Specific binding of 3H-adenosine to rat brain membranes.

The binding of 3H-adenosine to rat brain membranes was studied by a microcentrifugation technique. Specific binding of 3H-adenosine was rapid, reversible, saturable and dependent on pH and temperature. Scatchard plots of equilibrium binding data were nonlinear suggesting the existence of two different binding sites for adenosine. The dissociation constants (Kd) were 1.7 muM and 13.6 muM and the maximal number of binding sites (Bmax) 31 and 165 pmol adenosine bound per mg of membrane protein. Ten adenosine derivatives were studied for their ability to compete with 3H-adenosine binding. The phosphorylated adenosine compounds 5'-AMP, cyclic AMP and ATP were most potent in displacing 3H-adenosine from its binding sites and the IC50-values ranged from 11--25 muM. N6-Phenylisopropyladenosine produced only partial inhibition (30%) of 3H-adenosine binding and no stereospecific difference between the (-)- and (+)isomer was observed. Several methylxanthines known as adenosine antagonists competed for the 3H-adenosine binding sites parallel with their pharmacological potency. The results offer a first approach for the study of adenosine binding sites in brain membranes.