see also:

• pathology tests - urea and creatinine

• proteins conjugated with ubiquitin are destined for degradation
• dietary amino acids and the amino acids that form from catabolism of protein form an amino acid pool
• the fate of these amino acids may take several routes:
  • urinary excretion (although most filtered AA's are reabsorbed)
  • formation of body protein (some of which is lost as hair, urinary protein, stools, etc)
  • formation of hormones, neurotransmitters
  • formation of creatine ⇒ phosphyrylcreatine in muscle ⇒ creatinine ⇒ renal excretion
  • metabolism to purines & pyrimidines ⇒ RNA & DNA
    • excess pyrimidines ⇒ CO₂ + NH₃ ⇒ urea
    • excess purines ⇒ uric acid
  • transamination or amination to other AA's
  • oxidative deamination in liver:
    • ketogenic AA's (leucine, isoleucine, phenylalanine, tyrosine) ⇒ acetoacetate + NH⁴⁺ ⇒ urea
    • glucogenic AA's (alanine et al ) ⇒ glucose + NH⁴⁺ ⇒ urea
    • in addition, sulphur containing AA's (ie. cysteine, methionine) are the source of urinary sulphates