L-2-hydroxyglutarate production arises from non-canonical enzyme function at acidic pH

The metabolite 2-hydroxyglutarate (2HG) can be produced as either a D(R)- or L(S)- enantiomer, each of which inhibits alpha-ketoglutarate (αKG)-dependent enzymes involved in diverse biologic processes. Oncogenic mutations in isocitrate dehydrogenase produce D-2HG, which causes a pathologic blockade in cell differentiation. On the other hand, oxygen limitation leads to accumulation of L-2HG, which can facilitate physiologic adaptation to hypoxic stress in both normal and malignant cells. Here we demonstrate that purified lactate dehydrogenase (LDH) and malate dehydrogenase (MDH) catalyze stereospecific production of L-2HG via ‘promiscuous’ reduction of the alternative substrate αKG. Acidic pH enhances production of L-2HG by promoting a protonated form of αKG that binds to a key residue in the substrate-binding pocket of LDHA. Acid-enhanced production of L-2HG leads to stabilization of hypoxia-inducible factor 1 alpha (HIF-1α) in normoxia. These findings offer insights into mechanisms whereby microenvironmental factors influence production of metabolites that alter cell fate and function

Development and implementation of a novel assay for L-2-hydroxyglutarate dehydrogenase (L-2-HGDH) in cell lysates: L-2-HGDH deficiency in 15 patients with L-2-hydroxyglutaric aciduria

l-2-hydroxyglutaric aciduria (l-2-HGA) is a rare inherited autosomal recessive neurometabolic disorder caused by mutations in the gene encoding l-2-hydroxyglutarate dehydrogenase. An assay to evaluate l-2-hydroxyglutarate dehydrogenase (l-2-HGDH) activity in fibroblast, lymphoblast and/or lymphocyte lysates has hitherto been unavailable. We developed an l-2-HGDH enzyme assay in cell lysates based on the conversion of stable-isotope-labelled l-2-hydroxyglutarate to 2-ketoglutarate, which is converted into l-glutamate in situ. The formation of stable isotope labelled l-glutamate is therefore a direct measure of l-2-HGDH activity, and this product is detected by liquid chromatography-tandem mass spectrometry. A deficiency of l-2-HGDH activity was detected in cell lysates from 15 out of 15 l-2-HGA patients. Therefore, this specific assay confirmed the diagnosis unambiguously affirming the relationship between molecular and biochemical observations. Residual activity was detected in cells derived from one l-2-HGA patient. The l-2-HGDH assay will be valuable for examining in vitro riboflavin/FAD therapy to rescue l-2-HGDH activity. © 2009 SSIEM and Springer

Muscle weakness, cardiomyopathy, and l-2-hydroxyglutaric aciduria associated with a novel recessive SLC25A4 mutation

Background: Mutations in SLC25A4 (syn. ANT1, Adenine nucleotide translocase, type 1) are known to cause either autosomal dominant progressive external ophthalmoplegia (adPEO) or recessive mitochondrial myopathy, hypertrophic cardiomyopathy, and lactic acidosis. Methods and Results: Whole exome sequencing in a young man with myopathy, subsarcolemmal mitochondrial aggregations, cardiomyopathy, lactic acidosis, and L-2-hydroxyglutaric aciduria (L-2-HGA) revealed a new homozygous mutation in SLC25A4[c.653A>C, NM_001151], leading to the replacement of a highly conserved glutamine by proline [p.(Q218P); NP_001142] that most likely affects the folding of the ANT1 protein. No pathogenic mutation was found in L2HGDH, which is associated with “classic” L-2-HGA. Furthermore, L-2-HGDH enzymatic activity in the patient fibroblasts was normal. Long-range PCR and Southern blot confirmed absence of mtDNA-deletions in blood and muscle. Conclusion: The disturbed ADP/ATP transport across the inner mitochondrial membrane may lead to an accumulation of different TCA-cycle intermediates such as 2-ketoglutarate (2-KG) in our patient. As L-2-HG is generated from 2-KG we hypothesize that the L-2-HG increase is a secondary effect of 2-KG accumulation. Hence, our report expands the spectrum of laboratory findings in ANT1-related diseases and hints towards a connection with organic acidurias