ROLE OF HISTIDINE-RESIDUES IN THE ADENOSINE A(2A) RECEPTOR-LIGAND BINDING-SITE

The pH dependency of the binding of ligands to adenosine A(2a) recepto rs in rat striatal membranes was examined. For those agonists sensitiv e to adenosine deaminase a solubilised membrane preparation was used. A two- to fourfold increase in affinity was observed for CGS-21680, 5' -N-ethylcarboxamidoadenosine, adenosine, 3'-deoxyadenosine, 5'-deoxyad enosine, inosine, and N-6-methoxypurine riboside on lowering the ambie nt pH from 7.0 to 5.5. In contrast, no such pH dependency was observed with 2'-deoxyadenosine, although 2'-methoxyadenosine binding was pH d ependent. This effect on the affinity of CGS-21680 was reduced by diet hylpyrocarbonate and restored by hydroxylamine and implied a pK value of 7.0 for the histidine residue involved. No such dependence was obse rved with cyclopentyltheophylline or dimethylpropargylxanthine. It is concluded that one of the histidines conserved in the adenosine recept or binding site acts as a hydrogen bond donor to the oxygen of the 2'- hydroxyl group of adenosine agonists.