In humans, D-2-hydroxyglutaric acid is formed by a hydroxyacid-oxoacid transhydrogenase whereas in bacteria it is formed by a 2-hydroxyglutarate synthase. D-2-Hydroxyglutaric acid is also formed via the normal activity of hydroxyacid-oxoacid transhydrogenase during conversion of 4-hydroxybutyrate to succinate semialdehyde. The compound can be converted to alpha-ketoglutaric acid through the action of a 2-hydroxyglutarate dehydrogenase (EC 1.1.99.2). In humans, there are two such enzymes (D2HGDH and L2HGDH). Both the D and the L stereoisomers of hydroxyglutaric acid are found in body fluids. D-2-Hydroxyglutaric acid is a biochemical hallmark of the inherited neurometabolic disorder D-2-hydroxyglutaric aciduria (OMIM: 600721) and the genetic disorder glutaric aciduria II. D-2-Hydroxyglutaric aciduria (caused by loss of D2HGDH or gain of function of IDH) is rare, with symptoms including cancer, macrocephaly, cardiomyopathy, mental retardation, hypotonia, and cortical blindness. An elevated urine level of D-2-hydroxyglutaric acid has been reported in patients with spondyloenchondrodysplasia (OMIM: 271550). D-2-Hydroxyglutaric acid can be converted to alpha-ketoglutaric acid through the action of 2-hydroxyglutarate dehydrogenase (D2HGDH). Additionally, the enzyme D-3-phosphoglycerate dehydrogenase (PHGDH) can catalyze the NADH-dependent reduction of alpha-ketoglutarate (AKG) to D-2-hydroxyglutarate (D-2HG). Nyhan et al. (1995) described 3 female patients, 2 of them sibs, who were found to have excess accumulation of D-2-hydroxyglutaric acid in the urine. The phenotype was quite variable, even among the sibs, but included mental retardation, macrocephaly with cerebral atrophy, hypotonia, seizures, and involuntary movements. One of the patients developed severe intermittent vomiting and was given a pyloromyotomy. The electroencephalogram demonstrated hypsarrhythmia. There was an increased concentration of protein in cerebrospinal fluid, an unusual finding in inborn errors of metabolism. D-2-Hydroxyglutaric acid can also be produced via gain-of-function mutations in the cytosolic and mitochondrial isoforms of isocitrate dehydrogenase (IDH). IDH is part of the TCA cycle and this compound is generated in high abundance when IDH is mutated. Since D-2-hydroxyglutaric acid is sufficiently similar in structure to 2-oxoglutarate (2OG), it is able to inhibit a range of 2OG-dependent dioxygenases, including histone lysine demethylases (KDMs) and members of the ten-eleven translocation (TET) family of 5-methylcytosine (5mC) hydroxylases. This inhibitory effect leads to alterations in the hypoxia-inducible factor (HIF)-mediated hypoxic response and alterations in gene expression through global epigenetic remodeling. The net effect is that D-2-hydroxyglutaric acid causes a cascading effect that leads genetic perturbations and malignant transformation. Depending on the circumstances, D-2-hydroxyglutaric acid can act as an oncometabolite, a neurotoxin, an acidogen, and a metabotoxin. An oncometabolite is a compound that promotes tumour growth and survival. A neurotoxin is compound that is toxic to neurons or nerual tissue. An acidogen is an acidic compound that induces acidosis, which has multiple adverse effects on many organ systems. A metabotoxin is an endogenously produced metabolite that causes adverse health effects at chronically high levels.  As an oncometabolite, D-2-hydroxyglutaric acid is a competitive inhibitor of multiple alpha-ketoglutarate-dependent dioxygenases, including histone demethylases and the TET family of 5mC hydroxylases. As a result, high levels of 2-hydroxyglutarate lead to genome-wide histone and DNA methylation alterations, which in turn lead to mutations that ultimately cause cancer (PMID: 29038145). As a neurotoxin, D-2-hydroxyglutaric acid mediates its neurotoxicity through activation of N-methyl-D-aspartate receptors. D-2-Hydroxyglutaric acid is structurally similar to the excitatory amino acid glutamate and stimulates neurodegeneration by mechanisms similar to glutamate, NMDA, or mitochondrial toxins (PMID: 12153528). As an acidogen, D-2-hydroxyglutaric acid is classified as an alpha hydroxy acid belonging to the general class of compounds known as organic acids. Chronically high levels of D-2-hydroxyglutaric acid are a feature of the inborn error of metabolism called D-2-hydroxyglutaric aciduria. Abnormally high levels of organic acids in the blood (organic acidemia), urine (organic aciduria), the brain, and other tissues lead to general metabolic acidosis. Acidosis typically occurs when arterial pH falls below 7.35. In infants with acidosis, the initial symptoms include poor feeding, vomiting, loss of appetite, weak muscle tone (hypotonia), and lack of energy (lethargy). These can progress to heart abnormalities, kidney abnormalities, liver damage, seizures, coma, and possibly death. These are also the characteristic symptoms of untreated D-2-hydroxyglutaric aciduria. Many affected children with organic acidemias experience intellectual disability or delayed development. In adults, acidosis or acidemia is characterized by headaches, confusion, feeling tired, tremors, sleepiness, and seizures.