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 microsomes and in vivo in male Sprague-Dawley rats. Incubation of TCB with phenobarbital-induced rat liver microsomes resulted in metabolism of TCB to 3-hydroxy-TCB (3-OH-TCB) and 3,4-dihydroxy-TCB (3,4-diOH-TCB), which were further oxidized to form a reactive intermediate that bound to liver proteins. The predominant species observed in the Raney nickel assay for cysteinyl adducts was identified as 3,4-diOH-TCB, consistent with an adduct having the structure 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)PhBQ). Metabolism of 3-OH-TCB by phenobarbital-induced microsomes in the presence of either NADPH or cumene hydroperoxide as a cofactor resulted in the formation of adducts. Dose-dependent formation of cysteinyl adducts was observed in liver cytosolic protein from rats treated with a single dose of TCB (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 approximately 2.5-fold shorter than the endogenous half-life for liver cytosolic protein in rat liver, suggesting adduct instability. Saturable formation of TCB adducts was observed in liver cytosolic protein of rats receiving multiple doses of TCB over 5 days. The levels of Cl(4)PhBQ-derived adducts were 2.1-fold greater than the estimated steady-state levels predicted by the single-dose treatment [97.7 +/- 13.2 vs 45.7 +/- 3. 73 (pmol/g)/(mg/kg of body weight)], suggesting induction of metabolism. A single cysteinyl adduct, inferred to be 5-cysteinyl-3, 6-dichloro-4-(2',5'-dichlorophenyl)-1,2-benzoquinone, was detected in brain cytosolic protein of rats treated with multiple doses of TCB with levels of 15.2 (pmol/g)/(mg/kg of body weight). Implied involvement of a reactive quinone in the liver and brain of TCB-treated rats supports the idea that quinonoid metabolites may be important contributors to PCB-derived oxidative damage to genomic DNA.
Lin
P H
PH
Department of Environmental Sciences and Engineering, University of North Carolina, Chapel Hill, North Carolina 27599-7400, USA.
Sangaiah
R
R
Ranasinghe
A
A
Upton
P B
PB
La
D K
DK
Gold
A
A
Swenberg
J A
JA
D000818:Animals; D001921:Brain / Q000378:metabolism*; D003545:Cysteine / Q000378:metabolism; D003600:Cytosol / Q000378:metabolism; D004305:Dose-Response Relationship, Drug; D008401:Gas Chromatography-Mass Spectrometry; D006207:Half-Life; D008099:Liver / Q000378:metabolism*; D008297:Male; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D011078:Polychlorinated Biphenyls / Q000378:metabolism* / Q000494:pharmacology; D011485:Protein Binding / Q000187:drug effects; D011809:Quinones / Q000378:metabolism*; D051381:Rats; D017207:Rats, Sprague-Dawley
D011809:Quinones; C009407:2,5,2',5'-tetrachlorobiphenyl; D011078:Polychlorinated Biphenyls; D003545:Cysteine