NAPQI ultimately causes liver failure by disrupting the mitochondrial membrane potential in liver cells, thus preventing ATP synthesis by oxidative phosphorylation. The exact mechanism for how NAPQI causes cell death is complex and incompletely understood. However, the molecule is highly reactive and has been found to covalently bond with several proteins due to its electrophilicity and ability to irreversibly arylate thiol residues (19). In one study, researchers radioactively labelled APAP and administered a lethal dose to mice. Two hours post drug administration, the mice were killed for their tissues to be analyzed. Researchers found high levels of radioactivity in liver samples compared to other tissue, and more importantly, they noticed that the amount of APAP administered directly corresponded with the severity of liver necrosis and the amount of radioactivity found in liver cells (20). This provides evidence that irreversible binding of NAPQI to liver cell proteins causes hepatic necrosis.
Other studies have concluded that NAPQI concentration correlates with electron transport chain (ETC) activity. In particular, it is hypothesized that NAPQI inhibits succinate dehydrogenase (complex II) activity. In one experiment, researchers found that a10µL concentration of NAPQI decreased succinate dehydrogenase activity by 45%, while a 50µL concentration completely inhibited enzymatic activity. By comparison, the substance had little to no effect on the functionality of other ETC enzymes and ATP synthase. The structure of succinate dehydrogenase might provide a molecular explanation for its inhibition by NAPQI; it contains several cysteine-rich sulfur clusters whose sulfhydryl groups are potential sites for arylation by NAPQI (21). Overall, several studies indicate that high levels of APAP cause a loss of mitochondrial membrane potential, a decrease in ATP to ADP ratio in the cytoplasm, and the irreversible covalent attachment of NAPQI to liver proteins, but more research is needed to gain a full understanding of the effects of NAPQI on liver cells.