'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. It was first isolated in 1780 by a Swedish chemist,
Carl Wilhelm Scheele, and 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 CH
3CH(OH)COO
−. It is miscible with water or ethanol, and is
hygroscopic.
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.
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. 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. The concentration of
blood lactate is usually 1-2 mmol/L at rest, but can rise to over 20 mmol/L during intense exertion.
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 (Compound Sodium Lactate or Hartmann's Solution in the UK). This
intravenous fluid consists of
sodium and
potassium cations, with lactate and
chloride anions, 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.
Exercise and lactate
During power-intensive exercises such as sprinting, 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. 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.
Contrary to popular belief, this increased concentration of lactate does not directly cause
acidosis, nor is it responsible for
delayed onset muscle soreness.
[ Biochemistry of exercise-induced metabolic acidosis, R. Robergs, F. Ghiasvand, D. Parker, , , Am J Physiol Regul Integr Comp Physiol, 2004 ] 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 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,
aerobic metabolism cannot produce ATP quickly enough to supply the demands of the muscle. As a result,
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.
The effect of lactate on acidosis has been the topic of many recent conferences in the field of exercise physiology. Robergs et al. have accurately chased the proton movement that occurs during glycolysis. However, in doing so, they have suggested that [H
+] is an independent variable that determines its own concentration. A recent review by Lindinger et al. has been written to rebut the stoichiometric approach used by Robergs et al (2004).
[ In using this stoichiometric process, Robergs et al. have ignored the causitive factors (independent variables) of [H+]. These factors are strong ion difference [SID], PCO2, and weak acid buffers. Lactate is a strong anion, and causes a reduction in [SID] which causes and increase in [H+] to maintain electroneutrality. PCO2 also causes an increase in [H+]. During exercise, intramuscular [lactate] and PCO2 increase, causing an increase in [H+], and thus a decrease in pH.]
Lactic acid as a polymer precursor
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.
Lactic acid in food
Lactic acid is primarily found in sour milk products, such as: koumiss, leban, yogurt, kefir and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid.
Although it can be fermented from lactose (milk sugar), most commercially used lactic acid is derived by using bacteria such as ''Bacillus acidilacti'', ''Lactobacillus delbueckii'' or ''Lactobacillus bulgaricus'' to ferment carbohydrates from nondairy sources such as cornstarch, potatoes and molasses. Thus, although it is commonly known as "milk acid", products claiming to be vegan do sometimes feature lactic acid as an ingredient.
Lactic acid may also be found in various processed foods, usually either as a pH adjusting ingredient, or as a preservative (either as antioxidant or for control of pathogenic micro-organisms). It may also be used as a fermentation booster in rye and sourdough breads.[1]
See also
★ Cori cycle
★ Alanine cycle
References
1. "Food Applications". Galactic Div. of Finasucre. 2006. http://www.lactic.com/
External links
★ Lactic acid and running: myths, legends and reality
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