Glutamine liver transport

C. Ammonia is converted to a nontoxic form, mainly glutamine, for transport to the liver for further processing. 

Ammonia, produced in animal cells as a result of catabolism, must be transported to the liver for its eventual conversion to urea. Figure 20.14 shows schematically how amino acids are employed to carry ammonia in the form of amino groups to the liver where it is eventually converted to urea. In most tissues, the toxic ammonia is converted to the nontoxic, and electrically neutral, glutamine. Glutamine is transport in the blood to the liver where it is cleaved hydrolytically by glutaminase in the following reaction ... 

D. Role of Alanine and Glutamine in Transporting Amino Acid Nitrogen to the Liver... 

The proteins are broken down in a discrete fashion that remains unknown, but certainly does not involve lysosomal enzyme release or even lysosome formation in the muscle. The amino acids are freed, most are reutilized for protein synthesis, but alanine which constitutes only 5% of the muscle amino acid component and to a smaller extent glutamine are transported from muscle to liver where the amino acids are used in the process of gluconeogenesis. 

Tissue electrodes [2, 3, 4, 5, 45,57], In these biosensors, a thin layer of tissue is attached to the internal sensor. The enzymic reactions taking place in the tissue liberate products sensed by the internal sensor. In the glutamine electrode [5, 45], a thick layer (about 0.05 mm) of porcine liver is used and in the adenosine-5 -monophosphate electrode [4], a layer of rabbit muscle tissue. In both cases, the ammonia gas probe is the indicator electrode. Various types of enzyme, bacterial and tissue electrodes were compared [2]. In an adenosine electrode a mixture of cells obtained from the outer (mucosal) side of a mouse small intestine was used [3j. The stability of all these electrodes increases in the presence of sodium azide in the solution that prevents bacterial decomposition of the tissue. In an electrode specific for the antidiuretic hormone [57], toad bladder is placed over the membrane of a sodium-sensitive glass electrode. In the presence of the antidiuretic hormone, sodium ions are transported through the bladder and the sodium electrode response depends on the hormone concentration. 

Two amino acids—asparagine and glutamine—contain acid-amide groups in the side chains, from which NH3 can be released by hydrolysis (hydrolytic deamination). In the blood, glutamine is the most important transport molecule for amino nitrogen. Hydrolytic deamination of glutamine in the liver also supplies the urea cycle with NH3. 

The skeletal muscle is the most important site for degradation of the branched-chain amino acids (Val, Leu, lie see p. 414), but other amino acids are also broken down in the muscles. Alanine and glutamine are resynthesized from the components and released into the blood. They transport the nitrogen that arises during amino acid breakdown to the liver (alanine cycle see above) and to the kidneys (see p. 328). 

In most terrestrial animals, glutamine in excess of that required for biosynthesis is transported in the blood to the intestine, liver, and kidneys for processing. In these tissues, the amide nitrogen is released as ammonium ion in the mitochondria, where the enzyme glutaminase converts glutamine to glutamate and NHj (Fig. 18-8). The NHj from intestine and kidney is transported in the... 

FIGURE 18-8 Ammonia transport in the form of glutamine. Excess ammonia in tissues is added to glutamate to form glutamine, a process catalyzed by glutamine synthetase. After transport in the bloodstream, the glutamine enters the liver and NH) is liberated in mitochondria by the enzyme glutaminase. 

Ammonia transport in the blood from the peripheral tissues to the liver occurs by two major mechanisms glutamine can be synthesized from glutamate and ammonia (glutamine synthetase) or pyruvate can be transaminated to alanine. In the liver, the ammonia group is removed from glutamine by glutaminase and from alanine by transamination. 

Your liver is also a site for amino acid synthesis such as Serine, Glycine, Glutamic acid and Glutamine. This means that the liver will hang on to some amino acids for bio-synthesis while passing others onto the general circulation for transportation to other organs and tissue. 

Some of the amino acids undergo facilitated diffusion through selective transport proteins into the bloodstream, from which they are taken up by the liver and other organs. Others, particularly glutamate, glutamine, aspartate, and asparagine, are metabolized by the gut cells for energy. 

The mitochondrial translocators which have been most carefully assessed with respect to their role in control of metabolism are (1) the adenine nucleotide translocator with respect to its role in the control of respiration (2) the liver pyruvate transporter and the control of gluconeogenesis and (3) kidney glutamate and glutamine transport and their control of ammoniagenesis. 

The protein and amino-acid metabolism of the liver is characterized by three essential functions (1.) production and breakdown of proteins, (2.) production and breakdown of amino acids as well as regulation of their concentrations in the blood, and (i.) detoxification of ammonium via the synthesis of urea (= excretory form) and glutamine (= non-toxic transport or storage form) with simultaneous regulation of the acid-base balance. The breakdown of branched-chain amino acids occurs only in the musculature by way of deamination, (s. pp 38, 43)... 

Ammonia is produced by almost all cells in the body however, only the liver has the enzymatic machinery to convert it to urea. Therefore, extra-hepatic ammonia must be transported to the liver. However, anunonia in the blood is toxic to cells, and therefore the nitrogen from amino acid catabolism is transported in blood either as glutamine or alanine. Glutamine is synthesized from Glu and ammonia in an ATP-requiring reaction that is catalyzed by glutamine synthetase. Alanine is formed from pyruvate in a transamination reaction catalyzed by alanine transaminase (ALT). 

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