Malate dehydrogenase isoenzymes

Rocha, V., Ting, I. P. Tissue distribution of microbody, mitochondrial, and soluble malate dehydrogenase isoenzymes. Plant Physiol. 46,754-756 (1970)... 

Enzymes, measured in clinical laboratories, for which kits are available include y-glutamyl transferase (GGT), alanine transferase [9000-86-6] (ALT), aldolase, a-amylase [9000-90-2] aspartate aminotransferase [9000-97-9], creatine kinase and its isoenzymes, galactose-l-phosphate uridyl transferase, Hpase, malate dehydrogenase [9001 -64-3], 5 -nucleotidase, phosphohexose isomerase, and pymvate kinase [9001-59-6]. One example is the measurement of aspartate aminotransferase, where the reaction is followed by monitoring the loss of NADH ... 

In the malate shuttle (left)—which operates in the heart, liver, and kidneys, for example-oxaloacetic acid is reduced to malate by malate dehydrogenase (MDH, [2a]) with the help of NADH+HT In antiport for 2-oxogluta-rate, malate is transferred to the matrix, where the mitochondrial isoenzyme for MDH [2b] regenerates oxaloacetic acid and NADH+HT The latter is reoxidized by complex I of the respiratory chain, while oxaloacetic acid, for which a transporter is not available in the inner membrane, is first transaminated to aspartate by aspartate aminotransferase (AST, [3a]). Aspartate leaves the matrix again, and in the cytoplasm once again supplies oxalo-acetate for step [2a] and glutamate for return transport into the matrix [3b]. On balance, only NADH+H"" is moved from the cytoplasm into the matrix ATP is not needed for this. 

A single polypeptide chain can in theory exist in an infinite number of different conformations. However, one specific conformation generally appears to be the most stable for any given sequence of amino acids, and this conformation is assumed by the chain as it is synthesized within the cell. Thus, the primary structure of the polypeptide chain also determines its three-dimensional secondary and tertiary structures. It is conceivable that in some cases there may be several alternative conformations ("conforraers ) of a single chain that are of nearly equal stabilities and therefore these alternative forms may coexist. This possibility was first suggested to account for the heterogeneity noted in preparations of the cytoplasmic and mitochondrial isoenzymes of malate dehydrogenase and has also been proposed as an explanation of the multiple electrophoretic zones of erythrocyte acid phosphatase. However, no multiple enzyme forms have been shown unequivocally to be due to conformational isomerism. 

Product inhibition is a cause of nonlinearity of reaction progress curves during fixed-time methods of enzyme assay. For example, oxaloacetate produced by the action of aspartate aminotransferase inhibits the enzyme, particularly the mitochondrial isoenzyme. The inhibitory product may be removed as it is formed by a coupled enzymatic reaction malate dehydrogenase converts the oxaloacetate to malate and at the same time oxidizes NADH to NADL... 

Mitochondrial malate dehydrogenase (MDH) from several species has been shown to exist in several enzymically active forms which also appear to be conformational isoenzymes (Kitto et al, 1966, 1970). Kitto et al (1970) showed, in contrast to Epstein and Schechter (1968), that these MDH s were interconvertible in vitro, had the same amino acid compositions and molecular weights, but differed, once reversibly denatured, considerably in their heat stability. Similar interconversion of isoenzymes has also been observed with purified preparations of horse liver alcohol dehydrogenase (Lutstorf and von Wartburg, 1969). The question arises if such conformers are of any physiological or functional significance. It is possible that beeause of their differences in surface charge, the various conformative isoenzymes are differently bound within a cell. 

Mukerji,S.K., Ting, I. P. Malate dehydrogenase (decarboxylating) (NADP) isoenzymes of Opuntia stem tissue. Mitochondria, chloroplast, and soluble forms. Biochem. Biophys. Acta ]67,239-249 (1968 a)... 

These findings support the idea of a proteolytic mechanism for cata-bolite inactivation. The extremely high sensitivity of cytosolic malate dehydrogenase, in contrast to its mitochondrial isoenzyme, against proteolysis by the yeast proteinases A and B (7) also supports this idea. 

In the malate-aspartate shuttle (Figure 11.6), the acceptor of reducing equivalents is oxaloacetate which is reduced to malate by an isoenzyme of malate dehydrogenase specific to the cytoplasm. 

Both isoenzymes of malate dehydrogenase are NAD linked so that the shuttle in essence transfers electrons from the cytosolic pool of NADH to the mitochondrial pool of NAD". Utilization of the same coenzyme results in the direction of the shuttle being determined by the relative concentrations of NADH in each compartment. The shuttle is therefore reversible and can be used to transfer reducing power between the compartments as required. 

The main purpose of the glyoxylate cycle (Figure 12.7) which is located in glyoxysomes of plants is the synthesis of succinate from which carbohydrate may be produced. The reaction sequence utilizes organelle-specific isoenzymes of three enzymes of the tricarboxylate cycle citrate synthase, aconitate hydratase and malate dehydrogenase. These enzymes together with two enzymes... 

The fumarate produced in step [4] is converted via malate to oxaloacetate [6, 7], from which aspartate is formed again by transamination [9]. The glutamate required for reaction [9] is derived from the glutamate dehydrogenase reaction [8], which fixes the second NH4 " in an organic bond. Reactions [6] and [7] also occur in the tricarboxylic acid cycle. However, in urea formation they take place in the cytoplasm, where the appropriate isoenzymes are available. 

MDH appears in distinct forms in the cytoplasm and in the mitochondria of vertebrates (Thorne et al, 1963 Davidson and Cortner, 1967). Both isoenzymes show marked differences in their kinetic behavior. Kaplan (1963) proposed a possible metabolic significance of the two MDH s, suggesting that mitochondrial MDH oxidizes malate into oxalac-etate, whereas cytoplasmic MDH reduces oxalacetate back to malate. NADH2 set free in mitochondria by this reaction would be reoxidized in the respiratory chain and be available for oxidative phosphorylation. The two isoenzymes of NADP-isocitrate dehydrogenase (IDH) also differ in subcellular localization (Henderson, 1965). One possible explanation for the presence of a distinct form in mitochondria is that mitochondria have their own DNA and are therefore perhaps capable of coding for some mitochondrial proteins. 

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