Metallothionein-I/II knockout mice are sensitive to acetaminophen-induced hepatotoxicity

The purpose of this study was to examine whether intracellular metallothion ein (MT) protects against acetaminophen hepatotoxicity. MT-I/II knockout (M T-null) and control mice were given acetaminophen (150-500 mg/kg i.p.), and liver injury was assessed 24 h later. MT-null mice were more susceptible t han controls to acetaminophen-induced lethality and hepatotoxicity, as evid enced by elevated serum enzyme activities and histopathology. Zinc pretreat ment, a method of MT induction, protected against acetaminophen hepatotoxic ity in control mice, but not in MT-null mice. The susceptibility of MT-null mice to acetaminophen hepatotoxicity was not due to the increased acetamin ophen bioactivation, as cytochrome P-450 enzymes, and acetaminophen-reactiv e metabolites in bile and urine were not increased in MT-null mice. Western blots of liver cytosol indicated that acetaminophen covalent binding at 4h increased with acetaminophen dose, but there was nd consistent difference between control and MT-null mice. Acetaminophen injection depleted cellular glutathione similarly in both control and MT-null mice, but produced more lipid peroxidation in MT-null mice, as evidenced by the abundance of thioba rbiturate-reactive substances, and by immunohistochemical localization of 4 -hydroxynonenal and malondialdehyde protein adducts. MT-null hepatocytes we re more susceptible than control Cells to oxidative stress and cytotoxicity produced by N-acetylbenzoquinoneimine, a reactive metabolite of acetaminop hen, as determined by oxidation of 2',7'-dichlorofluorescin diacetate and l actate dehydrogenase leakage. In summary, this study demonstrated that MT d eficiency renders animals more vulnerable to acetaminophen-induced hepatoto xicity. The Increased sensitivity does not appear to be due to increased ac etaminophen activation, glutathione depletion, or covalent binding, but app ears to be associated with the antioxidant role of MT.