Amino acids nitrogen disposal from

Before the carbon skeletons of amino acids are oxidized, the nitrogen must be removed. Amino acid nitrogen forms ammonia, which is toxic to the body. In the liver, ammonia and the amino groups from amino acids are converted to urea, which is nontoxic, water-soluble, and readily excreted in the urine. The process by which urea is produced is known as the urea cycle. The liver is the organ responsible for producing urea. Branched-chain amino acids can be oxidized in many tissues, but the nitrogen must always travel to the liver for disposal. 

Most tissues transfer the amino acid nitrogen to the liver to dispose of as urea. They, therefore, produce either alanine (from the pyruvate-glucose-alanine cycle, in skeletal muscle, kidney, and intestinal mucosa) or glutamine (skeletal muscle, lungs, neural tissues) or serine (kidney), which are released into the blood and taken up by the liver. 

Urea is the form in which amino groups derived from amino acids are disposed of from the body. One nitrogen atom of the urea molecule comes from free ammonia, the other from aspartate. All but two of the reactions of the cycle occur in the cytosol of liver hepatocytes the other two occur in the mitochondria. Urea is transported to the kidney for excretion into the urine. Urea is produced by the liver even during starvation, as skeletal muscle proteins are broken down to release amino acids to act as gluconeogenic precursors. The amino group is removed from these amino acids and converted into urea, which is then excreted in the urine. 

Transaminases are used in the biosynthesis of amino acids and in the transfer of nitrogen atoms from amino acids when the disposal of nitrogen is necessary. 

The sequence begins with formation of an imine from the amino acid and the oxidized form of the vitamin, pyridoxal. This imine converts to an isomer via tautomerism simultaneous proton and double-bond shift (See Sections 13-7 and 13-8). Hydrolysis of the new imine gives pyridoxamine and the ketoacid. Depending on the body s metabolic needs, the pyridoxaniine thus formed may proceed to react with other ketoacids to produce required amino acids (the scheme shown above run in reverse), or it may serve to facilitate the ultimate disposal of the nitrogen by excretion. 

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