Lactic acid (IUPAC systematic name: 2-hydroxypropanoic acid), also known as milk acid, is a chemical compound that plays a role in several biochemical processes. Lactic acid is a carboxylic acid with a chemical formula of C3H6O3. It has a hydroxyl group adjacent to the carboxyl group, making it an alpha hydroxy acid (AHA). In solution, it can lose a proton from the acidic group, producing the lactate ion CH3CH(OH)COO?.
Lactic acid is chiral and has two optical isomers. One is known as L-(+)-lactic acid or (S)-lactic acid and the other, its mirror image, is D-(-)-lactic acid or (R)-lactic acid. L-(+)-Lactic acid is the biologically important isomer.
L-Lactate is produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation. Lactate is constantly produced during normal metabolism and exercise but does not increase in concentration until the rate of lactate production exceeds the rate of lactate removal. The rate of removal is governed by a number of factors including: monocarboxylate transporters, concentration and isoform of LDH and oxidative capacity of tissues. The concentration of blood lactate is usually 1-2 mmol/L at rest, but can rise to over 20 mmol/L during intense exertion.
Increases in lactate concentration typically occur under conditions where the rate of energy demand by tissues cannot be met by aerobic respiration i.e. tissues cannot get or process oxygen and substrates quickly enough. Under these conditions pyruvate dehydrogenase cannot convert pyruvate to acetyl-CoA quickly enough and pyruvate begins to build up. This would normally inhibit glycolysis and reduce ATP production, if not for lactate dehydrogenase reducing pyruvate to form lactate via the reaction:
pyruvate + NADH + H+ ? lactate + NAD+. The purpose of lactate production is to regenerate nicotinamide adenine dinucleotide (NAD+) needed for glycolysis and thus allow adenosine triphosphate (ATP) production to continue.
The increased lactate produced can be removed in a number of ways including: oxidation to pyruvate by well-oxygenated muscle cells which is then directly used to fuel the citric acid cycle and conversion to glucose via the Cori cycle in the liver through the process of gluconeogenesis.
Lactic acid fermentation is also performed by Lactobacillus bacteria. These bacteria can operate in the mouth; the acid they produce is responsible for the tooth decay known as caries.
In medicine, lactate is one of the main components of Ringer's lactate or lactated Ringer's solution. This intravenous fluid consists of sodium, chloride, potassium, and lactate in solution with distilled water in concentration so as to be isotonic compared to human blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery or a burn injury.