Investigation into the in vivo metabolism of glutathionyl conjugates of dopamine and norepinephrine: Possible insights into neurodegenerative diseases.

Abstract

The in vitro and in vivo hydrolyses of 5-S-Glu-NE were investigated in rat brain homogenates and by intracerebroventricular (icv) administration in rat brains. Under in vitro conditions, 5-S-Glu-NE was hydrolyzed to give rise to 5-S-cysteinylnorepinephrine (5-S-Cys-NE). Under in vivo conditions, 5-S-Cys-NE was further metabolized into 5-S-N-acetylcysteinylnorepinephrine (5-S-NAc-Cys-NE).

The in vitro hydrolyses of 5-S-glutationyldopamine (5-S-Glu-DA) and 2,5-bis-S-glutathionyldopamine (2,5-bis-S-Glu-DA) were investigated in rat brain homogenates. Under in vitro condition, 5-S-Glu-DA and 2,5-bis-S-Glu-DA were hydrolyzed to their corresponding cysteinyl conjugates of DA. Under in vivo condition, 5-S-Glu-DA was metabolized to 5-S-Cys-DA, and further metabolized into 5-S-N-acetylcysteinyldopamine (5-S-NAc-Cys-DA). 5-S-NAc-Cys-DA was formed after the icv administration of 5-S-Cys-DA. 5-S-Glu-DA, 5-S-Cys-DA and 5-S-NAc-Cys-DA were observed after the icv administration of 5-S-Cys-Gly-DA. (Abstract shortened by UMI.)

The effects of the icv administration of 5-S-Glu-NE on the neurotransmitter levels of serotonergic system, dopaminergic system and noradrenergic system were investigated. The levels of 5-HT and 5-HIAA were increased in all brain areas studied 30 min or 120 min after the icv administration of 5-S-Glu-NE. The levels of NE were increased in midbrain (30 min) and striatum (120 min) after administration. The levels of DA were decreased at hippocampus (120 min) and increased at cortex (120 min) after administration. The level of HVA, a metabolite of DA, was decreased at midbrain 120 min after administration.

In this study, the oxidation of NE in the presence of GSH at physiological pH was investigated. Under electrochemical conditions, four glutathionyl conjugates of NE were formed. These reaction products were isolated by preparative HPLC methods and structurally identified by spectroscopic methods, including NMR, MS, and UV-Vis. Other oxidation conditions investigated include autoxidation, hydroxyl radical (HO$\cdot$)-mediated and superoxide anion radical (O$\sbsp{2}{-1}\cdot$)-mediated oxidation of NE in the presence of glutathione at physiological pH.

Parkinson's disease (PD) and Alzheimer's disease (AD) are the two major neurodegenerative diseases.

Based on the in vitro study of catecholamine neurotransmitters, it was hypothesized that the chronic exposure to environmental toxicants or other factors could trigger the aberrant oxidation of neurotransmitters like dopamine (DA) and norepinephrine (NE). The oxidative intermediate o-quinone would be attacked by glutathione (GSH), a common antioxidant in the tissue, to form a series of glutathionyl conjugates. Preliminary study showed that some compounds derived from the oxidation of dopamine and norepinephrine in the presence of cysteine were toxic when administered in mice. These toxic metabolites may ultimately contribute to the neuronal degeneration.