Amino Acid Metabolism in Mammalian Systems

Amino acid metabolism in mammals encompasses the absorption, distribution, synthesis and degradation of the twenty standard amino acids that underlie protein synthesis, energy provision and cellular signalling. Dietary proteins are hydrolysed in the gastrointestinal tract to yield free amino acids and small peptides, which are transported across enterocytes into the portal circulation. From there, amino acids are taken up by liver, muscle, kidney and other tissues via specific transporter families. Within cells, amino groups are transferred in transamination reactions to form glutamate and aspartate, key hubs for further disposal or assimilation of nitrogen. Carbon skeletons derived from deamination enter central pathways such as the tricarboxylic acid cycle or gluconeogenesis, supporting energy production or glucose synthesis. Excess nitrogen is funnelled into the urea cycle in the liver and, to a lesser extent, the kidney, ensuring safe excretion. Interorgan exchange cycles—most notably the glutamine–alanine cycle between muscle and liver—coordinate nitrogen and carbon distribution during fasting, exercise and stress. Regulation of these pathways by hormonal cues (insulin, glucagon, catecholamines) and nutrient status maintains metabolic homeostasis. Disruptions to amino acid handling contribute to inherited metabolic disorders, insulin resistance and cancer, highlighting their clinical significance and potential as targets for nutritional or pharmacological intervention.

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Amino Acid Metabolism in Mammalian Systems publication trend

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Technical terms

Transamination: A biochemical reaction in which an amino group is transferred from an amino acid to an α-keto acid, forming a new amino acid and a new keto acid.

Gluconeogenesis: The metabolic pathway by which non-carbohydrate substrates (e.g. amino acids, lactate) are converted into glucose, primarily in liver and kidney.

Urea cycle: The hepatic and renal series of enzymatic steps that convert toxic ammonia into urea for excretion.

Glutamine–alanine cycle: An interorgan shuttle in which muscle exports nitrogen as alanine and the liver converts it back to glucose and urea.

Branched-chain amino acids (BCAAs): The amino acids leucine, isoleucine and valine, which are oxidised in peripheral tissues and play roles in energy homeostasis and signalling.

References

1. The Metabolic Responses to L-Glutamine of Livers from Rats with Diabetes Types 1 and 2. PLOS ONE (2016).
2. Uptake and Metabolism of Plasma Glutamine by the Small Intestine. Journal of Biological Chemistry (1974).
3. Studies of Hepatic Glutamine Metabolism in the Perfused Rat Liver with 15N-Labeled Glutamine*. Journal of Biological Chemistry (1999).