Ecto-adenosine deaminase: An ecto-enzyme and a costimulatory protein acting on a variety of cell surface receptors

Adenosine deaminase (ADA, E.C. 3.5.4.4) is an enzyme of the purine metaboli sm that converts adenosine (Ado) and 2'deoxyadenosine (dAdo) into inosine a nd 2'deoxyinosine, respectively. The enzyme has been the object of consider able interest because the congenital defect causes severe combined immunode ficiency (SCID). In the past 12 years, ADA, which was considered to be cyto solic, has been found on the surface of many cells; therefore, it has to be also considered as an ecto-enzyme (ecto-ADA). There is recent evidence abo ut a specific role for ecto-ADA that is different from that of intracellula r ADA. Apart from degrading extracellular Ado or dAdo, which are toxic for lymphocytes and other cells, ecto-ADA has got an extraenzymatic function by means of its interaction with cell surface proteins. The interactions in w hich ecto-ADA participates are summarized, and an overview of the known and suspected extraenzymatic roles for ecto-ADA is presented. Drug Dev. Res. 4 5:261-268, 1998. (C) 1998 Wiley-Liss, Inc.