Inhibitors of the α-ketoglutarate dehydrogenase complex alter [1-13C] glucose and [U-13C] glutamate metabolism in cerebellar granule neurons

Diminished activity of the alpha-ketoglutarate dehydrogenase complex (KGDHC), an important component of the tricarboxylic acid (TCA) cycle, occurs in several neurological diseases. The effect of specific KGDHC inhibitors [phosphonoethyl ester of succinyl phosphonate (PESP) and the carboxy ethyl ester of succinyl phosphonate (CESP)] on [1-C-13]glucose and [U-C-13]glutamate metabolism in intact cerebellar granule neurons was investigated. Both inhibitors decreased formation of [4-C-13]glutamate from [1-C-13]glucose, a reduction in label in glutamate derived from [1-C-13]glucose/[U-C-13]glutamate through a second turn of the TCA cycle and a decline in the amounts of gamma-aminobutyric acid (GABA), aspartate, and alanine. PESP decreased formation of [U-C-13]aspartate and total glutathione, whereas CESP decreased concentrations of valine and leucine. The findings are consistent with decreased KGDHC activity; increased alpha-ketoglutarate formation; increased transamination of alpha-ketoglutarate with valine, leucine, and GABA; and new equilibrium position of the aspartate aminotransferase reaction. Overall, the findings also suggest that some carbon derived from alpha-ketoglutarate may bypass the block in the TCA cycle at KGDHC by means of the GABA shunt and/or conversion of valine to succinate. The results suggest the potential of succinyl phosphonate esters for modeling the biochemical and pathophysiological consequences of reduced KGDHC activity in brain diseases. (c) 2006 Wiley-Liss, Inc.