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Lactic Acid (CAS: 50-21-5)
Colorless transparent viscous liquid
Chloride % ≤
Residue on ignition %≤
Heavy metals (Pb) %≤
Thermal stability ≥
Lactic acid is a chemical compound that plays a role in various biochemical processes and was first isolated in 1780 by the Swedish chemist Carl Wilhelm Scheele. Lactic acid is a carboxylic acid with the chemical formula 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 carboxyl group, producing the lactate ion CH3CH(OH)COO−. Compared to acetic acid, its pKa is 1 unit less, meaning lactic acid deprotonates ten times more easily than acetic acid does. This higher acidity is the consequence of the intramolecular hydrogen bridge between the α-hydroxyl and the carboxylate group, making the latter less capable of strongly attracting its proton.
Lactic acid is miscible with water and with 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.
In animals, L-lactate is constantly produced from pyruvate via the enzyme lactate dehydrogenase (LDH) in a process of fermentation during normalmetabolism 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.
In industry, lactic acid fermentation is performed by lactic acid bacteria, which convert glucose and sucrose to lactic acid. These bacteria can also grow 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 lactated Ringer's solution and Hartmann's solution. These intravenous fluids consist ofsodium and potassium cations along with lactate and chloride anions in solution with distilled water, generally in concentrations isotonic withhuman blood. It is most commonly used for fluid resuscitation after blood loss due to trauma, surgery, or burn injury.
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 produced from the pyruvate faster than the tissues can remove it, so lactate concentration begins to rise. The production of lactate is a beneficial process because it regenerates NAD+, which is used up in oxidation of 3-phosphoglyceraldehyde during creation 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 atoms that join to form NADH, and cannot regenerate NAD+ quickly enough)
The increased lactate produced can be removed in two ways:
Oxidation back to pyruvate by well-oxygenated muscle cells
Pyruvate is then directly used to fuel the Krebs cycle
Conversion to glucose via gluconeogenesis in the liver and release back into circulation; see Cori cycle
If blood glucose concentrations are high, the glucose can be used to build up the liver's glycogen stores.
C6H12O6 + 2 NAD+ + 2 ADP−3 + 2 HPO4−2 → 2 CH3COCOO− + 2 H+ + 2 NADH + 2 ATP−4 + 2 H2O → 2 CH3CHOHCOO− + 2 NAD+ + 2 ATP−4 + 2 H2O
However, lactate is continually formed even at rest and during moderate exercise. This occurs due to metabolism in red blood cells that lack mitochondria, and limitations resulting from the enzyme activity that occurs in muscle fibers having a high glycolytic capacity.
Strenuous anaerobic exercise causes a lowering of pH and soreness, called acidosis.
The effect of lactate production on acidosis has been the topic of many recent conferences in the field of exercise physiology. Robergs et al. have discussed the creation of H+ during exercise, and claim the idea that lactic acid causes acidosis is a "construct" or myth, pointing out that part of the H+ comes from ATP hydrolysis (ATP−4 + H2O → ADP−3 + HPO4−2 + H+), and that reducing pyruvate to lactate (pyruvate- + NADH + H+ → lactate- + NAD+) actually consumes H+. Lindinger et al. claimed that Robergs et al. ignored the causative factors of the increase in [H+]. The production of lactate- from a neutral molecule would increase [H+] to maintain electroneutrality. However, lactate- is produced from pyruvate-, which has the same charge: "Lactate− production is not associated with a stoichiometrically equivalent net production of protons (H+)".
It is pyruvate- production from neutral glucose that generates H+: C6H12O6 + 2 NAD+ + 2 ADP−3 + 2 HPO4−2 → 2 CH3COCOO− + 2 H+ + 2 NADH + 2 ATP−4+ 2 H2O
Subsequent lactate- production absorbs these protons: 4 CH3COCOO− + 2 H+ + 2 NADH + 2 ATP−4 + 2 H2O → 2 CH3CHOHCOO− + 2 NAD+ + 2 ATP−4 + 2 H2O
Although the reaction glucose → 2 lactate- + 2 H+ releases two H+ when viewed on its own, the H+ are absorbed in the creation of ATP. The absorbed acidity is released during subsequent hydrolysis of ATP: ATP−4 + H2O → ADP−3 + HPO4−2 + H+. Overall, [H+] does increase.
The creation of CO2 during respiration also causes an increase in [H+].
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 andcosmetics to adjust acidity and for its disinfectant and keratolytic properties.
Category: Food Additive Chemicals