Involvement of nitroxide radicals in cocaine-induced hepatotoxicity

Abstract

Cocaine administration can produce hepatotoxicity in non-induced mice of at least one strain, DBA/2Ha and hepatotoxicity in induced mice of several strains. Metabolic studies and the administration of metabolites indicate that the minor metabolic pathway, cocaine -» norcocaine -» /V-hydroxynorcocaine -» norcocaine nitroxide, is responsible for the observed cocaine-induced hepatotoxicity. In vitro experiments show that cytochrome P-450 can oxidize /V-hydroxynorcocaine to norcocaine nitroxide. Norcocaine nitroxide is unreactive towards cellular proteins or glutathione but does react directly with reduced pyridine nucleotides and is rapidly reduced enzymatically by the microsomal flavoproteins, NADPH-cytochrome P-450 reductase and FAD-containing monooxygenase. These reactions constitute a futile cycle in which NADPH is consumed and superoxide and hydrogen peroxide are generated. We postulate that the destruction of hydrogen peroxide by glutathione peroxidase results in the accumulation of excess oxidized glutathione which is actively excreted by the cell, since insufficient NADPH is available for glutathione reductase to maintain the GSH/GSSG ratio at an acceptable level. As reduced glutathione levels diminish, the cell can no longer protect itself against toxic lipid hydroperoxides which accumulate as a result of stimulation of lipid peroxidation (caused by the one electron cycling reaction, /V-hydroxynorcocaine to norcocaine nitroxide cycle). Finally, as glutathione is depleted below a certain level, the cell loses the ability to maintain the GSH/GSSG ratio in a range consistent with homeostasis resulting in loss of cellular function. Ultimately, necrosis results. This mechanism is consistent with all the information available concerning cocaine-induced hepatotoxicity. L'administration de cocaine peut produire de l'hépatotoxicité chez les souris non-induites de la souche DBA/ZHA ainsi que de l'hépatotoxicité chez plusieurs souches de souris induites. Des études métaboliques et l'administration des metabolites indiquent que la séquence métabolique mineure: cocaine -» norcocaine -» /V-hydroxynorcocaine -» nitroxyde de cocaine est responsable de rhépatotoxicité induite par la cocaine que Ton a observée. Des expériences in vitro montrent que le cytochrome P-450 peut oxyder la M-hydroxynorcocaine en nitroxyde de cocaine. Le nitroxyde de cocaine n'a aucun effet sur les protéines cellulaires ou sur le glutathion mais il réagit directement avec la pyridine réduite des nucléotides et elle est rapidement réduite enzymatiquement par les flavoprotéines microsomiques: la réductase du NADPH-cytochrome P-450 et la mono-oxygénase contenant le ADF. Ces réactions constituent un cycle non efficace au cours duquel le NADPH est consommé et où un superoxyde et le peroxyde d'hydrogène se forment. Nous pensons que la destruction du peroxyde d'hydrogène par la peroxydase du glutathion provient de l'accumulation d'un excès de glutathion oxydé qui est activement sécrété par la cellule, puisqu'une insuffisance de NADPH est nécessaire à la p...