Cell surface adenosine deaminase: Much more than an ectoenzyme

During the last 10 years, adenosine deaminase (ADA), an enzyme considered to be cytosolic, has been found on the cell surface of many cells, therefore it can be considered an ectoenzyme. EctoADA, which seems to be identical to intracellular ADA and has a globular structure, does not interact with membranes but with membrane proteins. Two of these cell surface receptors for ectoADA have been identified: CD26 and A(1) adenosine receptors (AIR). Apart from degradation of extracellular adenosine another functional role of ectoADA has been assigned. EctoADA is able to transmit signals when interacting with either CD26 or A(1)R. In this way, it acts as a co-stimulatory molecule which facilitates a variety of specific signalling events in different cell types. The heterogeneous distribution of the enzyme in the nervous system indicates that ectoADA may be a neuroregulatory molecule. On the other hand, ectoADA might act as a bridge between two different cells thus raising the possibility that it may be important for the development of the nervous system. (C) 1997 Elsevier Science Ltd. All rights reserved.