protein metabolism

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
  - glutamine, glycine, and aspartic acid along with active Vitamin B9/folate are used to make purines (adenine (A) and guanine (G) )
  - aspartic acid (aspartate) and glutamine are used to make pyrimidines (Cytosine (C), Thymine (T), Uracil (U) )
  - 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

the body operates a highly active internal Purine Salvage Pathway using enzymes like HGPRT. However, this pathway recycles over 90% of the purines generated from our own dying cells, while ignoring the purines from the food we eat. To build new DNA from scratch, our cells rely almost entirely on internal cell recycling or creating them entirely from scratch (de novo synthesis)
intestinal cells immediately convert almost 100% of dietary purines into uric acid - they are not re-used!
- hence a high-purine diet directly elevates blood uric acid levels and triggers gout - this is not due to a high amino acid diet
intestinal cells and the liver break dietary pyrimidines down into highly soluble compounds like beta-alanine and beta-amino-isobutyrate which are are easily converted into Acetyl-CoA and Succinyl-CoA which are fed directly into the citric acid cycle (Krebs cycle) to generate energy
see dietary_DNA_RNA