Influence of glutathione on the oxidation of 1-methyl-6-hydroxy-1,2,3,4-tetrahydro-beta-carboline: Chemistry of potential relevance to the addictive and neurodegenerative consequences of ethanol abuse

Recent evidence suggests that intraneuronal metabolism of ethanol by catalase/H2O2 and an ethanol-inducible form of cytochrome P450 together generate acetaldehyde and oxygen radicals including the hydroxyl radical (HO.). Within the cytoplasm of serotonergic neurons, these metabolic processes would thus provide acetaldehyde, which would react with unbound 5-hydroxytryptamine (5-HT) to give 1-methyl-6-hydroxy-1,2,3,4-tetrahydro-beta-carboline (1), known to be formed at elevated levels in the brain following ethanol drinking, and HO. necessary to oxidize this alkaloid. In this study, it is demonstrated that the HO.-mediated oxidation of 1 at physiological pH yields 1-methyl-1,2,3,4-tetrahydro-beta-carboline-5,6-dione (8) that reacts avidly with free glutathione (GSH), a significant constituent of axons and nerve terminals, to give diastereomers of 8-S-glutathionyl-1-methyl-1,2,3,4-tetrahydro-beta-carboline-5,6-dione (9A and 9B). In the presence of free GSH, ascorbic acid, other intraneuronal antioxidants/reductants, and molecular oxygen diastereomers, 9A/9B redox cycle in reactions that generate H2O2 and, via trace transition metal ion catalyzed decomposition of the latter compound, HO.. Further reactions of 9A/9B with GSH and/or HO. generate several additional glutathioxyl conjugates that also redox cycle in the presence of intraneuronal reductants and molecular oxygen forming H2O2 and HO.. Thus, intraneuronal formation of 1 and HO. as a consequence of ethanol drinking and resultant endogenous synthesis of 8, 9A, and 9B would, based on these in vitro chemical studies, be expected to generate elevated fluxes of H2O2 and HO. leading to oxidative damage to serotonergic axons and nerve terminals and the irreversible loss of GSH, both of which occur in the brain as a consequence of ethanol drinking. Furthermore, deficiencies of 5-HT and loss of certain serotonergic pathways in the brain have been linked to the preference for and addiction to ethanol.