EFFECT OF CHLORINATED DIBENZO-P-DIOXINS ON 7-ETHOXYCOUMARIN O-DEETHYLASE ACTIVITY IN RAT-LIVER MICROSOMES

The effect of dibenzo-p-dioxin (DD) and chlorinated dibenzo-p-dioxins (CDDs) on the cytochrome P450 (P450)-dependent monooxygenase, 7-ethoxy coumarin O-deethylase (ECOD) in rat liver microsomes was examined in u itro. ECOD activity was induced 21.9- and 5.8-fold over that of the co ntrol by 3-methylcholanthrene (MC)- and phenobarbital (PB)-pretreatmen t, respectively. Moreover, anti-P450IA1 immunoglobulin G (IgG) inhibit ed ECOD activity in liver microsomes of rats pretreated with MC, but w as not in that pretreated with PB. ECOD activities in MC and PB-pretre ated rat liver microsomes in the presence of 100 muM DD or CDDs were i nhibited to 18-70% and 31-91% of the control activity, respectively. I n the liver microsomes of rats pretreated with MC, the inhibition cons tant (Ki) for 1,2,4-tetrachlorodibenzo-p-dioxin (1,2,4-TrCDD) was the lowest (5.41 muM) among the compounds tested, which was the same as th at using beta-naphthoflavone as the positive control, followed by 2-mo nochlorodibenzo-p-dioxin (2-MCDD). On the other hand, in PB-pretreated rat liver microsomes, the apparent Ki values were 25.9 and 156 muM fo r 2-MCDD and 1,2,4-TrCDD, respectively. Thus, 1,2,4-TrDD strongly inhi bited ECOD activity in MC-pretreated rat liver microsomes, and 2-MCDD inhibited it in those of both MC- and PB-pretreated rats. Therefore, i n vitro, DD and CDDs could be classified into two groups: a MC-type su ch 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 compou nds in vitro is useful for predicting their toxicity.