Irreversible inhibition of mitochondrial complex I by 7-(2-aminoethyl)-3,4-dihydro-5-hydroxy-2H-1,4-benzothiazine-3-carboxyli c acid (DHBT-1): a putative nigral endotoxin of relevance to Parkinson's 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 disease might be elevated translocation of L-cysteine into neuromelanin-pigmented dopaminergic cell bodies in the substantia nigra. In vitro studies suggest that such an influx of L-cysteine would divert the neuromelanin pathway by scavenging dopamine-o-quinone, the proximate autoxidation product of dopamine, to give 5-S-cysteinyldopamine, which is oxidized further to 7-(2-aminoethyl)-3,4-dihydro-5-hydroxy-2H-1,4-benzothiazine-3-carboxylic acid (DHBT-1) and other cysteinyldopamines and dihydrobenzothiazines. In this study, it is demonstrated that DHBT-1 inhibits ADP-stimulated oxidation of malate and pyruvate (state 3 or complex I respiration) when incubated with intact rat brain mitochondria with an IC50 of approximatelly 0.80 mM. Incubation of DHBT-1 with freeze-thawed rat brain mitochondria in both the presence and absence of KCN and/or NADH causes an irreversible, time-dependent decrease of NADH-coenzyme Q1 reductase activity. Significantly lower concentrations of DHBT-1 are necessary to cause this effect when mitochondrial membranes are incubated in the absence of KCN and NADH. The irreversible inhibition of mitochondrial complex I caused by DHBT-1 under the latter conditions could be blocked only partially by glutathione, ascorbic acid, superoxide dismutase, or catalase. Together, these results suggest that DHBT-1 can cross the outer mitochondrial membrane and irreversibly inhibit complex I by a mechanism that is not primarily related to oxygen radical-mediated damage. Formation of DHBT-1 requires only dopamine, L-cysteine, and an oxidizing environment, conditions that may well exist in the cytoplasm of neuromelanin-pigmented dopaminergic neurons in the parkinsonian substantia nigra. The results of this study raise the possibility that DHBT-1 might be an endotoxin formed specifically in pigmented dopaminergic neurons that can contribute to irreversible damage to mitochondrial complex I and substantia nigra cell death in Parkinson's disease.
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Department of Chemistry and Biochemistry, University of Oklahoma, Norman 73019, U.S.A.
Dryhurst
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D000818:Animals; D042967:Electron Transport Complex I; D004731:Endotoxins / Q000378:metabolism; D004791:Enzyme Inhibitors / Q000494:pharmacology; D008297:Male; D008928:Mitochondria / Q000378:metabolism* / Q000502:physiology; D016660:NAD(P)H Dehydrogenase (Quinone) / Q000037:antagonists & inhibitors*; D009247:NADH, NADPH Oxidoreductases / Q000037:antagonists & inhibitors / Q000378:metabolism; D010101:Oxygen Consumption / Q000187:drug effects; D010300:Parkinson Disease / Q000378:metabolism; D051381:Rats; D017207:Rats, Sprague-Dawley; D013378:Substantia Nigra / Q000378:metabolism; D013843:Thiazines / Q000494:pharmacology*
C108502:7-(2-aminoethyl)-3,4-dihydro-5-hydroxy-2H-1,4-benzothiazine-3-carboxylic acid; D004731:Endotoxins; D004791:Enzyme Inhibitors; D013843:Thiazines; D009247:NADH, NADPH Oxidoreductases; D016660:NAD(P)H Dehydrogenase (Quinone); D042967:Electron Transport Complex I