Changes of superoxide dismutase activity and ascorbic acid in focal cerebral ischaemia in rats

Mitochondrial inhibition and oxidative stress: reciprocating players in neurodegeneration

Although the etiology for many neurodegenerative diseases is unknown, the common findings of mitochondrial defects and oxidative damage posit these events as contributing factors. The temporal conundrum of whether mitochondrial defects lead to enhanced reactive oxygen species generation, or conversely, if oxidative stress is the underlying cause of the mitochondrial defects remains enigmatic. This review focuses on evidence to show that either event can lead to the evolution of the other with subsequent neuronal cell loss. Glutathione is a major antioxidant system used by cells and mitochondria for protection and is altered in a number of neurodegenerative and neuropathological conditions. This review also addresses the multiple roles for glutathione during mitochondrial inhibition or oxidative stress. Protein aggregation and inclusions are hallmarks of a number of neurodegenerative diseases. Recent evidence that links protein aggregation to oxidative stress and mitochondrial dysfunction will also be examined. Lastly, current therapies that target mitochondrial dysfunction or oxidative stress are discussed.

Cooperative interaction between ascorbate and glutathione during mitochondrial impairment in mesencephalic cultures

A decrease in total glutathione, and aberrant mitochondrial bioenergetics have been implicated in the pathogenesis of Parkinson's disease. Our previous work exemplified the importance of glutathione (GSH) in the protection of mesencephalic neurons exposed to malonate, a reversible inhibitor of mitochondrial succinate dehydrogenase/complex II. Additionally, reactive oxygen species (ROS) generation was an early, contributing event in malonate toxicity. Protection by ascorbate was found to correlate with a stimulated increase in protein-glutathione mixed disulfide (Pr-SSG) levels. The present study further examined ascorbate-glutathione interactions during mitochondrial impairment. Depletion of GSH in mesencephalic cells with buthionine sulfoximine potentiated both the malonate-induced toxicity and generation of ROS as monitored by dichlorofluorescein diacetate (DCF) fluorescence. Ascorbate completely ameliorated the increase in DCF fluorescence and toxicity in normal and GSH-depleted cultures, suggesting that protection by ascorbate was due in part to upstream removal of free radicals. Ascorbate stimulated Pr-SSG formation during mitochondrial impairment in normal and GSH-depleted cultures to a similar extent when expressed as a proportion of total GSH incorporated into mixed disulfides. Malonate increased the efflux of GSH and GSSG over time in cultures treated for 4, 6 or 8 h. The addition of ascorbate to malonate-treated cells prevented the efflux of GSH, attenuated the efflux of GSSG and regulated the intracellular GSSG/GSH ratio. Maintenance of GSSG/GSH with ascorbate plus malonate was accompanied by a stimulation of Pr-SSG formation. These findings indicate that ascorbate contributes to the maintenance of GSSG/GSH status during oxidative stress through scavenging of radical species, attenuation of GSH efflux and redistribution of GSSG to the formation of mixed disulfides. It is speculated that these events are linked by glutaredoxin, an enzyme shown to contain both dehydroascorbate reductase as well as glutathione thioltransferase activities.

Hydrogen peroxide removal and glutathione mixed disulfide formation during metabolic inhibition in mesencephalic cultures

Compromised mitochondrial energy metabolism and oxidative stress have been associated with the pathophysiology of Parkinson's disease. Our previous experiments exemplified the importance of GSH in the protection of neurons exposed to malonate, a reversible inhibitor of mitochondrial succinate dehydrogenase/complex II. This study further defines the role of oxidative stress during energy inhibition and begins to unravel the mechanisms by which GSH and other antioxidants may contribute to cell survival. Treatment of mesencephalic cultures with 10 microM buthionine sulfoximine for 24 h depleted total GSH by 60%, whereas 3 h exposure to 5 mM 3-amino-1,2,4-triazole irreversibly inactivated catalase activity by 90%. Treatment of GSH-depleted cells with malonate (40 mM) for 6, 12 or 24 h both potentiated and accelerated the time course of malonate toxicity, however, inhibition of catalase had no effect. In contrast, concomitant treatment with buthionine sulfoximine plus 3-amino-1,2,4-triazole in the presence of malonate significantly potentiated toxicity over that observed with malonate plus either inhibitor alone. Consistent with these findings, GSH depletion enhanced malonate-induced reactive oxygen species generation prior to the onset of toxicity. These findings demonstrate that early generation of reactive oxygen species during mitochondrial inhibition contributes to cell damage and that GSH serves as a first line of defense in its removal. Pre-treatment of cultures with 400 microM ascorbate protected completely against malonate toxicity (50 mM, 12 h), whereas treatment with 1 mM Trolox provided partial protection. Protein-GSH mixed disulfide formation during oxidative stress has been suggested to either protect vulnerable protein thiols or conversely to contribute to toxicity. Malonate exposure (50 mM) for 12 h resulted in a modest increase in mixed disulfide formation. However, exposure to the protective combination of ascorbate plus malonate increased membrane bound protein-GSH mixed disulfides three-fold. Mixed disulfide levels returned to baseline by 72 h of recovery indicating the reversible nature of this formation. These results demonstrate an early role for oxidative events during mitochondrial impairment and stress the importance of the glutathione system for removal of reactive oxygen species. Catalase may serve as a secondary defense as the glutathione system becomes limiting. These findings also suggest that protein-GSH mixed disulfide formation under these circumstances may play a protective role.

Ascorbate availability and neurodegeneration in amyotrophic lateral sclerosis

Amyotrophic lateral sclerosis is a fatal neurodegenerative disease in which upper and lower motoneurons progressively deteriorate and die. Neuronal damage is most evident in the lower central nervous system, and death generally occurs following central respiratory failure. Proposed and demonstrated mechanisms for amyotrophic lateral sclerosis are diverse, and include altered superoxide dismutase and neurofilament proteins, autoimmune attack, and hyperglutamatergic activity. However, they do not account for the late onset of the disease, its earlier onset in males, and the differential vulnerability of neurons located in the brainstem and spinal cord. It is proposed here that, within the context of a specific defect such as altered superoxide dismutase, age-dependent decline in ascorbate availability triggers the disease. A role for ascorbate, which is found in millimolar levels in neurons, is suggested by a number of consistencies: 1) superoxide radicals being a common substrate for superoxide dismutase and ascorbate; 2) a close association between central nervous system ascorbate levels and injury tolerance; 3) a steady decline in ascorbate plasma levels and cellular availability with age; 4) plasma ascorbate levels being lower in males; 5) an association of ascorbate release with motor activity in central nervous system regions, in vivo; 6) the coupling of brain-cell ascorbate release with glutamate uptake; 7) possible roles for ascorbate modulation of N-methyl-D-aspartate receptor activity; 9) the ability of ascorbate to prevent peroxynitrite anion formation; and 10) evidence supporting the scorbutic guinea pig as a model for amyotrophic lateral sclerosis. Emphasis is placed on the probable competition between superoxide dismutase and ascorbate within the context of a primary defect of metal-binding or metal access in high-concentration proteins such as superoxide dismutase and human heavy neurofilaments. Finally, distinct features of alpha-motoneuronal physiology suggest that cell physiological characteristics such as high metabolic activity and extensive calcium dynamics may render neurons differentially vulnerable in amyotrophic lateral sclerosis.

Endogenous superoxide dismutase activity in reperfusion injuries

To elucidate the relationship between reperfusion injuries and free radicals, we monitored the endogenous superoxide dismutase (SOD) activity by intracerebral microdialysis. Six cats underwent a transient occlusion of the middle cerebral artery for 60 minutes after microdialysis probes were implanted bilaterally into the white matter under the ectosylvian gyrus. Dialysates were collected at 30 minute intervals over the course of 5 hours after reperfusion. The SOD activity of the dialysates was measured with electron spin resonance spectrometry. Regional cerebral blood flow was measured simultaneously and the water content of the white matter was assayed at the end of the experiment. After reperfusion, SOD activity increased significantly in the first 30 minutes compared with the preoperative value, and decreased over 4-4.5 hours and 4.5-5 hours in the occluded side. The water content in the occluded side was significantly higher than that in the contralateral side. The highest SOD activity during reperfusion and the water content in the occluded side seemed to correlate, although not significantly. A leakage of intracellular SOD or a reactive increase of SOD activity in response to the reperfusion injury are possible mechanisms of increase in extracellular SOD.