l-2-Hydroxyglutaric aciduria, a defect of metabolite repair

l-2-hydroxyglutaric aciduria is a metabolic disorder in which l-2-hydroxyglutarate accumulates as a result of a deficiency in FAD-linked l-2-hydroxyglutarate dehydrogenase, a mitochondrial enzyme converting l-2-hydroxyglutarate to α-ketoglutarate. The origin of the l-2-hydroxyglutarate, which accumulates in this disorder, is presently unknown. The oxidation–reduction potential of the 2-hydroxyglutarate/α-ketoglutarate couple is such that l-2-hydroxyglutarate could potentially be produced through the reduction of α-ketoglutarate by a NAD- or NADP-linked oxidoreductase. In fractions of rat liver cytosolic extracts that had been chromatographed on an anion exchanger we detected an enzyme reducing α-ketoglutarate in the presence of NADH. This enzyme co-purified with cytosolic l-malate dehydrogenase (cMDH) upon further chromatography on Blue Sepharose. Mitochondrial fractions also contained an NADH-linked, ‘α-ketoglutarate reductase’ which similarly co-purified with mitochondrial l-malate dehydrogenase (mMDH). Purified mMDH catalysed the reduction of α-ketoglutarate to l-2-hydroxyglutarate with a catalytic efficiency that was about 107-fold lower than that observed with oxaloacetate. For the cytosolic enzyme, this ratio amounted to 108, indicating that this enzyme is more specific. Both cMDH and mMDH are highly active in tissues and α-ketoglutarate is much more abundant than oxaloacetate and more concentrated in mitochondria than in the cytosol. As a result of this, the weak activity of mMDH on α-ketoglutarate is sufficient to account for the amount of l-2-hydroxyglutarate that is excreted by patients deficient in FAD-linked l-2-hydroxyglutarate dehydrogenase. The latter enzyme appears, therefore, to be responsible for a ‘metabolite repair’ phenomenon and to belong to the expanding class of ‘house-cleaning’ enzymes.