In vitro metabolism of [ 14C]4-chlorobiphenyl and [ 14C]2,2',5,5'-tetrachlorobiphenyl by hepatic microsomes from rats and pigeons. Evidence against an obligatory arene oxide in aromatic hydroxylation reactions

Abstract

1. 1. The catalytic activities of cytochromes P-450IA1 and P-450IIB1 in control and Aroclor 1254 treated rats and pigeons (1 mmol/kg) were assessed using [ 14C]4-chloro- and [ 14C]2,2',5,5'-tetrachlorobiphenyl as substrates. Treatment of rats resulted in increases of the total amount of chloroform-extractable metabolites of [ 14C]4-chlorobiphenyl from 37.2 (control) to 199.4 and 221.6 nmol/hr per mg microsomal protein at 48 and 120 hr post treatment. The portion of [ 14C]4-chloro-3',4'-dihydroxybiphenyl (M 4) and of a second unidentified dihydroxylated metabolite (M 3) increased during these incubations from 13.7% for controls to 53.5% at 48 hr and 69.12% at 120 hr post treatment. 2. 2. [ 14C]4-chloro-3'-hydroxybiphenyl (M 1) and [ 14C]4-chloro-4'-hydroxybiphenyl (M 2) were the major metabolites formed by pigeon hepatic microsomes; however, the amounts formed were 38.7- and 29.3-fold less, respectively, than in untreated rats. Treatment of pigeons with Aroclor 1254 increased the metabolite formation from 1.0 (control) to 13.6 and 22.4 nmol/hr per mg microsomal protein at 48 hr and 120 hr post treatment respectively; however, only small amounts of metabolites M 3 (0.5 nmol/hr per mg protein) and M 4 (2.0 nmol/hr per mg protein) were detected. 3. 3. Treatment of rats with Aroclor 1254 resulted in an approximately two-fold increase in the rate of metabolism of [ 14C]2,2',5,5'-tetrachlorobiphenyl, and the ratio of 3- to 4-hydroxylation increased from 0.45 (control) to 0.6 and 0.8 at 48 hr and 120 hr post treatment respectively. The rate of metabolism of [ 14C]2,2',5,5'-tetrachlorobiphenyl by control and Aroclor 1254 treated pigeons was up to 23-fold lower than in rats and there was no evidence for the formation of the diol metabolite M 3. However, as with rats, the ratio of meta- to para-carbon atom hydroxylation increased from 0.58 (controls) to 0.72 at 120 hr post treatment. 4. 4. From the evidence presented, it is suggested that cytochromes P-450IA1 and P-450IIB1 may not metabolize PCB-congeneric substrates via an obligatory arene oxide intermediate.