Modulation Of Liver GSH Homeostasis By GlcNAcCys And Its Mechanism

The human body is inevitably exposed to different toxins and drugs. Metabolisms of these exogenous toxins and drugs in liver by phaseâ… and phaseâ…¡enzymes will result in excessive productin of free radicals and reactive intermediates. Reactive intermediates can conjugate with important biological macromolecules and result in body damage, such as oxidation, aging, and liver diseases. Under normal physiological conditions, the body has evolved mechanisms to counteract the damage caused by reactive metabolites to keep the reactive intermediates in a steady, balanced and low level. However, accumulation of excessive toxic products in the body will induce tissue dysfunction. Liver, which is the main organ responsible for drug accumulation, biotransformation, and metabolism, is the first subject to the most of the exogenous toxins and drugs and is more susceptible to different diseases compared with other organs. Exogenous-induced liver damage has become the leading cause of liver diseases and is receiving more attentions.Glutathione (L-y-glutamyl-L-cysteinylglycine, GSH) is the ubiquitous low molecular weight cellular sulfhydryl and plays an important role in the body defense system. Glutathione exists in the reduced and the oxidized forms, which can be inter-converted by glutathione peroxidase (GPx) and glutathione reductase (GR). GSH also plays an important role in xenobiotic detoxification through direct sulfhydryl conjugation or indirectly as a co-factor of glutathione S-transferase (GST) and GPx. Excessive depletion of GSH has been shown to be related with apoptosis, redox imbalance, tissue dysfunctions, and diseases (such as cancer, liver disease, heart attack).Tissue GSH must be restored when it is depleted in order to protect cellular functions and integrity and to avoid further damages to other macromolecules.Evidences show that various ways are involved for maintenance of GSH homeostasis, including de novo synthesis and GPx mediated regeneration of GSH. The de novo synthesis of glutathione from constituent amino acids is via a two-step pathway catalyzed by glutamylcysteine ligase (GCL) and GSH synthase, with the former being the rate limiting step. The steady level of GSH can be modulated at any point by regulating GCL transcription, expression, post-translational modification. Many strategies have been reported for pharmacologically maintaining or increasing tissue GSH levels. These strategies can be ascribed to inducing GSH-related enzymes and/or increasing L-cysteine availability. Some compounds N-acetylcysteine amide, N-acetylcysteine, S-adenosyl methionine, Schisandrin B, resveratrol, L-2-oxo-4-thiazolidine carboxylate have been reported to induced enzyme expression and/or increasing tissue or intracellular GSH contents. However these compounds are not always applicable. For examples, L-cysteine can auto-oxidize to insoluble L-cystine and is reported to have neuro-toxicity. N-acetyl-L-cysteine can be administered orally or intravenously but must be enzymatically deacetylated in cells.We previously reported the preparation and characterization of a novel thiazolidine derivative N-acetyl-glucosamine-thiazolidine-4(R)-carboxylic acid (GlcNAcCys), which is cyclo-condensed from L-cysteine and N-acetyl-glucosamine. GlcNAc is the monosaccharide of chitin and shows many biological activities. GlcNAcCys can function as a L-cysteine prodrug to liberate free L-cysteine non-enzymatically in physiological pH in vitro and in vivo. With high reducing ability, GlcNAcCys can scavenge hydroxyl radicals and superoxide anion free radicals, protect biological macromolecules (deoxyribose, proteins and lipids) from free radical damage. In APAP induced liver damage, GlcNAcCys can increase liver sulfhydryl level. However, the mechanism underlying the increase in cellular sulfhydryl level by GlcNAcCys remains unclear.In the present study, we used APAP induced liver damage to further investigate the effect-time relationship by GlcNAcCys and use L-buthionine-[S,R]-sulfoximine (BSO, a specific inhibitor of GCL) induced liver GSH depletion mice model to further investigate the mechanism of GSH content improvement by GlcNAcCys. Our results demonstrated that GlcNAcCys could increase liver T-SH and GSH concentrations; induce the activities of hepatic antioxidant enzymes (GPx, GST). BSO increased c-fos, c-jun transcription but had no effect on mRNA level of GCL. GlcNAcCys treatment could induce the mRNA level of GCL. The augmented activities of hepatic antioxidant enzymes and increased expression of GCL appeared to be the predominant mechanism underlying GlcNAcCys mediated liver protection against chemically induced damages.