Animals, glutamate dehydrogenases

While reductive animation of glutamate via glutamate synthase appears to be the major pathway for incorporation of nitrogen into amino groups, some direct animation of pyruvate and other 2-oxoacids in reactions analogous to that of glutamate dehydrogenase occurs in bacteria.105 106 Another bacterial enzyme catalyzes reversible addition of ammonia to fumarate to form aspartate (p. 685). 

Clinical Biochemical Determinations of the Serum Serum lactic dehydrogenase (LDH) and glutamic-pyruvic transaminase (GPT) activities were measured on fresh, refrigerated serum within U8 h of sacrificing the animal. Lactic dehydrogenase was measured according to the method of Amador, Dorfman, and Wacker (12). Serum GPT activity was measured according to the method of Wroblewski and LaDue (13). 

Fig. 20 Regeneration of NAD(P)H by means of AMAPOR coupled with the reductive animation of 2-oxoglutarate to (S)-glutamate catalyzed by glutamate dehydrogenase (GluDH). MV methyl-viologen...

The effect of alcohol abuse, one of the most common aggravating factors in vitamin A toxicity, has been elucidated. Vitamin A toxicity was potentiated in patients who took 10 000 lU/day for sexual dysfunction, and this effect was attributed to excess alcohol consumption (95). In animals, potentiation of vitamin A toxicity by ethanol resulted in striking hepatic inflammation and necrosis accompanied by a rise in serum glutamate dehydrogenase and aspartate transaminase (96). 

O Brien, P. J., M. R. Slaughter, S. R. Policy, and K. Kramer. 2002. Advantages of glutamate dehydrogenase as blood biomarker of acute hepatic injury in rats. Laboratory Animals 36 313-321. 

Matsuda et al. (27) showed that the adenylosuccinate synthetase basic isozyme has a lower Km for aspartate, is more sensitive to inhibition by fructose 1,6-bisphosphate, and less sensitive to inhibition by nucleotides than the acidic isozyme. These properties could indicate that the basic isozyme is regulated coordinately with glycolysis (or gluconeogenesis) as proposed for the operation of the purine nucleotide cycle in skeletal muscle. The enzyme could also be affected by the availability of aspartate, as was found in Ehrlich ascites cells. The increase in basic isozyme activity, under conditions used in this study where the animal must rely on protein for most of its energy, is consistent with the idea that it is involved in the purine nucleotide cycle. This probably is not as an alternative to glutamate dehydrogenase in urea synthesis but is simply in amino acid catabolism. The small... 

The amino acids of animal tissue are involved in other reactions (1) oxidative deamination (2) non-oxidative decarboxylation (3) transamination (4) protein synthesis. Oxidative deamination is important only with respect to L-glutamate, which can be converted to 2-oxogJutarate and ammonia by glutamate dehydrogenase. Decarboxylation is confined to a few amino acids in animal tissue, notably glutamate, histidine, and (after hydroxylation) tryptophan and phenylalanine. In all cases, the products are potent pharmacological agents discussed under autocoid metabolism. Serine is also decarboxylated to ethanolamine, an important reaction which is referred to later in connection with transamination. 

There is a striking correlation between the form of nitrogen excretion and the pathway used by an animal to deaminate amino acids arising from proteolysis in ureoteles amino acids are transaminated with 2-oxo-glutarate to form glutamate, which in turn is attacked hy glutamate dehydrogenase, whereas in uricoteles amino acids are attack by amino acid oxidases. 

Glutamic dehydrogenase has been detected in bacteria, yeast, plants, and animal tissues. The enzyme has been purified extensively only from liver, and the properties of the ezyme from other sources are not known very precisely. It has been reported that glutamic dehydrogenase of plants requires DPN, while the enzyme of yeast and E. coli requires TPN. The mammalian enzyme uses both coenzymes. 

The concentration of ammonium ions is kept at a low level by other enzymes, also active in the liver, notably those concerned with the conversion of free ammonia to the normal waste products (urea or, uric acid) and also those concerned with the synthesis of glutamine, subsequently used in a variety of synthetic reactions. Ammonia is toxic in the central nervous system of higher animals and one fimction of the Uver is to keep the concentration in the bloodstream low. It is alleged that the main function of Uver glutamic dehydrogenase is in its involvement in the series of reactions by which excess L-amino acids are deaminated. The reactions are summarised in equations j (2-5). (See section VLB). Liver glutamic dehydrogenase is, therefore, l... 

However, it is well known that animals, with the exception of ruminants, cannot survive on ammonia as a source of nitrogen. The reason for this apparent anomaly is that although glutamic dehydrogenase is present in animals (predominantly in the Uver), the level of ammonia is too low to drive the reaction to the formation of glutamate. 

As we have just seen, L-glutamic acid is not deaminated by the action of the L-amino acid oxidase of animal tissues and bacteria. But, in the presence of a specific enzyme, glutamic dehydrogenase, it undergoes oxidative deamination in the presence of either DPN or TPN. This reversible reaction gives a-iminoglutaric acid. 

We have emphasized the specific character of macromolecules. Even in the arsenal of enzymes common to all cells we find indications of this specificity. The glucose dehydrogenase of vertebrate liver for example, is not inhibited by toluene, whilst that of E. colt is inhibited. Yeast alcohol dehydrogenase is completely inhibited by O-OOIM iodoacetate, whilst even at a concentration of O-OIM, animal alcohol dehydrogenase remains imaffected. Glutamic add dehydrogenase of yeast requires TPN as a coenzyme whilst the same enzyme from plants needs DPN. 

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