Lactic acid, also known as milk acid or 2-hydroxypropanoic 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 carboxylic acid, 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 inhibt 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.
During intense exercise, such as sprinting type activities, when the rate of demand for energy is high, lactate is produced faster than the ability of the tissues to remove it and lactate concentration begins to rise. This is a beneficial process since the regeneration of NAD+ ensures that energy production is maintained and exercise can continue. Contrary to popular belief, this increased concentration of lactate does not directly cause acidosis, nor is it responsible for muscle pain or "burning". This is because lactate itself is not capable of releasing a proton, and secondly, the acidic form of lactate (lactic acid) cannot be formed under normal circumstances in human tissues. Analysis of the glycolytic pathway in humans indicates that there are not enough hydrogen ions present in the glycolytic intermediates to produce lactic or any other acid.
The acidosis that is associated with increases in lactate concentration during heavy exercise arises from a completely separate reaction. When ATP is hydrolysed, a hydrogen ion is released. ATP-derived hydrogen ions are primarily responsible for the decrease in pH. During intense exercise, oxidative metabolism (aerobic) cannot produce ATP quick enough to supply the demands of the muscle. As a result, glycolysis (i.e. anaerobic metabolism) becomes the dominant energy producing pathway as it can form ATP at high rates. Due to the large amounts of ATP being produced and hydrolysed in a short period of time, the buffering systems of the tissues are overcome, causing pH to fall and creating a state of acidosis. This may be one factor, among many, that contributes to the acute muscular discomfort experienced shortly after intense exercise.
Although it is not firmly established, it is possible that lactate may contribute to an acidotic effect via the strong ion difference, however this has not been well investigated in exercise physiology research and so its contribution is still uncertain.
Lactic acid is popularly known as an AHA in the cosmetics industry. It is widely used as a milder alternative to glycolic acid. It is primarily used as an anti-aging chemical claimed to soften lines, reduce photodamage from the sun, improve skin texture and tone and improve overall appearance.
Several precautions should be taken when using lactic acid as a cosmetic agent because it can increase UV sensitivity to the sun.
Lactic acid is used in a variety of food stuffs to act as an acidity regulator. Although it can be fermented from lactose (milk sugar), most commercially used lactic acid is derived from bacteria such as Bascillus acidilacti, Lactobascillus delbueckii or L. bulgaricuswhey to ferment carbohydrates from sources such as cornstarch, potatoes or molasses. Thus, although it is commonly known as "milk acid", products claiming to be vegan do sometimes feature lactic acid as an ingredient.
Two molecules of lactic acid can be dehydrated to lactide, a cyclic lactone. A variety of catalysts can polymerise lactide to either heterotactic or syndiotactic Polylactide, which as biodegradable polyesters with valuable (inter alia) medical properties are currently attracting much attention.
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