Irreversible Inhibition of Mitochondrial Complex I by 7‐(2‐Aminoethyl)‐3,4‐Dihydro‐5‐Hydroxy‐2<i>H</i>‐1,4‐Benzothiazine‐3‐Carboxylic Acid (DHBT‐1): A Putative Nigral Endotoxin of Relevance to Parkinson's Disease

Abstract: 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‐2 H ‐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 IC 50 of ∼0.80 m M . 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 Q 1 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.