THE EFFECTS OF GLUTATHIONE AND ASCORBIC-ACID ON THE OXIDATIONS OF 6-HYDROXYDOPA AND 6-HYDROXYDOPAMINE

The interactions of ascorbic acid (AA) and reduced glutathione (GSH) i n the oxidations of the catecholaminergic neurotoxins 6-hydroxydopa (T OPA) and 6-hydroxydopamine (6-OHDA) were investigated by both high per formance liquid chromatography with electrochemical detection (HPLC-ED ) and spectrometric methods. These comparative studies showed TOPA and 6-OHDA to be extremely unstable, with 100% of the trihyroxyphenyls ox idized within 0.5 min at physiological pH in potassium phosphate buffe r. Neither AA nor GSH was found capable of significantly impeding the oxidations of these trihydroxyphenyls, or of regenerating these substa nces by reducing back their oxidation products, even though such a red ox exchange mechanism was demonstrated for AA and the dihydroxyphenyl dopamine. Although ineffective in keeping TOPA and 6-OHDA as reduced m olecules, GSH may nevertheless influence the neurotoxicity of trihydro xyphenyls by interacting with their oxidation products forming glutath ionyl conjugates, thereby switching the reaction pathway away from pot entially toxic eumelanin precursors and toward the production of pheom elanin. Electrochemical analyses established the formation of two oxid ation products derived from each trihydroxyphenyl, one detected at -10 0 mV and the other at +700 mV. AA had no effect on either oxidation pr oduct, whereas GSH significantly decreased the levels of both oxidatio n products. The component detected at +700 mV is the cyclized, reduced leukochrome. The identity of the component detected at -100 mV was no t established, but it is considered to be either the p-quinone or the cyclized, oxidized aminochrome.