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Concept
Lactic acid
Summary
Lactic acid is an organic acid. It has a molecular formula . It is white in the solid state and it is miscible with water. 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 loss of a proton to produce the lactate ion CH3CH(OH)CO2−. Compared to acetic acid, its pK_a 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 -lactic acid, (S)-lactic acid, or (+)-lactic acid, and the other, its mirror image, is -lactic acid, (R)-lactic acid, or (−)-lactic acid. A mixture of the two in equal amounts is called -lactic acid, or racemic lactic acid. Lactic acid is hygroscopic. -Lactic acid is miscible with water and with ethanol above its melting point, which is about . -Lactic acid and -lactic acid have a higher melting point. Lactic acid produced by fermentation of milk is often racemic, although certain species of bacteria produce solely -lactic acid. On the other hand, lactic acid produced by anaerobic respiration in animal muscles has the () enantiomer and is sometimes called "sarcolactic" acid, from the Greek sarx, meaning "flesh".
In animals, -lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation during normal metabolism and exercise. 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.
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