Cerebral citric acid cycle enzymes in methionine sulfoximine toxicity

The activity levels of pyruvate dehydrogenase, enzymes of the citric acid cycle, aspartate and alanine aminotransferases, and NADP+-isocitrate dehydrogenase were determined in the cerebral cortex, cerebellum, brain stem, corpus striatum, hippocampus, and midbrain regions of normal rats and rats injected with acute and subacute doses of methionine sulfoximine (MSI). In both conditions there was an elevation in the activities of pyruvate dehydrogenase and all the enzymes of the citric acid cycle except malate dehydrogenase, whereas the activities of aminotransferases and NADP+-isocitrate dehydrogenase were suppressed in all the cerebral regions. It is suggested that the operational rates of the citric acid cycle would be enhanced in MSI-induced hyperammonemia and that there might be a derangement in the transport of reducing equivalents across mitochondrial membranes. It has been suggested that the convulsant action of the drug is due to its effects on ionic gradients and may not be due to depletion of alpha-ketoglutarate from the citric acid cycle.