Acetaminophen (
APAP) overdose is the most frequent cause of
drug-induced
liver failure in the US.
Metallothionein (MT) expression attenuates
APAP-induced liver injury. However, the mechanism of this protection remains incompletely understood. To address this issue, C57BL/6 mice were treated with 100 micromol/kg ZnCl2 for 3 days to induce MT. Twenty-four hours after the last dose of
zinc, the animals received 300 mg/kg
APAP. Liver injury (plasma ALT activities, area of
necrosis), DNA fragmentation,
peroxynitrite formation (
nitrotyrosine staining), MT expression, hepatic
glutathione (GSH), and
glutathione disulfide (
GSSG) levels were determined after 6 h.
APAP alone caused severe liver injury with
oxidant stress (increased
GSSG levels),
peroxynitrite formation, and DNA fragmentation, all of which were attenuated by
zinc-induced MT expression. In contrast, MT knockout mice were not protected by
zinc.
Hydrogen peroxide-induced cell injury in primary hepatocytes was dependent only on the intracellular GSH levels but not on MT expression. Thus, the protective effect of MT in vivo was not due to the direct scavenging of
reactive oxygen species.
Zinc treatment had no effect on the early GSH depletion kinetics after
APAP administration, which is an
indicator of the metabolic activation of
APAP to its reactive metabolite N-acetyl-
p-benzoquinone imine (
NAPQI). However, MT was able to effectively trap
NAPQI by covalent binding. We conclude that MT scavenges some of the excess
NAPQI after GSH depletion and prevents covalent binding to cellular
proteins, which is the trigger for the propagation of the cell injury mechanisms through
mitochondrial dysfunction and nuclear DNA damage.