2-Hydroxybutyric acid is an organic acid that is involved in propanoate metabolism. It is produced in mammalian tissues (principaly hepatic) that catabolize L-threonine or synthesize glutathione. Oxidative stress or detoxification demands 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 forming methionine into the transsulfuration pathway forming cystathionine. 2-Hydroxybutyrate is released as a by-product when cystathionine is cleaved to cysteine that is incorporated into glutathione. 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 1970's that an increased NADH2/NAD ratio was the most important factor for the production of 2-hydorxybutyric acid (PMID: 168632) [HMDB]
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