Lactic acid fermentation

Lactic acid fermentation is a form of fermentation that occurs in animal cells in the absence of oxygen. Under these conditions, Glycolysis occurs normally, producing 2 molecules of ATP, 2 molecules of NADH and 2 molecules of pyruvate. However, the lack of O₂ prevents the NADH from being recycled to NAD⁺ (which is required for glycolyis) through the electron transport chain. Instead, it transfers electrons to pyruvate:

pyruvate + NADH $\longrightarrow$ lactate + NAD⁺

Lactate then diffuses out of the cell and into the blood. Certain cells, such as cardiac muscle cells, are highly permeable to lactate. Lactate is converted into pyruvate and metabolised normally (ie: via the citric acid cycle). Since these cells are highly oxygenated, it is unlikely that lactate would accumulate (as is the case in oxygen-starved muscle cells). This also allows circulating glucose to be available to muscle cells.

Any excess lactate is taken up by the liver, converted to pyruvate and then to glucose. This, along with the production of lactate from glucose in muscle cells constitutes the Cori cycle.

Phosphofructokinase (PFK), which catalyses an irreversible step in glycolysis, is inhibited by a low pH and this prevents the formation of excess lactate and/or lactic acidosis (sudden drop in blood pH). PFK

Lactic fermentation is much faster than the Krebs cycle or oxidative phophorylation, and is therefore the preferred pathway for energy generation in muscle cells under strenuous exercise.

Muscle cramps have been associated with levels of lactic acid in the blood, especially as a result of electrolyte imbalance caused by sweating. However, certain activities, which do not induce sweating or seem strenuous, still produce cramps (such as writing or playing an instrument) so it is unlikely that lactic acid is anything more than a minor factor.