Lactic acid
Lactic acid is an organic acid. It has the molecular formula CH3CH(OH)COOH. It is white in the solid state and it is miscible with water.[2] When in the dissolved state, it forms a colorless solution. Production includes both artificial synthesis as well as natural sources. Lactic acid is an alpha-hydroxy acid (AHA) due to the presence of a hydroxyl group adjacent to the carboxyl group. It is used as a synthetic intermediate in many organic synthesis industries and in various biochemical industries. The conjugate base of lactic acid is called lactate (or the lactate anion). The name of the derived acyl group is lactoyl.
In solution, it can ionize by a loss of a proton to produce the lactate ion CH
3CH(OH)CO−
2. Compared to acetic acid, its pKa is 1 unit less, meaning lactic acid is ten times more acidic than acetic acid. This higher acidity is the consequence of the intramolecular hydrogen bonding between the α-hydroxyl and the carboxylate group.
Lactic acid is chiral, consisting of two enantiomers. One is known as L-lactic acid, (S)-lactic acid, or (+)-lactic acid, and the other, its mirror image, is D-lactic acid, (R)-lactic acid, or (−)-lactic acid. A mixture of the two in equal amounts is called DL-lactic acid, or racemic lactic acid. Lactic acid is hygroscopic. DL-Lactic acid is miscible with water and with ethanol above its melting point, which is about 16 to 18 °C (61 to 64 °F). D-Lactic acid and L-lactic acid have a higher melting point. Lactic acid produced by fermentation of milk is often racemic, although certain species of bacteria produce solely D-lactic acid.[6] On the other hand, lactic acid produced by anaerobic respiration in animal muscles has the (L) enantiomer and is sometimes called "sarcolactic" acid, from the Greek sarx, meaning "flesh".
In animals, L-lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation during normal metabolism and exercise.[7] It does not increase in concentration until the rate of lactate production exceeds the rate of lactate removal, which is governed by a number of factors, including monocarboxylate transporters, concentration and isoform of LDH, and oxidative capacity of tissues.[7] The concentration of blood lactate is usually 1–2 mM at rest, but can rise to over 20 mM during intense exertion and as high as 25 mM afterward.[8][9] In addition to other biological roles, L-lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1 (HCA1), which is a Gi/o-coupled G protein-coupled receptor (GPCR).[10][11]
In industry, lactic acid fermentation is performed by lactic acid bacteria, which convert simple carbohydrates such as glucose, sucrose, or galactose to lactic acid. These bacteria can also grow in the mouth; the acid they produce is responsible for the tooth decay known as cavities.[12][13][14][15] In medicine, lactate is one of the main components of lactated Ringer's solution and Hartmann's solution. These intravenous fluids consist of sodium and potassium cations along with lactate and chloride anions in solution with distilled water, generally in concentrations isotonic with human blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery, or burns.
Lactic acid is produced in human tissues when the demand for oxygen is limited by the supply. This occurs during tissue ischemia when the flow of blood is limited. It may also occur more generally when demand is high such as with intense exercise.
History[edit]
Swedish chemist Carl Wilhelm Scheele was the first person to isolate lactic acid in 1780 from sour milk.[16] The name reflects the lact- combining form derived from the Latin word lac, meaning "milk". In 1808, Jöns Jacob Berzelius discovered that lactic acid (actually L-lactate) also is produced in muscles during exertion.[17] Its structure was established by Johannes Wislicenus in 1873.
In 1856, the role of Lactobacillus in the synthesis of lactic acid was discovered by Louis Pasteur. This pathway was used commercially by the German pharmacy Boehringer Ingelheim in 1895.
In 2006, global production of lactic acid reached 275,000 tonnes with an average annual growth of 10%.[18]
Biology[edit]
Molecular biology[edit]
L-Lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1 (HCA1), a Gi/o-coupled G protein-coupled receptor (GPCR).[10][11]