Muscle skeletal, lactate dehydrogenase

Figure 8.10 The quaternary structure of proteins. The enzyme lactate dehydrogenase (EC 1.1.1.27) has a relative molecular mass of approximately 140 000 and occurs as a tetramer produced by the association of two different globular proteins (A and B), a characteristic that results in five different hybrid forms of the active enzyme. The A and B peptides are enzymically inactive and are often indicated by M (muscle) and H (heart). The A4 tetramer predominates in skeletal muscle while the B4 form predominates in heart muscle but all tissues show most types in varying amounts.

Reasons for the presence of enzymes in the plasma Enzymes can normally be found in the plasma either because they were specifically secreted to fulfill a function in the blood, or because they were released by dead or damaged cells. Many diseases that cause tissue damage result in an increased release of intracellular enzymes into the plasma. The activities of many of these enzymes (for example, creatine kinase, lactate dehydrogenase, and alanine aminotransferase) are routinely determined for diagnostic purposes in diseases of the heart, liver, skeletal muscle, and other tissues. 

Elevated pyruvate concentration inhibits the heart muscle lactate dehydrogenase (LDH) isoenzyme but not the skeletal muscle LDH isoenzyme. 

The structure of an enzyme can also vary within a person, since different genes may encode enzymes that catalyse the same reaction. These enzymes are known as isozymes. Isozymes are often specific for different types of tissue. For example, lactate dehydrogenase (LDH) is produced in two forms, the M-type (muscle) and the H-type (heart). The M-type is predominates in tissue subject to anaerobic conditions, such as skeletal muscle and liver tissue, whereas the H-type predominates in tissue under aerobic conditions, such as the heart. Isozymes may be used as a diagnostic aid. For example, the presence of H-type LDH in the blood indicates a heart attack, since heart attacks cause the death of heart muscle with the subsequent release of H-type LDH into the circulatory system. 

An example of an enzyme which has different isoenzyme forms is lactate dehydrogenase (LDH) which catalyzes the reversible conversion of pyruvate into lactate in the presence of the coenzyme NADH (see above). LDH is a tetramer of two different types of subunits, called H and M, which have small differences in amino acid sequence. The two subunits can combine randomly with each other, forming five isoenzymes that have the compositions H4, H3M, H2M2, HM3 and M4. The five isoenzymes can be resolved electrophoretically (see Topic B8). M subunits predominate in skeletal muscle and liver, whereas H subunits predominate in the heart. H4 and H3M isoenzymes are found predominantly in the heart and red blood cells H2M2 is found predominantly in the brain and kidney while HM3 and M4 are found predominantly in the liver and skeletal muscle. Thus, the isoenzyme pattern is characteristic of a particular tissue, a factor which is of immense diagnostic importance in medicine. Myocardial infarction, infectious hepatitis and muscle diseases involve cell death of the affected tissue, with release of the cell contents into the blood. As LDH is a soluble, cytosolic protein it is readily released in these conditions. Under normal circumstances there is little LDH in the blood. Therefore the pattern of LDH isoenzymes in the blood is indicative of the tissue that released the isoenzymes and so can be used to diagnose a condition, such as a myocardial infarction, and to monitor the progress of treatment. 

D-7) Serum LDH (lactate dehydrogenase) eleva- tion. LDH is a widespread intracellular enzyme that acts at this step. Detection of high serum levels is rather nonspecific for localizing the site of damage, but its measurement is helpful in confirming myocardial infarction or injury to the liver, skeletal muscle, or certain other tissues. The presence of 5 different isozymes of LDH helps to further localize the injury, as they are fairly tissue specific. For instance, LDj and LD2 are elevated in myocardial infarction, LD2 and LD3 elevation occur in acute leukemia LDs elevation follows liver or skeletal muscle injury. 

Likewise, following the repeated infusion of doses of 1000-2000 mg/kg DCLHb daily for 7 days or of 400 mg/kg every 6h for 3 days, the concentrations of AST, lactate dehydrogenase (LDH) and creatine kinase (CK) were elevated in monkeys. Isoenzyme profiles for CK and LDH revealed predominant increases in the MM form of CK and the LD-5 form of LDH. The MM-CK originates predominantly from skeletal muscle and may also derive from the myocardium however, the MB isoenzyme, which emanates only from myocardium, was not elevated. The elevation of LD-5 was also consistent with a skeletal muscle source. 

There are five enzymes that are commonly used in diagnosis of liver disease Aspartate aminotransferase (AST EC 2.6.1.1), alanine aminotransferase (ALT EC 2.6.1.2), alkaline phosphatase (ALP 3.1.3.1), and y-glutamyl transferase (GGT EC 2.3.2.2), are commonly used to detect liver injury, and lactate dehydrogenase (LD EC 1.1.1.27) is occasionaEy used. ALT and GGT are present in several tissues, but plasma activities primarily reflect liver injury. AST is found in liver, muscle (cardiac and skeletal), and to a liipited extent iti fed cells. LD has wide tissue distribution, and is thus relatively nonspecific. ALP is found in a number of tissues, but in normal individuals primarEy reflects bone and liver sources. Thus based on tissue distribution, ALT and GGT would seem to be the most specific markers for liver injury. 

BIO. Brody, I. A., The significance of lactate dehydrogenase isoenzymes in abnormal human skeletal muscle. Neurology 14, 1091-1100 (1964). 

H17. Hultin, H. O., and Westort, C., Factors affecting the distribution of lactate dehydrogenase between particulate and non-particulate fractions of homogenized skeletal muscle. Arch. Biochem. Biophys. 117, 523-533 (1966). 

Lactate dehydrogenase Skeletal and heart muscle, liver, kidney, erythrocytes, pancreas, lungs... 

The M4 isozyme functions optimally in the anaerobic environment of hard-working skeletal muscle, whereas the H4 isozyme does so in the aerobic environment of heart muscle. Indeed, the proportions of these isozymes are altered in the development of the rat heart as the tissue switches from an anaerobic environment to an aerobic one (Figure 10.16A). Figure 10.16B shows the tissue-specific forms of lactate dehydrogenase in adult rat tissues. 

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