Vitamin aminotransferase

Most amino acids lose their nitrogen atom by a transamination reaction in which the -NH2 group of the amino acid changes places with the keto group of ct-ketoglutarate. The products are a new a-keto acid plus glutamate. The overall process occurs in two parts, is catalyzed by aminotransferase enzymes, and involves participation of the coenzyme pyridoxal phosphate (PLP), a derivative of pyridoxine (vitamin UJ. Different aminotransferases differ in their specificity for amino acids, but the mechanism remains the same. 

Vitamin Bg Status Is Assessed by Assaying Erythrocyte Aminotransferases... 

The most widely used method of assessing vitamin Bg status is by the activation of erythrocyte aminotransferases by pyridoxal phosphate added in vitro, expressed as the activation coefficient. 

In the bile-duct-ligated rat, hepatic mitochondrial lipid peroxides are increased and correlate with serum levels of alkaline phosphatase, bilirubin and alanine aminotransferase (Sokol et al., 1991). Dietary vitamin E deficiency resulted in relatively higher lipid peroxide and bilirubin... 

A group of enzymes which is particularly important in amino acid metabolism in the liver (and also in muscle) is the transaminases, (also called aminotransferases). These are vitamin B6 (pyridoxine) dependent enzymes which transfer an amino group from an amino acid to an oxo (keto) acid, thus ... 

Both muscle and liver have aminotransferases, which, unlike deaminases, do not release the amino groups as free ammonium ion. This class of enzymes transfers the amino group from one carbon skeleton (an amino acid) to another (usually a-ketoglutarate, a citric acid cycle intermediate). Pyridoxal phosphate (PLP) derived from vitamin is required to mediate the transfer. 

This vitamin Be-dependent enzyme [EC 2.6.1.14], also referred to as asparagine-oxo-acid aminotransferase, catalyzes the reversible reaction of asparagine and a 2-0X0 acid to yield 2-oxosuccinamate and an amino acid. 

L-Canaline is an ineffective antimetabolite of L-ornithine since it has little ability to antagonize ornithine-dependent reactions. On the other hand, it forms a covalently bound Schiff-base complex with the pyridoxal phosphate moiety of Bg-containing enzymes. As such it is a potent inhibitor of many decarboxylases and aminotransferases that utilize this vitamin. 

All aminotransferases have the same prosthetic group and the same reaction mechanism. The prosthetic group is pyridoxal phosphate (PLP), the coenzyme form of pyridoxine, or vitamin B6. We encountered pyridoxal phosphate in Chapter 15, as a coenzyme in the glycogen phosphorylase reaction, but its role in that reaction is not representative of its usual coenzyme function. Its primary role in cells is in the metabolism of molecules with amino groups. 

Pyridoxamine pyruvate aminotransferase, stereochemistry of 748 Pyridoxine (vitamin B6) 305s, 738... 

Transamination, the process whereby ammonia is reversibly transferred between amino acids and 2-oxoacids, is catalyzed by aminotransferases, which bind pyridoxal phosphate as a prosthetic group. Pyridoxal phosphate and pyridoxamine phosphate are the coenzyme forms of vitamin B6 (Fig. 15-1). 

Figure 9.4. Tryptophan load test for vitamin Be status. Tryptophan dioxygenase, EC 1.13.11.11 formylkynurenine formamidase, EC 3.5.1.9 kynurenine hydroxylase, EC 1.14.13.9 kynureninase, EC 3.7.1.3 kynurenine oxoglutarate aminotransferase, EC 2.6.1.7 andkynurenine glyoxylate aminotransferase, 2.6.1.63. Relative molecular masses (Mr) tryptophan, 204.2 kynurenine, 208.2 3-hydroxykynurenine, 223.2 kynurenic acid, 189.2 and xanthurenic acid, 205.2.

As shown in Table 9.5, there are a number of indices of vitamin Be status available plasma concentrations of the vitamin, urinary excretion of 4-pyridoxic acid, activation of erythrocyte aminotransferases by pyridoxal phosphate added in vitro, and the ability to metabolize test doses of tryptophan and methionine. None is wholly satisfactory and where more than one index has been used in population studies, there is poor agreement between the different methods (Bender, 1989b Bates et al., 1999a). 

Fasella P and Turano C (1970) Structure and catalytic role of the functional groups of aspartate aminotransferase. Vitamins and Hormones 157-94. 

All aminotransferases contain the prosthetic group pyridoxal phosphate (PLP), which is derived frompyridoxine (vitamin B ). Pyridoxal phosphate includes a pyridine ring that is slightly basic as well as a phenolic hydroxyl group... 

The conversion of amino acids to keto acids is usually catalyzed by enzymes called aiuinotransferases. Some aminotransferases can recognize a variety of different amino adds as substrates others are more specific in their action. Aminotransferases use vitamin as a cofactor, and the in the figures indicates that the enzyme is a vitamin B -requiring enzyme. Cofactors are small molecules that bind to specific enzymes and participate in the chemistry of cataly.sis. Some... 

Several tests are available for asses ing vitamin B status measurement of plasma levels of PLP, measurement of the percentage stimulation of red blood cell glula-matc-oxaloacetate aminotransferase, measurement of the daily excretion of urinary pyridoxic acid, and the tryptophan load test. 

The basal activity of aminotransferase fell by 50% during consumption of the Bt-deficient diet. The stimulation occurring with addition of PLP to the enzyme incubation mixtures rose in this period from 200% (twofold stimulation) to 400% (fourfold). The basal activity of the aminotransferase, and probably of ail enzymes of the body, varies from subject to subject. It may even vary with repeated enzyme assays using the same sample of red blood cells. Thus, the basal activity is not used to assess vitamin Be, status. The percentage stimulation is relatively constant in normal subjects and is thus a more useful indicator of B status. It should be noted that the storaffe of ivti blood teiis can lead to gradual release of the cofactor from the enzyme thus, an artefactual diagnosis of Bh deficiency is possible. 

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. 

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