The catechol-O-methyltransferase inhibitors tolcapone and entacapone uncouple and inhibit the mitochondrial respiratory chain in HepaRG cells

• Tolcapone and entacapone uncouple and impair respiration of liver mitochondria
• Tolcapone is a more potent inhibitor than entacapone.
• Mitochondrial impairment may explain hepatotoxicity associated with tolcapone.

The catechol-O-methyltransferase inhibitor tolcapone causes hepatotoxicity and mitochondrial damage in animal models. We studied the interaction of tolcapone with mitochondrial respiration in comparison to entacapone in different experimental models. In HepaRG cells (human cell-line), tolcapone decreased the ATP content (estimated IC50 100 ± 15 μM) and was cytotoxic (estimated IC50 333 ± 45 μM), whereas entacapone caused no cytotoxicity and no ATP depletion up to 200 μM. Cytochrome P450 induction did not increase the toxicity of the compounds. In HepaRG cells, tolcapone (not entacapone) inhibited maximal complex I- and complex II-linked oxygen consumption. In intact mouse liver mitochondria, tolcapone stimulated state 2 complex II-linked respiration and both compounds inhibited state 3 respiration of complex IV. Mitochondrial uncoupling was confirmed for both compounds by stimulation of complex I-linked respiration in the presence of oligomycin. Inhibition of complex I, II and IV for tolcapone and of complex I and IV for entacapone was directly demonstrated in disrupted mouse liver mitochondria. In HepaRG cells, tolcapone-induced inhibition of mitochondrial respiration was associated with increased lactate and ROS production and hepatocyte necrosis. In conclusion, both compounds uncouple oxidative phosphorylation and inhibit mitochondrial enzyme complexes. Tolcapone is a more potent mitochondrial toxicant than entacapone. Mitochondrial toxicity is a possible mechanism for tolcapone-associated hepatotoxicity.