The two stereoisomers of 2-hydroxyglutarate are normally converted to 2-oxoglutarate in the mitochondrial matrix, and can then be metabolized by the citric acid cycle. The physiological sources of 2-hydroxyglutarate have not been established although plausible hypotheses are that it is generated by lysine breakdown or as a byproduct of delta-aminolevulinate metabolism. The stereoisomers are oxidized to 2-oxoglutarate in FAD-dependent reactions catalyzed by the enzymes D2HGDH (specific for R(-)-2-hydroxyglutarate) and L2HGDH (specific for S(-)-2-hydroxyglutarate). An inherited deficiency in either enzyme is associated with accumulation of 2-hydroxyglutarate and variable neurological symptoms. R(-)-2-hydroxyglutarate also reacts reversibly with succinate semialdehyde to form 4-hydroxybutyrate and 2-oxoglutarate, catalyzed by ADHFE1. No deficiencies of this enzyme have been found in patients with elevated 2-hydroxyglutarate levels (Struys 2006).