Reactions of Cysteine and Cysteinyl Derivatives with Dopamine-o-quinone and Further Insights into the Oxidation Chemistry of 5-S-Cysteinyldopamine: Potential Relevance to Idiopathic Parkinson′s Disease

Recent studies suggest that elevated rates of autoxidation of dopamine (DA) in the cytoplasm of neuromelanin-pigmented dopaminergic cell bodies in the substantia nigra (SN) and/or reactions of its proximate oxidation product, DA-o-quinone, with glutathione (GSH) or L-cysteine (CySH) might yield endotoxins that play roles in the pathogenesis of idiopathic Parkinson′s Disease (PD). In this study the reactions between DA-o-quinone and CySH and some cysteine derivatives have been studied. The reactions between DA-o-quinone and CySH or cysteine methyl ester are rapid and give the corresponding 5-S-cysteinyl conjugates of DA as the predominant products. By contrast, the reaction between the sterically hindered D-penicillamine methyl ester (PME) and DA-o-quinone is much slower to give, initially, a mixture of the 2-S-, 5-S-, and 6-S-PME conjugates of DA. These conjugates are then further oxidized by unreacted DA-o-quinone to give a complex mixture of products which include 7-(2-aminoethyl)-5-hydroxy-2,2-dimethyl-1,4-benzothiazine-3-carboxylic acid methyl ester (13). Large increases in the 5-S-cysteinyldopamine (5-S-CyS-DA)/DA concentration ratio have been measured in the Parkinsonian SN and have been interpreted to reflect elevated rates of DA autoxidation in this structure that degenerates in PD. However, the present study reveals that at physiological pH 5-S-CyS-DA is not only more easily oxidized than DA but a bicyclic o-quinone imine (5) intermediate is formed that can chemically oxidize the former conjugate in a reaction that leads to 7-(2-aminoethyl)-3,4-dihydro-5-hydroxy-2H-1,4-benzothiazine-3-carboxylic acid (6). Compound 6 is lethal when administered into the brains of mice. However, this putative dihydrobenzothiazine endotoxin is even more easily oxidized than 5-S-CyS-DA in a rather complex reaction that ultimately forms 7-(2-aminoethyl)-5-hydroxy-1,4-benzothiazine-3-carboxylic acid (15). Although this benzothiazine could not be isolated it was identified by comparison of its electrochemical and spectroscopic properties in solution with those of 13. The results of this study suggest that benzothiazine 15 might serve as a better analytical marker molecule than 5-S-CyS-DA for either elevated rates of DA autoxidation and/or for roles of GSH and CySH in the neurodegenerative mechanisms in the SN that contribute to PD.