Effect of chlorinated dibenzo-p-dioxins on 7-ethoxycoumarin O-deethylase activity in rat liver microsomes

Abstract

The effect of dibenzo- p-dioxin (DD) and chlorinated dibenzo- p-dioxins (CDDs) on the cytochrome P450 (P450)-dependent monooxygenase, 7-ethoxycoumarin O-deethylase (ECOD) in rat liver microsomes was examined in vitro. ECOD activity was induced 21.9- and 5.8-fold over that of the control by 3-methylcholanthrene (MC)- and phenobarbital (PB)-pretreatment, respectively. Moreover, anti-P450IA1 immunoglobulin G (IgG) inhibited ECOD activity in liver microsomes of rats pretreated with MC, but was not in that pretreated with PB. ECOD activities in MC and PB-pretreated rat liver microsomes in the presence of 100 μM DD or CDDs were inhibited to 18–70% and 31–91% of the control activity, respectively. In the liver microsomes of rats pretreated with MC, the inhibition constant (Ki) for 1,2,4-tetrachlorodibenzo- p-dioxin (1,2,4-TrCDD) was the lowest (5.41 μM) among the compounds tested, which was the same as that using β-naphthoflavone as the positive control, followed by 2-monochlorodibenzo- p-dioxin (2-MCDD). On the other hand, in PB-pretreated rat liver microsomes, the apparent Ki values were 25.9 and 156 μM for 2-MCDD and 1,2,4-TrCDD, respectively. Thus, 1,2,4-TrCDD strongly inhibited ECOD activity in MC-pretreated rat liver microsomes, and 2-MCDD inhibited it in those of both MC- and PB-pretreated rats. Therefore, in vitro, DD and CDDs could be classified into two groups: a MC-type such as 1,2,4-TrCDD and a mixture type of MC and PB such as 2-MCDD. Our results suggested that the inhibition profile of ECOD by dioxin compounds in vitro is useful for predicting their toxicity.