Ammonia uptake and metabolism

Dynamics of [, N ]Ammonia Uptake and Metabolism by the Brain as Assessed by External Imaging... 

Table III. SLinetic Constants Used in a Computer Simulation Study for the Uptake and Metabolism of Blood-Bome [ N]Ammonia by the Rat Brain"...

FIGURE 34-3 Positron emission tomography using 13NH3 showing increased brain ammonia uptake in a patient with liver cirrhosis and mild hepatic encephalopathy. CMRA, cerebral metabolic ratio for ammonia HE, hepatic encephalopathy PS, permeability/surface area product. (With permission from reference [9].)... 

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.

Ammonia arises in the body principally from the oxidative deamination of amino acids. In addition to its uptake in the reactions mentioned above, ammonia is also excreted in the urine as ammonium salts. This is not derived directly from the blood ammonia but is formed by the kidney from glutamine by the action of glutaminase. In metabolic acidosis, ammonia production and excretion by the kidney is greatly increased, and conversely it is decreased in metabolic alkalosis. This may be an important means of excreting excess ammonia. It must be remembered that ammonia formed by the action of intestinal bacteria on the protein hydrolyzates in the intestine can be also absorbed. The contribution of the ammonia formed in this way to the total ammonia in the body is unknown. Since this ammonia drains into the portal circulation, it is promptly removed by the liver. 

The key role of insuhn in protein metabolism usually is evident only in diabetic patients with persistently poor glycemic control. Insulin stimulates amino acid uptake and protein synthesis and inhibits protein degradation in muscle and other tissues. The increased conversion of amino acids to glucose also results in increased production and excretion of urea and ammonia. In addition, there are increased circulating concentrations of branched-chain amino acids as a result of increased proteolysis, decreased protein synthesis, and increased release of branched-chain amino acids from the liver. 

As a final point, it should be noted that amino acids themselves exhibit some myocardial specificity (71, 72,73) and, thus, the metabolic-ally generated labeled amino acids contribute to total heart uptake and further confuse the issue. One can readily see that this use of N-labeled ammonia as a tracer for myocardial metabolism presents a very complex situation in which determination of even relative metabolic rates is di Scult at best and absolute measurements become virtually impossible. 

With whole-cell studies, we have unfortunately been unable to elucidate the [ N]ammonia uptake mechanism. The incomplete inhibition of assimilatory processes both by mutation and by inhibitors is the major obstacle. Vahd transport studies require that the accumulated substrate be unmodified by subsequent metabolism. However, these preliminary experiments have yielded interesting conclusions about the... 

Cyanobacteria may possess several enzymes directly involved in H2 metabolism nitrogenase(s) catalyzing the production of H2 concomitantly with the reduction of nitrogen to ammonia, an uptake hydrogenase, catalyzing the consumption of H2 produced by the nitrogenase, and a bidirectional hydrogenase, which has the capacity to both take up and produce H2 [3, 5],... 

The ratio of oxygen uptake to ammonia excretion in Black Sea mussels has been found to exhibit a distinct daily rhythm (Slatina, 1986). As a rule, the ammonia coefficient (O/N) tends to increase greatly at night. As this rise in energy metabolism does not result from an enhanced locomotor activity, it is difficult to explain or to find any analogue in fish. Farbridge and Leatherland (1987) demonstrated a strong effect of the lunar cycles on amino acid uptake by the scales, also on nucleic acids, metabolic reserves and plasma thyroid hormones in coho salmon. 

Dog No. 3 was omitted because of insufficient data. It is apparent that a marked uptake of ammonia occurs in tissues in metabolic acidosis. In alkalosis the uptake by tissues is only slightly greater than normal, and the arterial levels of ammonia are comparable with normal values. The elevation of blood ammonia which occurs on vigorous acidification of the animal might not be due to a drop in permeability with pH, but might be due to a phenomenon reported by Krebs and Henseleit (K4). Urea synthesis by liver slices is directly proportional to the pH and C02 content of the medium. Table 2, constructed from data reported in this paper, shows this dependence. 

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