2-Hydroxybutyric acid (CAS: 600-15-7), also known as alpha-hydroxybutyrate, is an organic acid derived from alpha-ketobutyrate. alpha-Ketobutyrate is produced by amino acid catabolism (threonine and methionine) and glutathione anabolism (cysteine formation pathway) and is metabolized into propionyl-CoA and carbon dioxide (PMID: 20526369). 2-Hydroxybutyric acid is formed as a byproduct from the formation of alpha-ketobutyrate via a reaction catalyzed by lactate dehydrogenase (LDH) or alpha-hydroxybutyrate dehydrogenase (alphaHBDH). alpha-Hydroxybutyric acid is primarily produced in mammalian hepatic tissues that catabolize L-threonine or synthesize glutathione. Oxidative stress or detoxification of xenobiotics in the liver can dramatically increase the rate of hepatic glutathione synthesis. Under such metabolic stress conditions, supplies of L-cysteine for glutathione synthesis become limiting, so homocysteine is diverted from the transmethylation pathway (which forms methionine) into the transsulfuration pathway (which forms cystathionine). alpha-Ketobutyrate is released as a byproduct when cystathionine is cleaved into cysteine that is incorporated into glutathione. Chronic shifts in the rate of glutathione synthesis may be reflected by urinary excretion of 2-hydroxybutyrate. 2-Hydroxybutyrate is an early marker for both insulin resistance and impaired glucose regulation that appears to arise due to increased lipid oxidation and oxidative stress (PMID: 20526369). 2-Hydroxybutyric acid is often found in the urine of patients suffering from lactic acidosis and ketoacidosis. 2-Hydroxybutyric acid generally appears at high concentrations in situations related to deficient energy metabolism (e.g. birth asphyxia) and also in inherited metabolic diseases affecting the central nervous system during neonatal development, such as "cerebral" lactic acidosis, glutaric aciduria type II, dihydrolipoyl dehydrogenase (E3) deficiency, and propionic acidemia. More recently it has been noted that elevated levels of alpha-hydroxybutyrate in the plasma is a good marker for early-stage type II diabetes (PMID: 19166731). It was concluded from studies done in the mid-1970s that an increased NADH2/NAD ratio was the most important factor for the production of 2-hydroxybutyric acid (PMID: 168632).