FUNCTION AND EXPRESSION OF YEAST MITOCHONDRIAL NAD-SPECIFIC AND NADP-SPECIFIC ISOCITRATE DEHYDROGENASES

The three isozymes of isocitrate dehydrogenase in Saccharomyces cerevi siae differ in subunit structure, subcellular location, and cofactor s pecificity. The two mitochondrial isozymes, IDH and IDP1, are NAD- and NADP-specific, respectively. Several lines of evidence presented here confirm the importance of IDH to respiratory processes. Expression of IDH RNA and protein is low with growth on glucose and is elevated wit h growth on non-fermentable carbon sources, a pattern of expression si milar to that seen for other tricarboxylic acid cycle enzymes. In addi tion, a disruption mutant lacking IDH activity exhibits reduced growth rates on non-fermentable carbon sources, and mitochondria isolated fr om this mutant are incapable of respiration with added citrate. In con trast, IDP1 expression levels appear to be unresponsive to carbon sour ce, and an IDP1 disruption mutant is not significantly impaired for gr owth or mitochondrial respiration. These results strongly suggest that IDP1 is incapable of participating in tricarboxylic acid cycle-based respiration despite its mitochondrial location. Analysis of the IDP1 a nd IDH disruption mutants for glutamate auxotrophy showed that either enzyme can contribute alpha-ketoglutarate for endogenous glutamate syn thesis. IDH expression levels were found to be repressed in response t o added glutamate during growth on glucose, while IDP1 expression leve ls remained unchanged. A double mutant lacking both IDP1 and IDH activ ities proved to be auxotrophic for glutamate during growth on glucose, but was capable of growth independent of added glutamate on non-ferme ntable carbon sources. These results suggest that the cytosolic NADP-s pecific IDP2 isozyme may provide alpha-ketoglutarate both for tricarbo xylic acid cycle carbon flux and for cytosolic glutamate synthesis dur ing growth on non-fermentable carbon sources in the absence of mitocho ndrial isocitrate dehydrogenase activity.