Lactate dehydrogenase (LDH) is an animal and plant enzyme which is also present in microorganisms such as bacteria. An enzyme is a type of protein which enables a chemical reaction to occur more quickly. In humans, lactate dehydrogenase helps cells convert glucose into energy in the absence of oxygen, a process known as anaerobic respiration. During yogurt making, bacteria carry out a similar process called fermentation, where glucose is converted into energy and lactic acid. In both cases, lactate dehydrogenase promotes reactions leading to the production of lactic acid.
There are two main forms of lactate dehydrogenase enzyme created in human cells. These enzyme isoforms are known as the H form and the M form. Inside human muscles, the M isoform mainly converts pyruvic acid into lactic acid. In the heart, the H isoform of lactate dehydrogenase enables the opposite reaction to the M form. Using the heart's constant oxygen supply, it converts lactic acid to pyruvic acid.
Pyruvic acid, or pyruvate, is formed during the first part of cellular respiration, known as glycolysis. When sufficient oxygen is present, aerobic respiration then takes place, which would normally take up the pyruvic acid and produce energy for the muscles to use. In extreme exertion, such as an athlete's sudden sprint, there may not be enough oxygen for aerobic respiration and glycolysis occurs alone instead.
During glycolysis, a substance known as reduced nicotinamide adenosine dinucleotide (NADH) is produced as well as pyruvic acid. Should a lack of oxygen continue, both of these substances will accumulate, and glycolysis may not go on due to a lack of NAD+, which is required at the start of the process. Lactate dehydrogenase converts NADH and pyruvic acid into lactic acid and NAD+, allowing the cycle of glycolysis to proceed. Eventually, lactic acid builds up in muscles, causing feelings of stiffness and soreness. When sufficient oxygen becomes available, perhaps when the athlete has crossed the finish line and slows down to recover, the body is able to convert lactic acid into pyruvic acid again.
Sometimes, doctors order blood tests for lactate dehydrogenase because raised LDH levels can indicate tissue damage. If the total LDH level is too high, further tests may be performed to determine which organs are affected. Following diagnosis, LDH levels can be used to monitor the progress of a disease. Some factors which are not serious can affect LDH levels, such as taking part in strenuous exercise or incorrect handling of a blood specimen.