In the human body, deamination takes place in the liver. It is the process by which amino acids are broken down. The amino group is removed from the amino acid and converted to ammonia. The rest of the amino acid is made up of mostly carbon and hydrogen, and is recycled or oxidized for energy. Ammonia is toxic to the human system, and enzymes convert it to urea or uric acid by addition of carbon dioxide molecules in the urea cycle. Urea and uric acid can safely diffuse into the blood and then be excreted in urine.
Deamination reactions in DNA
Spontaneous deamination is the hydrolysis reaction of cytosine into uracil, releasing ammonia in the process. This can occur in vitro through the use of bisulfite, which converts cytosine, but not 5-methylcytosine. This property has allowed researchers to sequence methylated DNA to distinguish non-methylated cytosine (shown up as uracil) and methylated cytosine (unaltered).
In DNA, this spontaneous deamination is corrected for by the removal of uracil (product of cytosine deamination and not part of DNA) and replacement with cytosine..
Spontaneous deamination of 5-methylcytosine results in thymine and ammonia. In DNA, this reaction cannot be corrected because the repair mechanisms do not recognize thymine as erroneous (as opposed to uracil), and, unless it affects the function of the gene, the mutation will persist. This flaw in the repair mechanism contributes to the rarity of CpG sites in the eukaryotic genome.