Quinone toxicity in hepatocytes: Studies on mitochondrial Ca 2+ release induced by benzoquinone derivatives

Abstract

Hepatocyte cytotoxicity caused by substituted benzoquinones was associated with increased cytosolic Ca 2+ concentration. p-Benzoquinone-induced hepatotoxicity was enhanced when the hepatocytes were loaded with Ca 2+ by preincubation with ATP. A similar order of potency of the substituted benzoquinones in releasing Ca 2+ from isolated mitochondria and inducing hepatocyte cytotoxicity was found; in decreasing order, this was 2-Br-, unsubstituted-, 2-CH 3-, 2,6-(CH 3O) 2-, 2,6-(CH 3) 2-, 2,5-(CH 3) 2-, 2,3,5-(CH 3) 3-, and 2,3,5,6-(CH 3) 4-benzoquinones (duroquinone). The cellular products of quinone metabolism, hydroquinones and glutathione conjugates, did not cause mitochondrial Ca 2+ release. Benzoquinone-induced mitochondrial Ca 2+ release was preceded by GSH conjugate formation and NAD(P)H oxidation but followed by mitochondrial swelling. With duroquinone, a slow GSH and NADPH oxidation preceded Ca 2+ release, but GSH oxidation did not occur with Se-deficient mitochondria lacking glutathione peroxidase activity. Cyanide-insensitive respiration was also observed with duroquinone but not with benzoquinone, suggesting that duroquinone undergoes redox cycling. GSH was depleted by both arylation and oxidation with 2,6-(CH 3O) 2-, 2,6-(CH 3) 2-, 2,5(CH 3) 2-, and 2,3,5-(CH 3) 3-benzoquinones. Benzoquinone concentrations that totally depleted GSH did not cause Ca 2+ release until intramitochondrial NAD(P)H was oxidized. Ca 2+ release was also prevented when NAD(P)H generation was stimulated by the presence of isocitrate or 3-hydroxybutyrate. This suggests that mitochondrial Ca 2+ release is associated with NAD(P)H oxidation catalyzed by NADH dehydrogenase with benzoquinone or by the glutathione peroxidase-glutathione reductase system with duroquinone.