Glutathione (GSH) conjugates with dopamine (DA)-derived quinones to form reactive or non-reactive GSH-conjugates

In this study we demonstrate for the first time that GSH could rapidly conjugate with dopamine (DA)-derived DA-o-quinones without enzymatic catalysis to form short-lived intermediate GSHconjugates (2-S-GSH-DA-o-quinone and 5-S-GSH-DA-o-quinone). These intermediate GSHconjugates are unstable and would finally form reactive or non-reactive GSH-conjugates dependent on ambient reductive forces. Under insufficient reductive forces, the intermediate GSH-conjugates could cyclize spontaneously to form reactive 7-S-GSH-aminochrome (7-S-GSHAM). The 7-S-GSH-AM is so reactive that it could further react with another GSH to form 4,7bi-GSH-5,6-dihydroindole. Its reactivity could also abrogate tyrosinase activity in solutions. In addition, the 7-S-GSH-AM could further undergo internal rearrangement to form non-reactive 7S-GSH-5,6-dihydroindole. From these novel findings, we propose two detrimental positive feedback loops involving accelerated DA oxidation, increased GSH consumption and impaired GSH detoxification efficiency, as the underlying chemical explanation for dopaminergic neuron degeneration in Parkinson’s disease. Parkinson’s disease (PD), characterized by tremor, rigidity, postural instability, and bradykinesia, is the most common serious movement disorder in the world, affecting about 1% of adults older than 60 years . The dopamine (DA) related oxidative stress was suggested to be the key event in the genesis of PD . DA oxidation could produce reactive oxygen species (ROS) and reactive DA quinones . The ROS could lead to comprehensive damage including proteins, lipids and nucleic acids in cells . On the other hand, DA quinones are highly reactive and could covalently react with the cysteine residues of functional proteins and irreversibly inhibit their activity 5, . The highly reactive DA-derived DA quinones rather than ROS have been suggested to play an important role in DA-induced toxicity related to PD pathogenesis . Various studies have shown that DA-derived DA quinones could impair mitochondria contributing to dopaminergic neuron demise . Our previous study has also demonstrated that DA could induce irreversible proteasome inhibition via DA-derived quinones rather than via the ROS . To date, GSH has been implicated to be an important endogenous protective factor against the toxicity of DA 14, . GSH could detoxify DA-related toxicity through conjugation with DAderived quinones, besides scavenging ROS 16, . In the presence of GSH transferase, GSH could conjugate with DA-derived quinone to form 5-S-GSH-DA . Furthermore GSH transferase could catalyze GSH conjugation with AM to form 4-S-GSH-5,6-dihydroxyindoline . In our recent study using an improved tandem HPLC procedure coupled with ESI-MS, we found that GSH could actually react with AM to form various GSH conjugates without any enzymatic catalysis . We have also shown in a separate study that DA-derived quinones are associated with cellular toxicity induced by mutant alpha-synuclein which is associated strongly with PD . Meanwhile, other investigators have reported that intrastriatal injection or administering of DA could significantly decrease GSH level in the brain of experimental animals as well as in cultured astroglial cells 20, . In addition, postmortem studies have implicated the decrease of GSH level in the substantia nigra (SN) of the brain to be the early event for PD onset since there was an obvious decrease (~ 40 %) of GSH in the SN of early and advanced PD patients compared with the aged controls 22, . Furthermore, when compared with aged controls, the highest level of GSH-DA conjugates were detected in the SN and putamen of postmortem PD patients . Taken together, these observations inferred that the increased production of DA-derived quinones and consumption of GSH via conjugation between GSH and DA-derived quinones are vital events highly related to PD. In this study, we report for the first time that GSH could rapidly conjugate with DA-derived quinones to form reactive or non-reactive GSH conjugates dependent on ambient reductive force and without any enzymatic catalysis. Based on our findings, two detrimental positive feedback loops leading to an imbalance between DA oxidation and GSH protective capacity are proposed to help us understand the chemical explanation underlying DA neuron degeneration. These detrimental feedback loops may act together in a vicious cycle aggravating GSH decrease in the brain, hence accounting for the onset of PD.