Lactate dehydrogenase subunit composition

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. 

Lactate dehydrogenase (LDH) is an oxidoreductase that catalyzes the conversion of lactate to pyruvate. It consists of four subunits that may be of two different types M and H ( muscle and heart formerly known as A and B, respectively). Five different isoenzymes are therefore possible, depending on the subunit composition ... 

Lactate dehydrogenase occurs as a tetramer with two kinds of subunits designated H for heart and M for muscle. Five different LDHs are separable and identifiable by electrophoresis. The composition of these and their major tissue locations are as follows ... 

The amino acid compositions of a number of lactate dehydrogenases (H4, M4, and X4) are given in Table II. Chemical modification of one cysteine per subunit (122) results in loss of activity (see Section II,B). Hence, cysteine in LDH has received particular attention. No disulfide bridges have been reported for LDH (123). The enzyme does not contain any metals 53,124)-... 

Reflect and Apply The M and H subunits of lactate dehydrogenase have very similar sizes and shapes but differ in amino acid composition. If the only difference between the two were that the H subunit had a glutamic acid in a position where the M subunit had a serine, how would the five isozymes of LDH separate on electrophoresis using a gel at pH 8.6 (See Chapter 5 for details on electrophoresis.)... 

A typical spectrum of individual lactate dehydrogenases (LDH) of muscles after electrophoresis on starch gel is shown in Fig. 81 (Markert and Appella, 1963). These five fractions, as mentioned previously, are determined by (he existence of two genetic loci for synthesis of two types of protein subunits, specific as regards amino acid composition and antigenicity, designated A and B, and by the presence of a tetramer structure of the principal protein (AAAA-LDH-5 AAAB-LDH-4 AABB-LDH-3 ABBB-LDH-2 BBBB-LDH-1). 

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