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

Abstract

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 alpha-ketoglutarate. The origin of the L-2-hydroxyglutarate, which accumulates in this disorder, is presently unknown. The oxidation-reduction potential of the 2-hydroxyglutarate/alpha-ketoglutarate couple is such that L-2-hydroxyglutarate could potentially be produced through the reduction of alpha-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 alpha-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, 'alpha-ketoglutarate reductase', which similarly co-purified with mitochondrial L-malate dehydrogenase (mMDH). Purified mMDH catalysed the reduction of alpha-ketoglutarate to L-2-hydroxyglutarate with a catalytic efficiency that was about 10(7)-fold lower than that observed with oxaloacetate. For the cytosolic enzyme, this ratio amounted to 10(8), indicating that this enzyme is more specific. Both cMDH and mMDH are highly active in tissues and alpha-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 alpha-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.

L-2-Hydroxyglutarate production arises from noncanonical enzyme function at acidic pH

“…The findings presented here substantiate prior genetic evidence implicating LDH and MDH as cellular sources of L-2HG 16–18,21 . In hypoxia, cells selectively produce L-2HG in a manner primarily dependent on LDHA and MDH2 17,18 .…”

“…3b–f). Collectively, these findings substantiate genetic evidence suggesting that LDH and MDH enzymes contribute to cellular production of L-2HG 16–18,21 .…”

“…Genetic evidence suggests that lactate dehydrogenase and malate dehydrogenase enzymes are major sources of hypoxia-induced L-2HG 16–18,21 , but there has been no demonstration that these enzymes directly catalyze L-2HG production. Here we demonstrate that purified LDH and MDH enzymes can catalyze stereospecific reduction of αKG to L-2HG and that acidic reaction conditions dramatically enhance LDH- and MDH-mediated production of L-2HG in vitro and in cells.…”

Saccharomyces cerevisiae Forms d-2-Hydroxyglutarate and Couples Its Degradation to d-Lactate Formation via a Cytosolic Transhydrogenase

“…Finally, there is evidence that wildtype IDH1 and IDH2 also slowly form D-2HG from ␣-ketoglutarate (15). A known source of L-2HG in mammalian cells is a low side activity of malate dehydrogenase, converting ␣-ketoglutarate to L-2HG at the expense of NADH (16,17). Such an L-2HG-producing side activity has also been demonstrated for the mitochondrial malate dehydrogenases of Arabidopsis thaliana (18).…”

Identification of novel L2HGDH gene mutations and update of the pathological spectrum

“…It is now known that the mutated gene in L-2-HGA affects subject maps at chromosome 14q22 2,3 and encodes a FAD-dependent enzyme 2 known as L-2-hydroxyglutaric acid dehydrogenase. 2,4,5 Once clarified the molecular basis involved in L-2-HGA, the reports of co-segregating mutations started to increase and are now available for several populations. At the time of this study, 32 distinct mutations found in a total of 50 affected families are known from several distinct populations.…”