Abstract

Adenosine deaminase (ADA) is an enzyme of the purine metabolism that has been largely considered to be cytosolic. Recently, it has been demonstrated that the enzyme appears on the surface of lymphocytes where it interacts with the T-cell activation antigen CD26. ADA also appears on the surface of nonlymphoid cells anchored to adenosine A₁ receptors. Here it is demonstrated that cell surface ADA in ADA⁺/CD26⁻ T lymphocytes anchors to adenosine receptors of the A_2B subtype (A_2BR). An interaction between A_2BR and cell surface ADA has been demonstrated in transfected Chinese hamster ovary cells and Jurkat J32 T lymphocytes. This has been proved by coimmunoprecipitation, binding of exogenous ADA to A_2BR⁺ cells, and coimmunolocalization. The specificity of the interaction has also been demonstrated by the lack of interaction with other members of the G protein-coupled receptor superfamily. Binding of ADA to A_2BR increases the affinity of the agonist 5′-N-ethylcarboxamidoadenosine and cAMP production. This effect occurs even when ADA devoid of enzyme activity is used. Therefore, in lymphocytes, cell surface ADA, apart from degrading extracellular adenosine, regulates those actions of adenosine that are mediated via adenosine receptors of the A_2B subtype.