Glutamate dehydrogenase nucleotides

Figure 8.29 The initial reactions of glutamine metabolism in kidney, intestine and cells of the immune system. The initial reaction in all these tissues is the same, glutamine conversion to glutamate catalysed by glutaminase the next reactions are different depending on the function of the tissue or organ. In the kidney, glutamate dehydrogenase produces ammonia to buffer protons. In the intestine, the transamination produces alanine for release and then uptake and formation of glucose in the liver. In the immune cells, transamination produces aspartate which is essential for synthesis of pyrimidine nucleotides required for DNA synthesis otherwise it is released into the blood to be removed by the enterocytes in the small intestine or by cells in the liver.

Somewhat surprisingly, within the mitochondria the ratio [NAD+]/[NADH] is 100 times lower than in the cytoplasm. Even though mitochondria are the site of oxidation of NADH to NAD+, the intense catabolic activity occurring in the (3 oxidation pathway and the citric acid cycle ensure extremely rapid production of NADH. Furthermore, the reduction state of NAD is apparently buffered by the low potential of the (3-hydroxybutyrate-acetoacetate couple (Chapter 18, Section C,2). Mitochondrial pyridine nucleotides also appear to be at equilibrium with glutamate dehydrogenase.169... 

Ammonia is produced by oxidative and nonoxidative deaminations catalyzed by glutaminase and glutamate dehydrogenase (Chapter 17). Ammonia is also released in the purine nucleotide cycle. This cycle is prominent in skeletal muscle and kidney. Aspartate formed via transamination donates its a-amino group in the formation of AMP the amino group is released as ammonia by the formation of IMP. 

A related fluorescent nucleotide affinity label, 5 -p-fluorosulfonylbenzoyl-2-aza-1,7V -ethenoadenosine (5 -FSBaeA), in which a nitrogen replaces the carbon atom at the 2-position of 5 -FSBeA, has also been prepared (205, 206). 5 -FSBaeA has different spectral properties, with an emission peak at 490 nm and an excitation maximum at 356 nm. Thus, it can be used in a complementary manner to 5 -FSBeA as a covalently bound chromophore. 5 -FSBaeA reacts at a GTP regulatory site of glutamate dehydrogenase, and energy transfer experiments have led to an estimated distance between the catalytic and GTP sites in... 

Fig. 38.5. Summary of the sources of NH4 for the urea cycle. All of the reactions are irreversible except glutamate dehydrogenase (GDH). Only the dehydratase reactions, which produce NH4 from serine and threonine, require pyridoxal phosphate as a cofactor. The reactions that are not shown occurring in the muscle or the gut can all occur in the liver, where the NH4 generated can be converted to urea. The purine nucleotide cycle of the brain and muscle is further described in Chapter 41.

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... 

Amlnation introduction of an amino group ( NH2) into an organic compound. This may be accomplished by reductive A. or by Transamination (see). Reductive A. requires a reduced pyridine nucleotide as a reducing agent, e.g. the most common reductive A. of 2-oxoglutarate by L-glutamate dehydrogenase (EC 1.4.1.4) requires NADPH. 

The third reaction, the oxidation of cysteine sulfinate, proceeds differently in Proteus vulgaris and in animal tissues. As shown by Singer and Kearney 18,20) the Proteus enzyme system oxidizes it directly to cysteate. This oxidation requires a pyridine nucleotide coenzyme which appears to be closely related to DPN+ 23). On the other hand, in rat liver mitochondria a soluble enzyme system oxidizes L-cysteine sulfinate in the presence of DPN+ to j8-sulfinylpyruvate and NH3 20). This reaction is analogous to that catalyzed by glutamic dehydrogenase [see Eq. (7)]. There is some indication that kidney n-amino acid oxidase can also oxidize cysteine sulfinate 24). Among these pathways of cysteine sulfinate metabolism the transaminative one is of the greatest importance. 

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