Adenosine deaminase

Adenosine deaminase (also known as ADA) is an enzyme (EC 3.5.4.4)involved in purine metabolism. It is needed for the breakdown of adenosine from food and from the turnover of nucleic acids in tissues. ADA irreversibly deaminates adenosine, converting it to the related nucleoside inosine by the removal of an amine group. Inosine can then be deribosylated (removed from ribose) by another enzyme called purine nucleoside phosphorylase (PNP), converting it to hypoxanthine.

Mutations in the gene for adenosine deaminase causing it to not be expressed are one cause of severe combined immunodeficiency (SCID). Mutations causing it to be overexpressed are one cause of hemolytic anemia. There is some evidence that a different allelle (ADA2) may lead to autism.

There are 2 isoforms of ADA: ADA1 and ADA2. ADA1 is found in most body cells, particularly lymphocytes and macrophages, where it is present not only in the cytosol but also as the ecto- form on the cell membrane attached to a protein called CD26. ADA2 has only been found in the macrophage where it co-exists with ADA1 where the two isoforms regulate the ratio of adenosine to deoxyadenosine to potentiate the killing of parasites.

ADA2 is the predominant form present in human plasma and is increased in many diseases, particularly those associated with the immune system: for example rheumatoid arthritis, psoriasis and sarcoidosis. The plasma AD2 isoform is also increased in most cancers. Total plasma ADA can be measured using high performance liquid chromatography, enzymatic or colorimetric techniques. Perhaps the simplest system is the measurement of the ammonia released from adenosine when broken down to inosine. After incubation of plasma with a buffered solution of adenosine the ammonia is reacted with a Berthelot reagent to form a blue colour which is proportionate to the amount of enzyme activity. To measure ADA2 then erythro-9-(2-hydroxy-3-nonyl) adenine (EHNA) is added to the reaction mixture so as to inhibit the ADA1. It is the absence of ADA1 that causes SCID.

An RNA-specific ADA also exists, ie ADAR, see http://en.wikipedia.org/wiki/ADARs