Adenosine deaminase (sometimes known as ADA) is an enzyme involved in purine metabolism. It is needed for the breakdown of adenosine from food and from the turnover of nucleic acids in tissues. Adenosine cannot be deribosylated (removed from ribose) without first converting it to the related nucleoside inosine, which is then broken down further. Adenosine deaminase converts adenosine to inosine by the removal of an amine group. Mutations in the gene for adenosine deaminase are one cause of severe combined immunodeficiency.