title: Role of CYP2E1 and saturation kinetics in the bioactivation of thioacetamide: Effects of diet restriction and phenobarbital.
abstract: Thioacetamide (TA) undergoes saturation toxicokinetics in ad libitum (AL) fed rats. Diet restriction (DR) protects rats from lethal dose of TA despite increased bioactivation-mediated liver injury via CYP2E1 induction. While a low dose (50 mg TA/kg) produces 6-fold higher initial injury, a 12-fold higher dose produces delayed and mere 2.5-fold higher injury. The primary objective was to determine if this less-than-expected increase in injury is due to saturation toxicokinetics. Rats on AL and DR for 21 days received either 50 or 600 mg TA/kg i.p. T(1/2) and AUCs for TA and TA-S-oxide were consistent with saturable kinetics. Covalent binding of (14)C-TA-derived-radiolabel to liver macromolecules after low dose was 2-fold higher in DR than AL rats. However, following lethal dose, no differences were found between AL and DR. This lack of dose-dependent response appears to be due to saturation of bioactivation at the higher dose. The second objective was to investigate the effect of phenobarbital pretreatment (PB) on TA-initiated injury following a sub-lethal dose (500 mg/kg). PB induced CYP2B1/2 approximately 350-fold, but did not increase covalent binding of (14)C-TA, TA-induced liver injury and mortality, suggesting that CYP2B1/2 has no major role in TA bioactivation. The third objective was to investigate the role of CYP2E1 using cyp2e1 knockout mice (KO). Injury was assessed over time (0-48 h) in wild type (WT) and KO mice after LD(100) dose (500 mg/kg) in WT. While WT mice exhibited robust injury which progressed to death, KO mice exhibited neither initiation nor progression of injury. These findings confirm that CYP2E1 is responsible for TA bioactivation.
authors.affiliation: Department of Toxicology, College of Pharmacy, The University of Louisiana Monroe, 700 University Avenue, Sugar Hall # 306, Monroe, LA 71209-0470, USA.
authors.forename: Jaya
authors.initials: J
authors.lastname: Chilakapati
authors.forename: Midhun C
authors.initials: MC
authors.lastname: Korrapati
authors.forename: Kartik
authors.initials: K
authors.lastname: Shankar
authors.forename: Ronald A
authors.initials: RA
authors.lastname: Hill
authors.forename: Alan
authors.initials: A
authors.lastname: Warbritton
authors.forename: John R
authors.initials: JR
authors.lastname: Latendresse
authors.forename: Harihara M
authors.initials: HM
authors.lastname: Mehendale
mesh_terms: D000410:Alanine Transaminase / Q000097:blood; D000818:Animals; D001189:Aryl Hydrocarbon Hydroxylases / Q000378:metabolism; D001219:Aspartate Aminotransferases / Q000097:blood; D001711:Biotransformation / Q000187:drug effects; D031204:Caloric Restriction; D002273:Carcinogens / Q000493:pharmacokinetics* / Q000633:toxicity; D056486:Chemical and Drug Induced Liver Injury / Q000473:pathology; D019362:Cytochrome P-450 CYP2B1 / Q000378:metabolism; D019392:Cytochrome P-450 CYP2E1 / Q000235:genetics / Q000378:metabolism*; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D006207:Half-Life; D007700:Kinetics; D008297:Male; D051379:Mice; D018345:Mice, Knockout; D008862:Microsomes, Liver / Q000187:drug effects / Q000378:metabolism; D010634:Phenobarbital / Q000494:pharmacology*; D051381:Rats; D017207:Rats, Sprague-Dawley; D013250:Steroid Hydroxylases / Q000378:metabolism; D013454:Sulfoxides / Q000378:metabolism; D013853:Thioacetamide / Q000493:pharmacokinetics* / Q000633:toxicity
chemical_list: D002273:Carcinogens; D018076:DNA, Complementary; D013454:Sulfoxides; D013853:Thioacetamide; D013250:Steroid Hydroxylases; D019392:Cytochrome P-450 CYP2E1; D001189:Aryl Hydrocarbon Hydroxylases; D019362:Cytochrome P-450 CYP2B1; C025619:steroid 16-beta-hydroxylase; D001219:Aspartate Aminotransferases; D000410:Alanine Transaminase; D010634:Phenobarbital