Influence of glutathione on the oxidation chemistry of 5-S-cysteinyldopamine: potentially neuroprotective reactions of relevance to Parkinsonʹs disease

In recent reports from this laboratory we have hypothesized that a key step underlying the degeneration of pigmented dopaminergic neurons in the substantia nigra pars compacta (SNc) in Parkinsonʹs disease is an accelerated rate of oxidation of intraneuronal dopamine in the presence of l-cysteine (CySH) to form initially 5-S-cysteinyldopamine (5-S-CyS-DA). 5-S-CyS-DA, however, is more easily oxidized than dopamine in a reaction which leads to the dihydrobenzothiazine (DHBT) 7-(2-aminoethyl)-3,4-dihydro-5-hydroxy-2H-1,4-benzothiazine-3-carboxylic acid (DHBT-1), a putative endogenously-formed metabolite that may be responsible for inhibition of mitochondrial complex I and α-ketoglutarate dehydrogenase, characteristic defects in the parkinsonian SNc. In this investigation it is demonstrated that glutathione (GSH) dramatically attenuates the oxidative transformation of 5-S-CyS-DA into DHBT-1 by two major pathways. In one pathway GSH displaces the cysteinyl residue from the o-quinone proximate oxidation product of 5-S-CyS-DA forming the corresponding glutathionyl conjugate that is attacked by GSH, to form 2,5-di-S-glutathionyldopamine, or by released CySH to give 2-S-cysteinyl-5-S-glutathionyldopamine. The former is the precursor of 2,5,6-tris-S-glutathionyldopamine, a major reaction product. However, intramolecular cyclization of the o-quinone proximate product of 2-S-cysteinyl-5-S-glutathionyldopamine is the first step in a pathway leading to glutathionyl conjugates of 8-(2-aminoethyl)-3,4-dihydro-5-hydroxy-2H-1,4-benzothiazine-3-carboxylic acid (DHBT-5). The second pathway involves nucleophilic addition of GSH to the o-quinone proximate oxidation product of 5-S-CyS-DA forming 2-S-glutathionyl-5-S-cysteinyldopamine the precursor of a number of glutathionyl conjugates of DHBT-1. These results raise the possibility that strategies which elevate intraneuronal levels of GSH in dopaminergic SNc cells in Parkinsonʹs disease patients may block formation of the putative mitochondrial toxin DHBT-1 and hence be neuroprotective.