CYTOTOXICITY OF MENADIONE AND RELATED QUINONES IN FRESHLY ISOLATED RAT HEPATOCYTES - EFFECTS ON THIOL HOMEOSTASIS AND ENERGY-CHARGE

The cytotoxic events in freshly isolated rat hepatocytes following exp osure over 2 h to menadione (2-methyl-1,4-naphthoquinone) and mio clos ely related quinones, 2,3-dimethyl-1,4-naphthoquinone (DMNQ) and 1,4-n aphthoquinone (NQ), were examined. These quinones differ in their aryl ation capacity (NQ> menadione >> DMNQ) and in their potential to induc e redox cycling (NQ approximate to menadione >> DMNQ). The glutathione status (reduced and oxidized glutathione) of the hepatocytes was dete rmined using HPLC after derivatization with monobromobimane. Protein t hiols were measured spectrophotometrically and the energy charge of th e cells was determined with HPLC using ion pair chromatography. The le akage of lactate dehydrogenase was used as a marker for cell viability . All three quinones caused alterations of the glutathione status of t he exposed cells but the effects were markedly different. Exposure to DMNQ resulted in a slow decrease of reduced glutathione and an increas e of mixed disulfides. The other two quinones caused an almost complet e depletion of reduced glutathione within 5 min. Hepatocytes exposed t o NQ accumulated oxidized glutathione whereas menadione-exposed hepato cytes showed increased levels of mixed disulfides. We did not find any effects of DMNQ (200 mu M) on protein thiols, energy charge or cell v iability. There was a clear difference in the effects of menadione and NQ on protein thiols, energy charge and cell viability: exposure to N Q resulted in a more extensive decrease of protein thiols and energy c harge and an earlier onset of lactate dehydrogenase leakage. From our results we conclude that the arylation capacity of a quinone is a dete rmining factor in the cytotoxic potential of such compounds and that t he decrease of protein thiols and of the energy are critical events pr eceding loss of cell viability.