Intertissue Relationships in the Metabolism of Amino Acids

• The body maintains a large free amino acid pool in the blood, even during fasting, allowing tissues continuous access to these building blocks.
• Amino acids are used for gluconeogenesis by the liver, as a fuel source for the gut, and as neurotransmitter precursors in the nervous system. They are also required by all organs for protein synthesis.
• During an overnight fast and during hypercatabolic states, degradation of labile protein (primarily from skeletal muscle) is the major source of free amino acids.
• The liver is the major site of urea synthesis. Nitrogen from other tissues travels to the liver in the form of glutamine and alanine.
• Branched-chain amino acids are oxidized primarily in the skeletal muscle.
• Glutamine in the blood serves a number of roles:
  • The kidney utilizes the ammonium ion carried by glutamine for excretion in the urine to act as a buffer against acidotic conditions.
  • The kidney and the gut utilize glutamine as a fuel source.
  • All tissues utilize glutamine for protein synthesis.
• The body can enter a catabolic state characterized by negative nitrogen balance under the following conditions:
  • Sepsis (any of various pathogenic organisms or their toxins in the blood or tissues)
  • Trauma
  • Injury
  • Burns
• The negative nitrogen balance results from increased net protein degradation in skeletal muscle, brought about by release of glucocorticoids. The released amino acids are used for protein synthesis and cell division in cells involved in the immune response and wound healing.