BRAIN MITOCHONDRIA CATALYZE THE OXIDATION OF IHYDRO-5-HYDROXY-2H-1,4-BENZOTHIAZINE-3-CARBOXYLIC ACID (DHBT-1) TO INTERMEDIATES THAT IRREVERSIBLY INHIBIT COMPLEX I AND SCAVENGE GLUTATHIONE, POTENTIAL RELEVANCE TO THE PATHOGENESIS OF PARKINSONS-DISEASE

We have proposed that a very early step in the pathogenesis of idiopat hic Parkinson's disease is the elevated translocation of L-cysteine in to neuromelanin-pigmented dopaminergic neurons in the substantia nigra . This influx of L-cysteine was proposed to divert the normal neuromel anin pathway by scavenging dopamine-o-quinone, formed by autoxidation of cytoplasmic dopamine, to give initially 5-S-cysteinyldopamine, whic h is further oxidized to ihydro-5-hydroxy-2H-1,4-benzothiazine-3-carbo xylic acid (DHBT-1). In a recent report, it was demonstrated that DHBT -1 evokes inhibition of complex I respiration when incubated with inta ct rat brain mitochondria and a time-dependent irreversible inhibition of NADH-coenzyme Q(1) (CoQ(1)) reductase when incubated with mitochon drial membranes. In this study, it is established that the time depend ence of NADH-CoQ(1) reductase inhibition reflects the oxidation of DHB T-1, catalyzed by an unknown constituent of the inner mitochondrial me mbrane, to an o-quinone imine intermediate that rearranges to inoethyl )-5-hydroxy-1,4-benzothiazine-3-carboxylic acid (BT-I) and decarboxyla tes to 7-(2-aminoethyl)-5-hydroxy-1,4-benzothiazine (BT-2), which are further catalytically oxidized to o-quinone imine intermediates. The e lectrophilic o-quinone imine intermediates formed in these mitochondri a-catalyzed oxidations of DHBT-1, BT-I, and BT-2 are proposed to bind covalently to key sulfhydryl residues at the complex I site, thus evok ing irreversible inhibition of NADH-CoQ1 reductase, Evidence for this mechanism derives from the fact that greater than equimolar concentrat ions of glutathione completely block inhibition of NADH-CoQ(1) reducta se by DHBT-1, BT-l,and BT-2 by scavenging their electrophilic o-quinon e imine metabolites to form glutathionyl conjugates, The results of th is investigation may provide insights into the irreversible loss of gl utathione and decreased mitochondrial complex I activity, which are bo th anatomically specific to the substantia nigra and exclusive to Park inson's disease.