EFFECT OF HYPERTHYROIDISM ON THE IN-VITRO METABOLISM AND COVALENT BINDING OF 1,1-DICHLOROETHYLENE IN RAT-LIVER MICROSOMES

Hyperthyroidism potentiates the in vivo hepatotoxicity of I,l-dichloro ethylene (DCE) in rats, with a concomitant increase in [C-14]-DCE cova lent binding. The enhanced injury produced in hyperthyroid livers by D CE could be due to alterations in either the bioactivation or detoxica tion phases of DCE metabolism. Previous in vitro studies suggested tha t hyperthyroidism did not potentiate DCE hepatotoxicity by increasing DCE oxidation to intermediates which were able to covalently bind. Sev eral factors, however, that could contribute to the magnitude of DCE b ioactivation or covalent binding were not examined. Our objectives wer e to characterize the effects of hyperthyroidism in male Sprague-Dawle y rats on: (1) covalent binding of [C-14]-DCE to microsomes and other subcellular fractions, (2) microsomal mixed-function oxidase (MFO) and glutathione S-transferase (GST) activities, and (3) inactivation of m icrosomal enzyme activities by presumptive DCE reactive intermediates. Hyperthyroid (HYPERT) and euthyroid (EUT) rats received 3 sc injectio ns of thyroxine (100 mu g/100 g) or vehicle, respectively, al 48-h int ervals; microsomes and other subcellular fractions were isolated from HYPERT and EUT livers 24 h after the last injection. [C-14]-DCE-derive d covalent binding was consistently greater in EUT than HYPERT microso mes. The absence of NADH, and the addition of low concentrations (0.1 and 0.5 mM), but not higher concentrations (> 1 mM), of glutathione (C GH) diminished covalent binding to a greater extent in HYPERT than EUT microsomes. Covalent binding in mitochondrial, nuclear, and cytosolic fractions pi EUT and HYPERT livers was equivalent. Regression analysi s oi covalent binding to liver cell fractions from both EUT and HYPERT rats showed a significant correlation with P-450 content. Hyperthyroi dism decreased microsomal, but not mitochondrial, cytochrome P-450 con tent, and MFO activities for 7-ethoxycoumarin and benzphetamine were s imilarly decreased. Hyperthyroidism also diminished microsomal GST act ivity, and altered GST kinetics for both GSH and 1-chloro-2,4-dinitrob enzene (CDNB). The magnitude of inactivation of MFO and CST activities in the presence of DCE (presumably by DCE reactive intermediates) was comparable between EUT and HYPERT microsomes. When covalent binding w as standardized to cytochrome P-450 concentrations in microsomes and m itochondria, HYPERT fractions exhibited slightly greater covalent bind ing than EUT fractions, suggesting that hyperthyroidism does not reduc e the capacity of P-450 hemoproteins to bioactivate DCE. Thus, potenti ation of DCE hepatotoxicity by hyperthyroidism may be predominantly du e to diminished Phase II constituents, and major increases in reactive intermediate/conjugates that covalently bind to and impair critical c ellular molecules.