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Introduction

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High-Yield Terms

  • Aminotransferase: any of a family of enzymes that catalyze the transfer of an amino group between a α-amino acid and an α-keto acid

  • AST and ALT: aspartate aminotransferase (AST; also called serum glutamate-oxaloacetate aminotransferase, [SGOT]) and alanine transaminase (ALT; also called serum glutamate-pyruvate aminotransferase [SGPT]), the two prevalent liver enzymes whose elevations in the blood have been used as clinical markers of tissue damage

  • Glucose-alanine cycle: mechanism for skeletal muscle to eliminate nitrogen while replenishing its glucose supply. Glucose oxidation produces pyruvate which can undergo transamination to alanine; the alanine then enters the blood stream and is transported to the liver where it is converted back to pyruvate, which is then a source of carbon atoms for gluconeogenesis

  • Urea: nitrogen compound composed of 2 amino groups (−NH2) joined by a carbonyl (−C=O) functional group; is the main nitrogen-containing substance in human urine

  • Kwashiorkor: an acute form of childhood protein-energy malnutrition with adequate caloric intake; characterized by edema, irritability, anorexia, ulcerating dermatoses, and an enlarged liver with fatty infiltrates

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Nitrogen Distribution From Biosphere

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Humans are totally dependent on other organisms for converting atmospheric nitrogen into forms available to the body. Nitrogen fixation is carried out by bacterial nitrogenases forming reduced nitrogen, NH4+, which can then be used by all organisms to form amino acids (Figure 29-1).

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FIGURE 29-1:

Overview of the flow of nitrogen in the biosphere. Nitrogen, nitrites, and nitrates are acted upon by bacteria (nitrogen fixation) and plants and we assimilate these compounds as protein in our diets. Ammonia incorporation in animals occurs through the actions of glutamate dehydrogenase and glutamine synthase. Glutamate plays the central role in mammalian nitrogen flow, serving as both a nitrogen donor and nitrogen acceptor. Reproduced with permission of the medical biochemistry page, LLC.

Graphic Jump Location
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Reduced nitrogen enters the human body as dietary free amino acids, protein, and the ammonia produced by intestinal tract bacteria. A pair of principal enzymes, glutamate dehydrogenase and glutamine synthetase, incorporates this ammonia into carbon skeletons generating the amino acids glutamate and glutamine, respectively. Amino and amide groups from these 2 amino acids are then freely transferred to other carbon skeletons by transamination (Figure 29-2) and transamidation reactions.

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FIGURE 29-2:

Transamination. The reaction is freely reversible with an equilibrium constant close to unity. Murray RK, Bender DA, Botham KM, Kennelly PJ, Rodwell VW, Weil PA. Harper's Illustrated Biochemistry, 29th ed. New York: McGraw-Hill; 2012.

Graphic Jump Location
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Aminotransferases exist for all amino acids except threonine and lysine. The most common compounds involved as a donor/acceptor pair in transamination reactions are glutamate and α-ketoglutarate (2-oxoglutarate), which participate in reactions with many different aminotransferases. Serum aminotransferases AST and ALT have been used ...

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