Gh. Gunasena et Mf. Kanz, EFFECT OF HYPERTHYROIDISM ON THE IN-VITRO METABOLISM AND COVALENT BINDING OF 1,1-DICHLOROETHYLENE IN RAT-LIVER MICROSOMES, Journal of toxicology and environmental health, 52(2), 1997, pp. 169-188
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.