The pharmacology of adenosine. Pharmacotherapy (1990) 10; 157-74 Adenosine deaminase (ADA): an enzyme (EC 3. 5. 4. 4) essential in the cellular metabolism of purines which deaminates adenosine and 2'-deoxyadenosine to inosine or 2'-deoxyinosine. ADA deficiency has been identified as forming the metabolic basis for 20-30% of cases with recessively inherited severe combined immunodeficiency (SCID).