected nucleophilic compounds (glutathione, L-cysteine or L-lysine) significantly reduced [14C]4-CP binding to mouse liver microsomes. An epoxide hydrolase inhibitor, cyclohexene oxide, did not alter the extent of irreversible binding, whereas scavengers of superoxide anions or agents that are reported to reduce accumulation of active semiquinone and quinone species (L-ascorbic acid, superoxide dismutase or epinephrine) decreased the binding of [14C]4-CP to mouse liver microsomal proteins by 56, 31 and 92% respectively. The data suggest that semiquinone and quinone species of 4-CP may be the chemically reactive intermediates leading to the in vivo reduction of liver thiol levels. Since 4-CP is a minor contaminant and possible metabolite of clofibrate and chemically related hypolipidemic agents, 4-CP and its metabolites may be partly responsible for some of the hepatotoxic effects seen after long-term administration of this therapeutic class of drugs.