Lactate dehydrogenase

Lactate dehydrogenase is an enzyme found in nearly all living cells. LDH catalyzes the conversion of pyruvate to lactate and back, as it converts NAD⁺ to NADH and back. A dehydrogenase is an enzyme that transfers a hydride from one molecule to another.
LDH exists in four distinct enzyme classes. This article is specifically about the NAD-dependent L-lactate dehydrogenase. Other LDHs act on D-lactate and/or are dependent on cytochrome c: D-lactate dehydrogenase and L-lactate dehydrogenase.
LDH is expressed extensively in body tissues, such as blood cells and heart muscle. Because it is released during tissue damage, it is a marker of common injuries and disease such as heart failure.

Reaction

Lactate dehydrogenase catalyzes the interconversion of pyruvate and lactate with concomitant interconversion of the cofactors NADH and NAD⁺. It converts pyruvate, the final product of glycolysis, to lactate when oxygen is absent or in short supply, and it performs the reverse reaction during the Cori cycle in the liver. At high concentrations of lactate, the enzyme exhibits feedback inhibition, and the rate of conversion of pyruvate to lactate is decreased. It also catalyzes the dehydrogenation of 2-hydroxybutyrate, but this is a much poorer substrate than lactate.

Active site

LDH in humans uses His as the proton acceptor, and works in unison with the coenzyme, and substrate binding residues. The His active site, is not only found in the human form of LDH, but is found in many different animals, showing the convergent evolution of LDH. The two different subunits of LDH both retain the same active site and the same amino acids participating in the reaction. The noticeable difference between the two subunits that make up LDH's tertiary structure is the replacement of alanine with a glutamine. This tiny but notable change is believed to be the reason the H subunit can bind NAD faster, and the M subunit's catalytic activity isn't reduced in the presence of acetylpyridine adenine dinucleotide, whereas the H subunit's activity is reduced fivefold.

Isoenzymes

Enzymatically active lactate dehydrogenase is consisting of four subunits. The two most common subunits are the LDH-M and LDH-H peptides, named for their discovery in muscle and heart tissue, and encoded by the LDHA and LDHB genes, respectively. These two subunits can form five possible tetramers : LDH-1, LDH-5, and the three mixed tetramers. These five isoforms are enzymatically similar but show different tissue distribution.

LDH-1 —in the heart and in RBC, as well as the brain
LDH-2 —in the reticuloendothelial system
LDH-3 —in the lungs
LDH-4 —in the kidneys, placenta, and pancreas
LDH-5 —in the liver and striated muscle, also present in the brain

LDH-2 is usually the predominant form in the serum. An LDH-1 level higher than the LDH-2 level suggests myocardial infarction. The use of this phenomenon to diagnose infarction has been largely superseded by the use of Troponin I or T measurement.
There are two more mammalian LDH subunits that can be included in LDH tetramers: LDHC and LDHBx. LDHC is a testes-specific LDH protein, that is encoded by the LDHC gene. LDHBx is a peroxisome-specific LDH protein. LDHBx is the readthrough-form of LDHB. LDHBx is generated by translation of the LDHB mRNA, but the stop codon is interpreted as an amino acid-encoding codon. In consequence, translation continues to the next stop codon. This leads to the addition of seven amino acid residues to the normal LDH-H protein. The extension contains a peroxisomal targeting signal, so that LDHBx is imported into the peroxisome.

Protein families

The family also contains L-lactate dehydrogenases that catalyse the conversion of pyruvate to L-lactate, the last step in anaerobic glycolysis. Malate dehydrogenases that catalyse the interconversion of malate to oxaloacetate and participate in the citric acid cycle, and L-2-hydroxyisocaproate dehydrogenases are also members of the family. The N-terminus is a Rossmann NAD-binding fold and the C-terminus is an unusual alpha+beta fold.

Interactive pathway map

Enzyme regulation

This protein may use the morpheein model of allosteric regulation.

Ethanol-induced hypoglycemia

is dehydrogenated to acetaldehyde by alcohol dehydrogenase, and further into acetyl CoA by acetaldehyde dehydrogenase. During this reaction 2 NADH are produced. If large amounts of ethanol are present, then large amounts of NADH are produced, leading to a depletion of NAD⁺. Thus, the conversion of pyruvate to lactate is increased due to the associated regeneration of NAD⁺. Therefore, anion-gap metabolic acidosis may ensue in ethanol poisoning.
The increased NADH/NAD+ ratio also can cause hypoglycemia in an fasting individual who has been drinking and is dependent on gluconeogenesis to maintain blood glucose levels. Alanine and lactate are major gluconeogenic precursors that enter gluconeogenesis as pyruvate. The high NADH/NAD+ ratio shifts the lactate dehydrogenase equilibrium to lactate, so that less pyruvate can be formed and, therefore, gluconeogenesis is impaired.

Substrate regulation

LDH is also regulated by the relative concentrations of its substrates. LDH becomes more active under periods of extreme muscular output due to an increase in substrates for the LDH reaction. When skeletal muscles are pushed to produce high levels of power, the demand for ATP in regards to aerobic ATP supply leads to an accumulation of free ADP, AMP, and Pi. The subsequent glycolytic flux, specifically production of pyruvate, exceeds the capacity for pyruvate dehydrogenase and other shuttle enzymes to metabolize pyruvate. The flux through LDH increases in response to increased levels of pyruvate and NADH to metabolize pyruvate into lactate.

Transcriptional regulation

LDH undergoes transcriptional regulation by PGC-1α. PGC-1α regulates LDH by decreasing LDH A mRNA transcription and the enzymatic activity of pyruvate to lactate conversion.

Genetics

The M and H subunits are encoded by two different genes:

The M subunit is encoded by LDHA, located on chromosome 11p15.4.
The H subunit is encoded by LDHB, located on chromosome 12p12.2-p12.1.
A third isoform, LDHC or LDHX, is expressed only in the testis ; its gene is likely a duplicate of LDHA and is also located on the eleventh chromosome.
The fourth isoform is localized in the peroxisome. It is tetramer containing one LDHBx subunit, which is also encoded by the LDHB gene. The LDHBx protein is seven amino acids longer than the LDHB protein. This amino acid extension is generated by functional translational readthrough.

Mutations of the M subunit have been linked to the rare disease exertional myoglobinuria, and mutations of the H subunit have been described but do not appear to lead to disease.

Mutations

In rare cases, a mutation in the genes controlling the production of lactate dehydrogenase will lead to a medical condition known as lactate dehydrogenase deficiency. Depending on which gene carries the mutation, one of two types will occur: either lactate dehydrogenase-A deficiency or lactate dehydrogenase-B deficiency. Both of these conditions affect how the body breaks down sugars, primarily in certain muscle cells. Lactate dehydrogenase-A deficiency is caused by a mutation to the LDHA gene, while lactate dehydrogenase-B deficiency is caused by a mutation to the LDHB gene.
This condition is inherited in an autosomal recessive pattern, meaning that both parents must contribute a mutated gene in order for this condition to be expressed.
A complete lactate dehydrogenase enzyme consists of four protein subunits. Since the two most common subunits found in lactate dehydrogenase are encoded by the LDHA and LDHB genes, either variation of this disease causes abnormalities in many of the lactate dehydrogenase enzymes found in the body. In the case of lactate dehydrogenase-A deficiency, mutations to the LDHA gene result in the production of an abnormal lactate dehydrogenase-A subunit that cannot bind to the other subunits to form the complete enzyme. This lack of a functional subunit reduces the amount of enzyme formed, leading to an overall decrease in activity. During the anaerobic phase of glycolysis, the mutated enzyme is unable to convert pyruvate into lactate to produce the extra energy the cells need. Since this subunit has the highest concentration in the LDH enzymes found in the skeletal muscles, high-intensity physical activity will lead to an insufficient amount of energy being produced during this anaerobic phase. This in turn will cause the muscle tissue to weaken and eventually break down, a condition known as rhabdomyolysis. The process of rhabdomyolysis also releases myoglobin into the blood, which will eventually end up in the urine and cause it to become red or brown: another condition known as myoglobinuria. Some other common symptoms are exercise intolerance, which consists of fatigue, muscle pain, and cramps during exercise, and skin rashes. In severe cases, myoglobinuria can damage the kidneys and lead to life-threatening kidney failure. In order to obtain a definitive diagnosis, a muscle biopsy may be performed to confirm low or absent LDH activity. There is currently no specific treatment for this condition.
In the case of lactate dehydrogenase-B deficiency, mutations to the LDHB gene result in the production of an abnormal lactate dehydrogenase-B subunit that cannot bind to the other subunits to form the complete enzyme. As with lactate dehydrogenase-A deficiency, this mutation reduces the overall effectiveness in the enzyme. However, there are some major differences between these two cases. The first is the location where the condition manifests itself. With lactate dehydrogenase-B deficiency, the highest concentration of B subunits can be found within the cardiac muscle, or the heart. Within the heart, lactate dehydrogenase plays the role of converting lactate back into pyruvate so that the pyruvate can be used again to create more energy. With the mutated enzyme, the overall rate of this conversion is decreased. However, unlike lactate dehydrogenase-A deficiency, this mutation does not appear to cause any symptoms or health problems linked to this condition. At the present moment, it is unclear why this is the case. Affected individuals are usually discovered only when routine blood tests indicate low LDH levels present within the blood.