Glutamate: oxaloacetate aminotransferase

Serum aminotransferases such as serum glutamate-oxaloacetate-aminotransferase (SCOT) have been used as clinical markers of tissue damage, with increasing serum levels indicating an increased extent of damage. 

This chapter focuses initially on the catabolism of the amino acids. Aminotransferases can be used to catalyse the first step in the breakdown of nearly all of the amino acids. Lysine catabolism, in contrast, does not begin with an aminotrans-feraseamino acids can be catabolized via more than one pathway. Glutamate catabolism, for example, can begin by reactions catalyzed by glutamate oxaloacetate aminotransferase or glutamate dehydrogenase. 

PLP is the cofactor for a large number of enzymes used in the metabolism of amino acids and related compounds. Some of these enzymes are listed in Table 9.3. In the aminotransferases, the cofaefor form shifts between PLP and PME In glutamate-oxaloacetate aminotransferase, for example, glutamate reacts with the enzyme bound cofactor and is converted to ot-ketoglutarate. Its amino group remains bound to the cofactor, which is changed to the pyridoxamine phosphate form ... 

Glutamate-oxaloacetate aminotransferase Interconversion of aspartate and oxaloacetate... 

The glutamate-oxaloacetate aminotransferase stimulation test involves the reconstitution of PLP with the apoenzyme. The enzyme activity in broken red blood cells is measured with and without PLP added. Addition of PLP would be expected to result in little or no stimulation of enzyme activity if the subject had been consuming a Bg-adequate diet, whereas an increase in enzyme activity with the addition of pure PLP to the enzyme assay mixtures would indicate that the subject had been consuming a Bg-deficient diet. Consumption of a Bg-deficient diet allows continued synthesis of the apoenzyme in the cell, but not conversion of the apoenzyme to the holoenzyme. A marked increase occurring with the addition of PLP could indicate that the subject had been consuming a Bg-deficient diet or that absorption of dietary vitamin Bg was impaired or defective in some way. 

Some studies suggest that the principal pathway of glutamate utilization in liver mitochondria is by transamination (81). GDH decreases the distribution coefficient of glutamate-oxaloacetate aminotransferase on Sephadex G-20Q, possibly by forming a complex with that enzyme (82). In addition, in the presence of NADPH and NHC, GDH appears to catalyze the conversion of the pyridoxal phosphate form of the aminotransferase to the pyridoxamine form, which catalyzes the formation of a-amino acids from a-keto acids (83). This reaction is interesting in view of the inhibition of GDH by pyridoxal phosphate (54) (See Section V,A). If the complex exists in mitochondria, it may provide an efficient mode of dehydrogenation of amino acids that are not normally good substrates of GDH (82). 

On the other hand, there is considerable positive evidence for PLP involvement in plant transaminases. Cruickshank and Isherwood (1958) found that partially purified wheat germ glutamate oxaloacetate aminotransferase was stimulated 50-80% by PLP whereas glutamaterpyruvate aminotransferase was not. However in mung bean mitochondria the reverse was observed (Bone and Fowden, 1960). Cauliflower glutamate oxaloacetate aminotransferase showed no PLP activation in crude extracts, but a two- to three-fold stimulation of activity was seen after ammonium sulfate fractionation. Pyridoxamine-P activated the latter enzyme much more slowly than PLP (Davies and Ellis, 1961). 

Gel electrophoresis evidence suggested that different aspartate aminotransferase isoenzymes were present in chloroplasts and mitochondria from spinach leaves. In addition, two electrophoretically distinct forms of aspartate aminotransferase were seen in the peroxisomal fraction (Yamazaki and Tolbert, 1970). Kanamori and Matsumoto (1974) found two isoenzymes of glutamate oxaloacetate aminotransferase in roots of rice seedlings, whereas there were three isoenzymes in the shoots. However, in the roots, where the soluble and mitochondrial enzymes both consisted of multiple forms, the electrophoretic pattern was similar for both, i.e., the electrophoretic forms were not organelle specific. Other workers concluded that extensively purified mitochondrial and soluble alanine aminotransferases in tomato fruits were the same protein (Gazeu-Reyjal and Crouzet, 1976) and that there were not organelle-specific isoenzymes. 

Glutamate-oxaloacetate aminotransferase (GOT) and GPT, possible aliphatic aminotransferases in the amination of 5-keto-octanal instead of L-alanine 5-keto-octanal aminotransferase, were excluded from that function by further work which separated the activities of the former from that of the latter, in mm shown to be composed of two isozymes, although no individual function was described. 

FIGURE 14.22 Glutamate aspartate aminotransferase, an enzyme conforming to a double-displacement bisnbstrate mechanism. Glutamate aspartate aminotransferase is a pyridoxal phosphate-dependent enzyme. The pyridoxal serves as the —NH, acceptor from glntamate to form pyridoxamine. Pyridoxamine is then the amino donor to oxaloacetate to form asparate and regenerate the pyridoxal coenzyme form. (The pyridoxamine enzyme is the E form.)... 

EC2.6.1.1 L-aspartate aminotransferase (glutamate oxaloacetate transaminase) (aminotransferase (transaminase))... 

ALT = alanine aminotransferase (newer name) GPT - glutamate-pyruvate transaminase (older name) AST = aspaitate aminotransferase (newer name) GOT = glutamate-oxaloacetate transaminase (older name)... 

Aminotransferases have received many names as fashions in nomenclature have changed. Two obsolete names are still used in chnical practice glutamate-oxaloacetate transaminase (abbreviated to GOT) is now aspartate aminotransferase, and glutamate-pyruvate transaminase (GPT) is now alanine aminotransferase. The new abbreviations are AST and ALT, respectively. 

There are numerous transminases, each specific to a given substrate pair. Some may be primarily mitochondrial others, cytosolic. For example, glutamate-oxaloacetate transminase (GOT), also called aspartate aminotransferase (AST), is primarily a mitochondrial enzyme. AST is extensively used in the diagnosis of heart and liver disorders (see Chapter 5). The AST reaction is represented by Equation (20.7). 

Guanosine monophosphate Glutamate-oxaloacetate transaminase (aspartate aminotransferase)... 

E-11) SCOT (serum glutamate oxaloacetate transaminase), more recently called AST (aspartate aminotransferase), acts at this step. Both names make sense, depending on which way you read the chemical reaction. The enzyme is found in many areas of the body, but Is most useful as a marker of hver or cardiac injury. It leaks out of the damaged cell and increases in the smim after myocardial infarction and liver injury (for instance, hepatitis) and may pros ids clua as to the existence cf theeo coiuhtions. 

Aspartate transaminase (AST) (also known as glutamic oxaloacetic transaminase, or GOT), the most active of the aminotransferases, is found in most cells. Because AST isozymes occur in both mitochondria and the cytoplasm and the reaction that it catalyzes is reversible, this enzymatic activity significantly influences the flow of carbon and nitrogen within the cell. For example, excess glutamate is converted via AST to aspartate. Aspartate is then used as a source of both nitrogen (for... 

GLDH = glutamate dehydrogenase MDH = malate dehydrogenase SDH = succinic dehydrogenase GOT = glutamic oxaloacetic transaminase (aspartate aminotransferase)... 

Abbreviations ALT, alanine aminotransferase AST, aspartate aminotransferase CYP, cytochrome P GST, glutathione-L-transferase SGOT, serum glutamic oxaloacetic transaminase SGPT, serum glutamic pyruvic transaminase. 

Myocardial infarction occurs when the blood supply to the heart muscle is blocked for an extended time. If this lack of blood supply, called ischemia, is prolonged, the myocardium suffers irreversible cell damage and muscle death, or infarction. When this happens, the concentration of cardiac enzymes in the blood rises dramatically as the dead cells release their contents into the bloodstream. Although many enzymes are liberated, three are of prime importance. These three enzymes, creatine phosphokinase (CPK), lactate dehydrogenase (LDH), and aspartate aminotransferase/serum glutamate-oxaloacetate transaminase (AST/SGOT), show a characteristic sequential rise in blood serum level following myocardial infarction and then return to normal. This enzyme profile, shown in the ac-... 

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