Myocardial infarction, lactate dehydrogenase isoenzymes

Jaffe AS, Landt Y, Parvin CA,et al. Comparative sensitivity of cardiac troponin I and lactate dehydrogenase isoenzymes for diagnosing acute myocardial infarction. Clin Chem 42 1770-1776,1996. 

ST90 Jensen, A.E., Reikvam, A., Nordgird, S., Pedersen, K.G. and Asberg, A. (1990). Diagnostic accuracy of Kodak creatine kinase MB, Stratus creatine kinase MB, and lactate dehydrogenase isoenzyme 1 in serum after acute myocardial infarction. Clin. Chem. 36, 1847-1848. 

Isoenzymes Isoenzymes are different forms of an enzyme which catalyze the same reaction, but which exhibit different physical or kinetic properties. The isoenzymes of lactate dehydrogenase (LDH) can be separated electrophoretically and can be used clinically to diagnose a myocardial infarction. 

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. 

A profile of the serum isoenzymes of lactate dehydrogenase, (a) The pattern of LDH isoenzymes from a normal individual, (b) The pattern of LDH isoenzymes of an individual suffering from a myocardial infarction. 

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