IRREVERSIBLE INHIBITION OF MITOCHONDRIAL COMPLEX-I BY IHYDRO-5-HYDROXY-2H-1,4-BENZOTHIAZINE-3-CARBOXYLIC ACID (DHBT-1) - A PUTATIVE NIGRAL ENDOTOXIN OF RELEVANCE TO PARKINSONS-DISEASE

Based on a number of lines of evidence, we have proposed recently that a very early step in the pathogenesis of idiopathic Parkinson's disea se might be elevated translocation of L-cysteine into neuromelanin-pig mented dopaminergic cell bodies in the substantia nigra. In vitro stud ies suggest that such an influx of L-cysteine would divert the neurome lanin pathway by scavenging dopamine-o-quinone, the proximate autoxida tion product of dopamine, to give 5-S-cysteinyldopamine, which is oxid ized further to ihydro-5-hydroxy-2H-1,4-benzothiazine-3-carboxylic aci d (DHBT-1) and other cysteinyldopamines and dihydrobenzothiazines. In this study, it is demonstrated that DHBT-1 inhibits ADP-stimulated oxi dation of malate and pyruvate (state 3 or complex I respiration) when incubated with intact rat brain mitochondria with an IC50 of similar t o 0.80 mM. Incubation of DHBT-1 with freeze-thawed rat brain mitochond ria in both the presence and absence of KCN and/or NADH causes an irre versible, time-dependent decrease of NADH-coenzyme Q(1) reductase acti vity. Significantly lower concentrations of DHBT-1 are necessary to ca use this effect when mitochondrial membranes are incubated in the abse nce of KCN and NADH. The irreversible inhibition of mitochondrial comp lex I caused by DHBT-1 under the latter conditions could be blocked on ly partially by glutathione, ascorbic acid, superoxide dismutase, or c atalase. Together, these results suggest that DHBT-1 can cross the out er mitochondrial membrane and irreversibly inhibit complex I by a mech anism that is not primarily related to oxygen radical-mediated damage. Formation of DHBT-1 requires only dopamine, L-cysteine, and an oxidiz ing environment, conditions that may well exist in the cytoplasm of ne uromelanin-pigmented dopaminergic neurons in the parkinsonian substant ia nigra. The results of this study raise the possibility that DHBT-1 might be an endotoxin formed specifically in pigmented dopaminergic ne urons that can contribute to irreversible damage to mitochondrial comp lex I and substantia nigra cell death in Parkinson's disease.