Identification of specific [3H]adenine-binding sites in rat brain membranes

We recently reported that adenine acts as a neurotrophic factor independent of adenosine or P2 receptors in cultured Purkinje cells Watanabe S. et al. (2003) J. Neurosci. Res. 74, 754-759], suggesting the presence of specific receptors for adenine in the brain. In this study, the characterization of adenine-binding activity in the rat brain was performed to further characterize the receptor-like adenine-binding sites. Specific binding sites for (3)H]adenine were detected in membrane fractions prepared from rat brains. The kinetics of (3)H]adenine binding to membranes was described by the association and dissociation rate constants, 8.6 x 10(5) M(-1) min(-1) and 0.118 +/- 0.045 min(-1), respectively. A single binding site for (3)H]adenine with a K (D) of 157.1 +/- 20.8 nM and a B (max) of 16.3 +/- 1.1 pmol/mg protein (n = 6) was demonstrated in saturation experiments. A displacement study involving various related compounds showed that the (3)H]adenine binding was highly specific for adenine. It was also found that (3)H]adenine-binding activity was inhibited by adenosine, although other adenosine receptor ligands were ineffective as to (3)H]adenine binding. The brain, especially the cerebellum and spinal cord, showed the highest (3)H]adenine-binding activity of the tissues examined. These results are consistent with the presence of a novel adenine receptor in rat brain membranes.