The structure and function of mitochondrial respiratory-chain enzyme proteins were studied postmortem in the substantia nigra of nine patients with Parkinson's disease and nine matched controls. Total protein and mitochondrial mass were similar in the two groups. NADH-ubiquinone reductase (Complex I) and NADH cytochrome c reductase activities were significantly reduced, whereas succinate cytochrome c reductase activity was normal. These results indicated a specific defect of Complex I activity in the substantia nigra of patients with Parkinson's disease. This biochemical defect is the same as that produced in animal models of parkinsonism by 1-methyl-4-phenyl-1,2,3,6-tetrahydropyridine (MPTP) and adds further support to the proposition that Parkinson's disease may be due to an environmental toxin with action(s) similar to those of MPTP.

No abstract

Disrupted energy metabolism, in particular reduced activity of cytochrome oxidase (EC 1.9.3.1), a-ketoglutarate dehydrogenase (EC 1.2.4.2) and pyruvate dehydrogenase (EC 1.2.4.1) have been reported in post-mortem Alzheimer's disease brain. b-Amyloid is strongly implicated in Alzheimer's pathology and can be formed intracellularly in neurones. We have investigated the possibility that b-amyloid itself disrupts mitochondrial function. Isolated rat brain mitochondria have been incubated with the b-amyloid alone or together with nitric oxide, which is known to be elevated in Alzheimer's brain. Mitochondrial respiration, electron transport chain complex activities, a-ketoglutarate dehydrogenase activity and pyruvate dehydrogenase activity have been measured. b-Amyloid caused a signi®cant reduction in state 3 and state 4 mitochondrial respiration that was further diminished by the addition of nitric oxide. Cytochrome oxidase, a-ketoglutarate dehydrogenase and pyruvate dehydrogenase activities were inhibited by b-amyloid. The K m of cytochrome oxidase for reduced cytochrome c was raised by b-amyloid. We conclude that b-amyloid can directly disrupt mitochondrial function, inhibits key enzymes and may contribute to the de®ciency of energy metabolism seen in Alzheimer's disease.

Abstract:Within the CNS and under normal conditions, nitric oxide ('NO) appears to be an important physiological signalling molecule. Its ability to increase cyclic GMP concentration suggests that 'No is implicated in the regulation of important metabolic pathways in the brain. Under certain circumstances N0 synthesis may be excessive and N0 may become neurotoxic. Excessive glutamatereceptor stimulation may lead to neuronal death through a mechanism implicating synthesis of both 'No and superoxide (02') and hence peroxynitrite (ONOO ) formation. In response to lipopolysaccharide and cytokines, glial cells may also be induced to synthesize large amounts of 'No, which may be deleterious to the neighbouring neurones and oligodendrocytes. The precise mechanism of 'No neurotoxicity is not fully understood. One possibility is that it may involve neuronal energy deficiency. This may occur by ONOO interfering with key enzymes of the tricarboxylic acid cycle, the mitochondrial respiratory chain, mitochondrial calcium metabolism, or DNA damage with subsequent activation of the energy-consuming pathway involving poly(ADPribose) synthetase. Possible mechanisms whereby ONOO impairs the mitochondrial respiratory chain and the relevance for neurotoxicity are discussed. The intracellular content of reduced glutathione also appears important in determining the sensitivity of cells to ONOOproduction. lt is concluded that neurotoxicity elicited by excessive 'NO production may be mediated by mitochondrial dysfunction leading to an energy deficiency state.

The effect of the neurotoxic nitric oxide derivative, the peroxynitrite anion (ONOO−), on the activity of the mitochondrial respiratory chain complexes in cultured neurones and astrocytes was studied. A single exposure of the neurones to ONOO− (initial concentrations of 0.01–2.0 mM) caused, after a subsequent 24‐h incubation, a dose‐dependent decrease in succinate‐cytochrome c reductase (60% at 0.5 mM) and in cytochrome c oxidase (52% at 0.5 mM) activities. NADH‐ubiquinone‐1 reductase was unaffected. In astrocytes, the activity of the mitochondrial complexes was not affected up to 2 mM ONOO−. Citrate synthase was unaffected in both cell types under all conditions studied. However, lactate dehydrogenase activity released to the culture medium was increased by ONOO− in a dose‐dependent manner (40% at 0.5 mM ONOO−) from the neurones but not from the astrocytes. Neuronal glutathione concentration decreased by 39% at 0.1 mM ONOO−, but astrocytic glutathione was not affected up to 2 mM ONOO−. In isolated brain mitochondria, only succinate‐cytochrome c reductase activity was affected (22% decrease at 1 mM ONOO−). We conclude that the acute exposure of ONOO− selectively damages neurones, whereas astrocytes remain unaffected. Intracellular glutathione appears to be an important factor for ameliorating ONOO−‐mediated mitochondrial damage. This study supports the hypothesis that the neurotoxicity of nitric oxide is mediated through mitochondrial dysfunction.