Calcium inhibition of the NAD+-linked isocitrate dehydrogenase from blowfly flight muscle mitochondria

Alpha-ketoglutarate dehydrogenase: a target and generator of oxidative stress

Alpha-ketoglutarate dehydrogenase (alpha-KGDH) is a highly regulated enzyme, which could determine the metabolic flux through the Krebs cycle. It catalyses the conversion of alpha-ketoglutarate to succinyl-CoA and produces NADH directly providing electrons for the respiratory chain. alpha-KGDH is sensitive to reactive oxygen species (ROS) and inhibition of this enzyme could be critical in the metabolic deficiency induced by oxidative stress. Aconitase in the Krebs cycle is more vulnerable than alpha-KGDH to ROS but as long as alpha-KGDH is functional NADH generation in the Krebs cycle is maintained. NADH supply to the respiratory chain is limited only when alpha-KGDH is also inhibited by ROS. In addition being a key target, alpha-KGDH is able to generate ROS during its catalytic function, which is regulated by the NADH/NAD+ ratio. The pathological relevance of these two features of alpha-KGDH is discussed in this review, particularly in relation to neurodegeneration, as an impaired function of this enzyme has been found to be characteristic for several neurodegenerative diseases.

Brain alpha-ketoglutarate dehydrogenase complex: kinetic properties, regional distribution, and effects of inhibitors

The substrate and cofactor requirements and some kinetic properties of the alpha-ketoglutarate dehydrogenase complex (KGDHC; EC 1.2.4.2, EC 2.3.1.61, and EC 1.6.4.3) in purified rat brain mitochondria were studied. Brain mitochondrial KGDHC showed absolute requirement for alpha-ketoglutarate, CoA and NAD, and only partial requirement for added thiamine pyrophosphate, but no requirement for Mg2+ under the assay conditions employed in this study. The pH optimum was between 7.2 and 7.4, but, at pH values below 7.0 or above 7.8, KGDHC activity decreased markedly. KGDHC activity in various brain regions followed the rank order: cerebral cortex greater than cerebellum greater than or equal to midbrain greater than striatum = hippocampus greater than hypothalamus greater than pons and medulla greater than olfactory bulb. Significant inhibition of brain mitochondrial KGDHC was noted at pathological concentrations of ammonia (0.2-2 mM). However, the purified bovine heart KGDHC and KGDHC activity in isolated rat heart mitochondria were much less sensitive to inhibition. At 5 mM both beta-methylene-D,L-aspartate and D,L-vinylglycine (inhibitors of cerebral glucose oxidation) inhibited the purified heart but not the brain mitochondrial enzyme complex. At approximately 10 microM, calcium slightly stimulated (by 10-15%) the brain mitochondrial KGDHC. At concentrations above 100 microM, calcium (IC50 = 1 mM) inhibited both brain mitochondrial and purified heart KGDHC. The present results suggest that some of the kinetic properties of the rat brain mitochondrial KGDHC differ from those of the purified bovine heart and rat heart mitochondrial enzyme complexes. They also suggest that the inhibition of KGDHC by ammonia and the consequent effect on the citric acid cycle fluxes may be of pathophysiological and/or pathogenetic importance in hyperammonemia and in diseases (e.g., hepatic encephalopathy, inborn errors of urea metabolism, Reye's syndrome) where hyperammonemia is a consistent feature. Brain accumulation of calcium occurs in a number of pathological conditions. Therefore, it is possible that such a calcium accumulation may have a deleterious effect on KGDHC activity.