Amino Acid Metabolism in the Liver: Nutritional and Physiological Significance
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The liver plays a central role in amino acid (AA) metabolism in humans and other animals. In all mammals, this organ synthesizes many AAs (including glutamate, glutamine, alanine, aspartate, asparagine, glycine, serine, and homoarginine), glucose, and glutathione (a major antioxidant). Similar biochemical reactions occur in the liver of birds except for those for arginine and glutamine hydrolysis, proline oxidation, and gluconeogenesis from AAs. In contrast to mammals and birds, the liver of fish has high rates of glutamate and glutamine oxidation for ATP production. In most animals (except for cats and possibly some of the other carnivores), the liver produces taurine from methionine or cysteine. However, the activity of this pathway is limited in human infants (particularly preterm infants) and is also low in adult humans as compared with rats, birds and livestock species (e.g., pigs, cattle and sheep). The liver exhibits metabolic zonation and intracellular compartmentation for ureagenesis, uric acid synthesis, and gluconeogenesis, as well as AA degradation and syntheses. Capitalizing on these extensive bases of knowledge, dietary supplementation with functional AAs (e.g., methionine, N-acetylcysteine, and glycine) to humans and other animals can alleviate or prevent oxidative stress and damage in the liver. Because liver diseases are common problems in humans and farm animals (including fish), much research is warranted to further both basic and applied research on hepatic AA metabolism and functions.