![]() It is important to remember that the synthesis of urea is an anabolic process that requires ATP. The initial free ammonia diffuses through the mitochondrial membrane and is fixed with carbon dioxide (in the form of bicarbonate) during the initial step in this process (figures 5.15 and 5.16). The urea cycle occurs in the liver and spans both the mitochondria and the cytosolic compartments. A functioning urea cycle is essential for the disposal of nitrogen from catabolic processes, and if dysfunction occurs the accumulation of ammonia can be life threatening. Excess dietary amino acids, which are not stored, will also require deamination, and the carbons can be stored as either glycogen or fat.Īmmonia freed in the liver by glutaminase (or glutamate dehydrogenase) will readily enter the urea cycle to be incorporated into urea. Nitrogen metabolism, unlike glucose metabolism, is fairly consistent in the fed and fasted states. This is in contrast to glutamine synthetase, which is primarily used by peripheral tissues as a means of generating glutamine to remove ammonia from the tissues to the liver (figure 5.14). The free ammonia can enter into the urea cycle, and the remaining glutamate can be transaminated to generate \(\alpha\)-ketoglutarate. Glutaminase, is active in the liver and responsible for deaminating glutamine as it is shuttled into the liver. The other key enzyme in nitrogen metabolism is glutaminase. The glucose is released from the liver and oxidized by the skeletal muscle. The alanine is released and transported to the liver where it will undergo another transamination to generate pyruvate, which is used as a substrate for glucose production (gluconeogenesis). Alanine aminotransferase (AST) will transaminate glutamate with pyruvate to generate alanine (and \(\alpha\)-ketoglutarate). In this process, ammonia from amino acid degradation is transaminated to form glutamate. In skeletal muscle, the alanine-glucose cycle is commonly used for the transport of nitrogen from the skeletal muscle to the liver. This reaction, catalyzed by glutamine synthetase, facilitates the synthesis and subsequent movement of excess nitrogen from peripheral tissues to the liver (figure 5.14). In peripheral tissues, glutamate generated from transamination or from the GDH reaction can be used to fix an additional ammonia to generate glutamine. The direction of the reaction will be influenced by several factors including cellular needs, the levels of NAD + or NADP +, and levels of ammonia (figure 5.14).įigure 5.14: Movement of ammonia from peripheral tissues to the liver. ![]() In figure 5.14, in the skeletal muscle, glutamate dehydrogenase is illustrated fixing ammonia to \(\alpha\)-ketoglutarate to generate glutamate, while in the liver it is shown freeing ammonia in the reverse reaction. Glutamate dehydrogenase (GDH) is present in most tissues and is one of the few enzymes able to fix or free ammonia. In addition to transaminases, there are three other enzymes that play essential roles in nitrogen transport. ![]() Glutamate dehydrogenase, glutamine synthetase, and glutaminase ![]()
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