Lactic acid

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Lactic acid
7 Milchsaure.svg
Lactic-acid-from-xtal-3D-bs-17.png
Names
Preferred IUPAC name
2-Hydroxypropanoic acid [1]
Other names
  • Lactic acid [1]
  • Milk acid
Identifiers
3D model (JSmol)
3DMet
1720251
ChEBI
ChEMBL
ChemSpider
ECHA InfoCard 100.000.017 OOjs UI icon edit-ltr-progressive.svg
EC Number
  • 200-018-0
E number E270 (preservatives)
362717
KEGG
PubChem CID
RTECS number
  • OD2800000
UNII
UN number 3265
  • InChI=1S/C3H6O3/c1-2(4)3(5)6/h2,4H,1H3,(H,5,6)/t2-/m0/s1 Yes check.svgY
    Key: JVTAAEKCZFNVCJ-REOHCLBHSA-N Yes check.svgY
  • CC(O)C(=O)O
Properties
C3H6O3
Molar mass 90.078 g·mol−1
Melting point 18 °C (64 °F; 291 K)
Boiling point 122 °C (252 °F; 395 K) at 15 mmHg
Miscible [2]
Acidity (pKa)3.86, [3] 15.1 [4]
Thermochemistry
1361.9 kJ/mol, 325.5 kcal/mol, 15.1 kJ/g, 3.61 kcal/g
Pharmacology
G01AD01 ( WHO ) QP53AG02 ( WHO )
Hazards
GHS labelling:
GHS-pictogram-acid.svg [5]
H315, H318 [5]
P280, P305+P351+P338 [5]
Related compounds
Other anions
Lactate
Related compounds
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
X mark.svgN  verify  (what is  Yes check.svgYX mark.svgN ?)

Lactic acid is an organic acid. It has a 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 loss of a proton to produce the lactate ion CH
3
CH(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.[ citation needed ] 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. [6] 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. [6] 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. [7] [8] 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). [9] [10]

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 caries. [11] [12] [13] [14] 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.

History

Swedish chemist Carl Wilhelm Scheele was the first person to isolate lactic acid in 1780 from sour milk. [15] 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. [16] 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%. [17]

Production

Lactic acid is produced industrially by bacterial fermentation of carbohydrates, or by chemical synthesis from acetaldehyde. [18] As of 2009, lactic acid was produced predominantly (70–90%) [19] by fermentation. Production of racemic lactic acid consisting of a 1:1 mixture of D and L stereoisomers, or of mixtures with up to 99.9% L-lactic acid, is possible by microbial fermentation. Industrial scale production of D-lactic acid by fermentation is possible, but much more challenging.

Fermentative production

Fermented milk products are obtained industrially by fermentation of milk or whey by Lactobacillus bacteria: Lactobacillus acidophilus , Lacticaseibacillus casei (Lactobacillus casei), Lactobacillus delbrueckii subsp. bulgaricus (Lactobacillus bulgaricus), Lactobacillus helveticus , Lactococcus lactis , Bacillus amyloliquefaciens , and Streptococcus salivarius subsp. thermophilus (Streptococcus thermophilus).

As a starting material for industrial production of lactic acid, almost any carbohydrate source containing C
5
(Pentose sugar) and C
6
(Hexose sugar) can be used. Pure sucrose, glucose from starch, raw sugar, and beet juice are frequently used. [20] Lactic acid producing bacteria can be divided in two classes: homofermentative bacteria like Lactobacillus casei and Lactococcus lactis, producing two moles of lactate from one mole of glucose, and heterofermentative species producing one mole of lactate from one mole of glucose as well as carbon dioxide and acetic acid/ethanol. [21]

Chemical production

Racemic lactic acid is synthesized industrially by reacting acetaldehyde with hydrogen cyanide and hydrolysing the resultant lactonitrile. When hydrolysis is performed by hydrochloric acid, ammonium chloride forms as a by-product; the Japanese company Musashino is one of the last big manufacturers of lactic acid by this route. [22] Synthesis of both racemic and enantiopure lactic acids is also possible from other starting materials (vinyl acetate, glycerol, etc.) by application of catalytic procedures. [23]

Biology

Molecular biology

L-Lactic acid is the primary endogenous agonist of hydroxycarboxylic acid receptor 1 (HCA1), a Gi/o-coupled G protein-coupled receptor (GPCR). [9] [10]

Exercise and lactate

During power exercises such as sprinting, when the rate of demand for energy is high, glucose is broken down and oxidized to pyruvate, and lactate is then produced from the pyruvate faster than the body can process it, causing lactate concentrations to rise. The production of lactate is beneficial for NAD+ regeneration (pyruvate is reduced to lactate while NADH is oxidized to NAD+), which is used up in oxidation of glyceraldehyde 3-phosphate during production of pyruvate from glucose, and this ensures that energy production is maintained and exercise can continue. During intense exercise, the respiratory chain cannot keep up with the amount of hydrogen ions that join to form NADH, and cannot regenerate NAD+ quickly enough.

The resulting lactate can be used in two ways:

However, lactate is continually formed at rest and during all exercise intensities. Lactate serves as a metabolic fuel being produced and oxidatively disposed in resting and exercising muscle. Some causes of this are metabolism in red blood cells that lack mitochondria, and limitations resulting from the enzyme activity that occurs in muscle fibers having high glycolytic capacity. [24] Lactic acidosis is a physiological condition characterized by accumulation of lactate (especially L-lactate), with formation of an excessively low pH in the tissues a form of metabolic acidosis.

Lactic acidosis during exercise may occur due to the H+ from ATP hydrolysis (ATP4− + H2O → ADP3− + HPO2−
4
+ H+), and that reducing pyruvate to lactate (pyruvate + NADH + H+ → lactate + NAD+) actually consumes H+. [25] The causative factors of the increase in [H+] result from the production of lactate from a neutral molecule, increasing [H+] to maintain electroneutrality. [26] A contrary view is that lactate is produced from pyruvate, which has the same charge. It is pyruvate production from neutral glucose that generates H+:

   C6H12O6 + 2 NAD+ + 2 ADP3− + 2 HPO2−
4
2 CH
3
COCO
2
+ 2 H+ + 2 NADH + 2 ATP4− + 2 H2O
Subsequent lactate production absorbs these protons:
2 CH
3
COCO
2
+ 2 H+ + 2 NADH
2 CH
3
CH(OH)CO
2
+ 2 NAD+
Overall:
C6H12O6 + 2 NAD+ + 2 ADP3− + 2 HPO2−
4
2 CH
3
COCO
2
+ 2 H+ + 2 NADH + 2 ATP4− + 2 H2O
2 CH
3
CH(OH)CO
2
+ 2 NAD+ + 2 ATP4− + 2 H2O

Although the reaction glucose → 2 lactate + 2 H+ releases two H+ when viewed on its own, the H+ are absorbed in the production of ATP. On the other hand, the absorbed acidity is released during subsequent hydrolysis of ATP: ATP4− + H2O → ADP3− + HPO2−
4
+ H+. So once the use of ATP is included, the overall reaction is

C6H12O6 → 2 CH
3
COCO
2
+ 2 H+

The generation of CO2 during respiration also causes an increase in [H+].

Neural tissue energy source

Although glucose is usually assumed to be the main energy source for living tissues, there are a few reports that indicate that it is lactate, and not glucose, that is preferentially metabolized by neurons in the brain of several mammalian species (the notable ones being mice, rats, and humans). [27] [28] [ non-primary source needed ] According to the lactate-shuttle hypothesis, glial cells are responsible for transforming glucose into lactate, and for providing lactate to the neurons. [29] [30] Because of this local metabolic activity of glial cells, the extracellular fluid immediately surrounding neurons strongly differs in composition from the blood or cerebrospinal fluid, being much richer with lactate, as was found in microdialysis studies. [27]

Brain development metabolism

Some evidence suggests that lactate is important at early stages of development for brain metabolism in prenatal and early postnatal subjects, with lactate at these stages having higher concentrations in body liquids, and being utilized by the brain preferentially over glucose. [27] It was also hypothesized that lactate may exert a strong action over GABAergic networks in the developing brain, making them more inhibitory than it was previously assumed, [31] acting either through better support of metabolites, [27] or alterations in base intracellular pH levels, [32] [33] or both. [34]

Studies of brain slices of mice show that β-hydroxybutyrate, lactate, and pyruvate act as oxidative energy substrates, causing an increase in the NAD(P)H oxidation phase, that glucose was insufficient as an energy carrier during intense synaptic activity and, finally, that lactate can be an efficient energy substrate capable of sustaining and enhancing brain aerobic energy metabolism in vitro. [35] The study "provides novel data on biphasic NAD(P)H fluorescence transients, an important physiological response to neural activation that has been reproduced in many studies and that is believed to originate predominantly from activity-induced concentration changes to the cellular NADH pools." [36]

Lactate can also serve as an important source of energy for other organs, including the heart and liver. During physical activity, up to 60% of the heart muscle's energy turnover rate derives from lactate oxidation. [15]

Blood testing

Reference ranges for blood tests, comparing lactate content (shown in violet at center-right) to other constituents in human blood Blood values sorted by mass and molar concentration.png
Reference ranges for blood tests, comparing lactate content (shown in violet at center-right) to other constituents in human blood

Blood tests for lactate are performed to determine the status of the acid base homeostasis in the body. Blood sampling for this purpose is often arterial (even if it is more difficult than venipuncture), because lactate levels differ substantially between arterial and venous, and the arterial level is more representative for this purpose.

Reference ranges
Lower limitUpper limitUnit
Venous4.5 [37] 19.8 [37] mg/dL
0.5 [38] 2.2 [38] mmol/L
Arterial4.5 [37] 14.4 [37] mg/dL
0.5 [38] 1.6 [38] mmol/L

During childbirth, lactate levels in the fetus can be quantified by fetal scalp blood testing.

Polymer precursor

Two molecules of lactic acid can be dehydrated to the lactone lactide. In the presence of catalysts lactide polymerize to either atactic or syndiotactic polylactide (PLA), which are biodegradable polyesters. PLA is an example of a plastic that is not derived from petrochemicals.

Pharmaceutical and cosmetic applications

Lactic acid is also employed in pharmaceutical technology to produce water-soluble lactates from otherwise-insoluble active ingredients. It finds further use in topical preparations and cosmetics to adjust acidity and for its disinfectant and keratolytic properties.

Lactic acid containing bacteria have shown promise in reducing oxaluria with its descaling properties on calcium compounds. [39]

Foods

Lactic acid is found primarily in sour milk products, such as kumis, laban, yogurt, kefir, and some cottage cheeses. The casein in fermented milk is coagulated (curdled) by lactic acid. Lactic acid is also responsible for the sour flavor of sourdough bread.

In lists of nutritional information lactic acid might be included under the term "carbohydrate" (or "carbohydrate by difference") because this often includes everything other than water, protein, fat, ash, and ethanol. [40] If this is the case then the calculated food energy may use the standard 4 kilocalories (17 kJ) per gram that is often used for all carbohydrates. But in some cases lactic acid is ignored in the calculation. [41] The energy density of lactic acid is 362 kilocalories (1,510 kJ) per 100 g. [42]

Some beers (sour beer) purposely contain lactic acid, one such type being Belgian lambics. Most commonly, this is produced naturally by various strains of bacteria. These bacteria ferment sugars into acids, unlike the yeast that ferment sugar into ethanol. After cooling the wort, yeast and bacteria are allowed to “fall” into the open fermenters. Brewers of more common beer styles would ensure that no such bacteria are allowed to enter the fermenter. Other sour styles of beer include Berliner weisse, Flanders red and American wild ale. [43] [44]

In winemaking, a bacterial process, natural or controlled, is often used to convert the naturally present malic acid to lactic acid, to reduce the sharpness and for other flavor-related reasons. This malolactic fermentation is undertaken by lactic acid bacteria.

While not normally found in significant quantities in fruit, lactic acid is the primary organic acid in akebia fruit, making up 2.12% of the juice. [45]

As a food additive it is approved for use in the EU, [46] USA [47] and Australia and New Zealand; [48] it is listed by its INS number 270 or as E number E270. Lactic acid is used as a food preservative, curing agent, and flavoring agent. [49] It is an ingredient in processed foods and is used as a decontaminant during meat processing. [50] Lactic acid is produced commercially by fermentation of carbohydrates such as glucose, sucrose, or lactose, or by chemical synthesis. [49] Carbohydrate sources include corn, beets, and cane sugar. [51]

Forgery

Lactic acid has historically been used to assist with the erasure of inks from official papers to be modified during forgery. [52]

Cleaning products

Lactic acid is used in some liquid cleaners as a descaling agent for removing hard water deposits such as calcium carbonate, forming the lactate, calcium lactate. Owing to its high acidity, such deposits are eliminated very quickly, especially where boiling water is used, as in kettles. It also is gaining popularity in antibacterial dish detergents and hand soaps replacing Triclosan.

See also

Related Research Articles

<span class="mw-page-title-main">Glycolysis</span> Catabolic pathway

Glycolysis is the metabolic pathway that converts glucose into pyruvate. The free energy released in this process is used to form the high-energy molecules adenosine triphosphate (ATP) and reduced nicotinamide adenine dinucleotide (NADH). Glycolysis is a sequence of ten reactions catalyzed by enzymes.

Pyruvic acid (CH3COCOOH) is the simplest of the alpha-keto acids, with a carboxylic acid and a ketone functional group. Pyruvate, the conjugate base, CH3COCOO, is an intermediate in several metabolic pathways throughout the cell.

<span class="mw-page-title-main">Cellular respiration</span> Process to convert glucose to ATP in cells

Cellular respiration is the process by which biological fuels are oxidised in the presence of an inorganic electron acceptor such as oxygen to produce large amounts of energy, to drive the bulk production of ATP. Cellular respiration may be described as a set of metabolic reactions and processes that take place in the cells of organisms to convert chemical energy from nutrients into adenosine triphosphate (ATP), and then release waste products.

<span class="mw-page-title-main">Adenosine monophosphate deaminase deficiency type 1</span> Medical condition

Adenosine monophosphate deaminase deficiency type 1 or AMPD1, is a human metabolic disorder in which the body consistently lacks the enzyme AMP deaminase, in sufficient quantities. This may result in exercise intolerance, muscle pain and muscle cramping. The disease was formerly known as myoadenylate deaminase deficiency.

Anaerobic glycolysis is the transformation of glucose to lactate when limited amounts of oxygen (O2) are available. Anaerobic glycolysis is only an effective means of energy production during short, intense exercise, providing energy for a period ranging from 10 seconds to 2 minutes. This is much faster than aerobic metabolism. The anaerobic glycolysis (lactic acid) system is dominant from about 10–30 seconds during a maximal effort. It replenishes very quickly over this period and produces 2 ATP molecules per glucose molecule, or about 5% of glucose's energy potential (38 ATP molecules). The speed at which ATP is produced is about 100 times that of oxidative phosphorylation.

<span class="mw-page-title-main">Lactic acid fermentation</span> Metabolic process

Lactic acid fermentation is a metabolic process by which glucose or other six-carbon sugars are converted into cellular energy and the metabolite lactate, which is lactic acid in solution. It is an anaerobic fermentation reaction that occurs in some bacteria and animal cells, such as muscle cells.

Digestion is the breakdown of carbohydrates to yield an energy-rich compound called ATP. The production of ATP is achieved through the oxidation of glucose molecules. In oxidation, the electrons are stripped from a glucose molecule to reduce NAD+ and FAD. NAD+ and FAD possess a high energy potential to drive the production of ATP in the electron transport chain. ATP production occurs in the mitochondria of the cell. There are two methods of producing ATP: aerobic and anaerobic. In aerobic respiration, oxygen is required. Using oxygen increases ATP production from 4 ATP molecules to about 30 ATP molecules. In anaerobic respiration, oxygen is not required. When oxygen is absent, the generation of ATP continues through fermentation. There are two types of fermentation: alcohol fermentation and lactic acid fermentation.

Gluconeogenesis (GNG) is a metabolic pathway that results in the generation of glucose from certain non-carbohydrate carbon substrates. It is a ubiquitous process, present in plants, animals, fungi, bacteria, and other microorganisms. In vertebrates, gluconeogenesis occurs mainly in the liver and, to a lesser extent, in the cortex of the kidneys. It is one of two primary mechanisms – the other being degradation of glycogen (glycogenolysis) – used by humans and many other animals to maintain blood sugar levels, avoiding low levels (hypoglycemia). In ruminants, because dietary carbohydrates tend to be metabolized by rumen organisms, gluconeogenesis occurs regardless of fasting, low-carbohydrate diets, exercise, etc. In many other animals, the process occurs during periods of fasting, starvation, low-carbohydrate diets, or intense exercise.

<span class="mw-page-title-main">Lactic acidosis</span> Metabolic medical condition

Lactic acidosis is a medical condition characterized by a build-up of lactate in the body, with formation of an excessively low pH in the bloodstream. It is a form of metabolic acidosis, in which excessive acid accumulates due to a problem with the body's oxidative metabolism.

Carbohydrate metabolism is the whole of the biochemical processes responsible for the metabolic formation, breakdown, and interconversion of carbohydrates in living organisms.

<span class="mw-page-title-main">Cori cycle</span> Lactate degrading metabolic pathway

The Cori cycle, named after its discoverers, Carl Ferdinand Cori and Gerty Cori, is a metabolic pathway in which lactate, produced by anaerobic glycolysis in muscles, is transported to the liver and converted to glucose, which then returns to the muscles and is cyclically metabolized back to lactate.

In oncology, the Warburg effect is the observation that most cancer cells produce energy predominantly not through the 'usual' citric acid cycle and oxidative phosphorylation in the mitochondria as observed in normal cells, but through a less efficient process of 'aerobic glycolysis' consisting of high level of glucose uptake and glycolysis followed by lactic acid fermentation taking place in the cytosol, not the mitochondria, even in the presence of abundant oxygen. This observation was first published by Otto Heinrich Warburg, who was awarded the 1931 Nobel Prize in Physiology for his "discovery of the nature and mode of action of the respiratory enzyme". The precise mechanism and therapeutic implications of the Warburg effect, however, remain unclear.

<span class="mw-page-title-main">Lactic acid bacteria</span> Order of bacteria

Lactobacillales are an order of gram-positive, low-GC, acid-tolerant, generally nonsporulating, nonrespiring, either rod-shaped (bacilli) or spherical (cocci) bacteria that share common metabolic and physiological characteristics. These bacteria, usually found in decomposing plants and milk products, produce lactic acid as the major metabolic end product of carbohydrate fermentation, giving them the common name lactic acid bacteria (LAB).

<span class="mw-page-title-main">Mixed acid fermentation</span> Biochemical conversion of six-carbon sugars into acids in bacteria

In biochemistry, mixed acid fermentation is the metabolic process by which a six-carbon sugar is converted into a complex and variable mixture of acids. It is an anaerobic (non-oxygen-requiring) fermentation reaction that is common in bacteria. It is characteristic for members of the Enterobacteriaceae, a large family of Gram-negative bacteria that includes E. coli.

<span class="mw-page-title-main">Fermentation</span> Metabolic process

Fermentation is a metabolic process that produces chemical changes in organic substrates through the action of enzymes. In biochemistry, it is narrowly defined as the extraction of energy from carbohydrates in the absence of oxygen. In food production, it may more broadly refer to any process in which the activity of microorganisms brings about a desirable change to a foodstuff or beverage. The science of fermentation is known as zymology.

The Pasteur effect describes how available oxygen inhibits ethanol fermentation, driving yeast to switch toward aerobic respiration for increased generation of the energy carrier adenosine triphosphate (ATP).

Cellular waste products are formed as a by-product of cellular respiration, a series of processes and reactions that generate energy for the cell, in the form of ATP. One example of cellular respiration creating cellular waste products are aerobic respiration and anaerobic respiration.

<span class="mw-page-title-main">Lactate dehydrogenase</span> Class of enzymes

Lactate dehydrogenase (LDH or LD) is an enzyme found in nearly all living cells. LDH catalyzes the conversion of lactate to pyruvate and back, as it converts NAD+ to NADH and back. A dehydrogenase is an enzyme that transfers a hydride from one molecule to another.

Inborn errors of carbohydrate metabolism are inborn error of metabolism that affect the catabolism and anabolism of carbohydrates.

The lactate shuttle hypothesis describes the movement of lactate intracellularly and intercellularly. The hypothesis is based on the observation that lactate is formed and utilized continuously in diverse cells under both anaerobic and aerobic conditions. Further, lactate produced at sites with high rates of glycolysis and glycogenolysis can be shuttled to adjacent or remote sites including heart or skeletal muscles where the lactate can be used as a gluconeogenic precursor or substrate for oxidation. The hypothesis was proposed by professor George Brooks of the University of California at Berkeley.

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