Heart disease lactate dehydrogenase isoenzymes

These data give the clinician an accurate picture of the nature of the diseased tissue. For a heart attack victim, only creatine phosphokinase isoenzyme 2 (CPK II, the predominant heart isoenzyme) will be elevated in the three days following the heart attack. CPK I (brain) and CPK III (skeletal muscle) levels will remain unchanged. Similarly, only LDH 1, the lactate dehydrogenase isoenzyme made in heart muscle, will be elevated. The levels of LDH 2-5 will remain within normal values. 

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. 

Determination of LDH Lactate dehydrogenase (LDH) catalyzes the equilibrium reaction of pyruvate to lactate. The activity of serum LDH is due to the presence of the enzyme released from damaged organs and tissues such as liver, heart, skeletal muscle, erythrocytes, etc. because LDH is located in the cytoplasm of the cells. Therefore, the activity of LDH is useful for screening for the existence of cell injuries, estimation of damaged tissues, and evaluation of treatment of diseases. LDH has five isoenzymes, and their patterns are of diagnostic importance. 

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