Formation of quinonoid-derived protein adducts in the liver and brain of Sprague-Dawley rats treated with 2,2 ',5,5 '-tetrachlorobiphenyl

A possible role for metabolic activation of 2,2',5,5'-tetrachlorobiphenyl ( TCB) to quinonoid metabolites was investigated in vitro in rat liver micros omes and in vivo in male Sprague-Dawley rats. Incubation of TCB with phenob arbital-induced rat liver microsomes resulted in metabolism of TCB to 3-hyd roxy-TCB (3-OH-TCB) and 3,4-dihydroxy-TCB (3,4-diOH-TCB), which were furthe r oxidized to form a reactive intermediate that bound Do liver proteins, Th e predominant species observed in the Raney nickel assay for cysteinyl addu cts was identified as 3,4-diOH-TCB, consistent with an adduct having the st ructure 5-cysteinyl-3,6-dichloro-4-(2',5'-dichlorophenyl)-1,2-benzoquinone. This adduct may arise via the Michael addition of the sulfhydryl group of cysteine to 3,6-dichloro-4-(2',5'-dichlorophenyl)-1,2-benzoquinone, (Cl(4)P hBQ). Metabolism of 3-OH-TCB by phenobarbital-induced microsomes in the pre sence of either NADPH or cumene hydroperoxide as a cofactor resulted in the formation of adducts. Dose-dependent formation of cysteinyl adducts was ob served in liver cytosolic protein from rats treated with a single dose of T CB (0-200 mg/kg) by gavage. By regression analysis, the TCB adducts decayed with a half-life of 2.03 +/- 0.131 days (mean +/- SE), which is similar to 2.5-fold shorter than the endogenous half-life for liver cytosolic protein in rat liver, suggesting adduct instability, Saturable formation of TCB ad ducts was observed in liver cytosolic protein of rats receiving multiple do ses of TCB over 5 days, The levels of Cl(4)PhBQ-derived adducts were 2.1-fo ld greater than the estimated steady-state levels predicted by the single-d ose treatment [97.7 +/- 13.2 vs 45.7 +/- 3.73 (pmol/g)/(mg/kg of body weigh t)], suggesting induction of metabolism, A single cysteinyl adduct, inferre d to be 5-cysteinyl-3,6-dichloro-4-(2',5'-dichlorophenyl)-1,2-benzoquinone quinone, was detected in brain cytosolic protein of rats treated with multi ple doses of TCB with levels of 15.2 (pmol/g)/(mg/kg of body weight). Impli ed involvement of a reactive quinone in the liver and brain of TCB-treated rats supports the idea that quinonoid metabolites may be important contribu tors to PCB-derived oxidative damage to genomic DNA.