Robust protein nitration contributes to acetaminophen-induced mitochondrial dysfunction and acute liver injury

Acetaminophen (APAP), a widely used analgesic/antipyretic agent, can cause liver injury through increased nitrative stress, leading to protein nitration. However, the identities of nitrated proteins and their roles in hepatotoxicity are poorly understood. Thus, we aimed at studying the mechanism of APAP-induced hepatotoxicity by systematic identification and characterization of nitrated proteins in the absence or presence of an antioxidant, N-acetylcysteine (NAC). The levels of nitrated proteins markedly increased at 2h in mice exposed to a single APAP dose (350mg/kg ip), which caused severe liver necrosis at 24h. Protein nitration and liver necrosis were minimal in mice exposed to nontoxic 3-hydroxyacetanilide or animals co-treated with APAP and NAC. Mass-spectral analysis of the affinity-purified nitrated proteins identified numerous mitochondrial and cytosolic proteins, including mitochondrial aldehyde dehydrogenase, Mn-superoxide dismutase, glutathione peroxidase, ATP synthase, and 3-ketoacyl-CoA thiolase, involved in antioxidant defense, energy supply, or fatty acid metabolism. Immunoprecipitation followed by immunoblot with anti-3-nitrotyrosine antibody confirmed that the aforementioned proteins were nitrated in APAP-exposed mice but not in NAC-cotreated mice. Consistently, NAC cotreatment significantly restored the suppressed activity of these enzymes. Thus, we demonstrate a new mechanism by which many nitrated proteins with concomitantly suppressed activity promotes APAP-induced mitochondrial dysfunction and hepatotoxicity.