Solubilized Rabbit Striatal A2a-Adenosine Receptors: Stability and Antagonist Binding

The A2a-adenosine binding subunit from rabbit striatal membranes was solubilized using 1% 3- [(3-cholamido-propyl)dimethylammonioJ- 1 -propanesulfonate and was characterized using the antagonist radioligand [3H]8-[4-[[[[2 - aminoethyl)amino]carbonyl]methyl]oxy]phenyl] -1 ,3-dipropylxanthine (XAC). The solubilized receptor was very stable, with 55% of the specific [3H]XAC binding remaining after storage for 15 days at 4°C. The dissociation constant (Kd) for binding of [3H]XAC to solubilized A2 receptors was determined in saturation studies to be 4.0 nM, with a Bmax of 600 fmol/mg protein. Xanthine inhibitors displaced the specific binding of the adenosine antagonist [3H]XAC (in the presence of 50 nM 8-cyclopentyl-1,3-dipropylxanthine) at 25°C, with Ki values consonant with the expected affinities at A2a receptors. Binding of [3H]XAC (1 nM) or the adenosine agonist [3H]2-(carboxyethylphenylethylamino)adenosine-5′-N-ethyl carboxamide (5 nM) to A2a receptors was diminished in the presence of 0.1 M Na+ in both membranes and solubilized preparations. Agonist binding was increased (by 280% for membranes and 180% for solubilized receptors), and antagonist binding was decreased in the presence of 10 mM Mg2+. Displacement of [3H]XAC by the agonist (R)-N6-phenylisopropyladenosine was biphasic, corresponding to high (IC50 = 188 nM, RH 30%) and low (IC50 = 9730 nM, RL = 70%) affinity sites. Preincubation with 100 μM GTP (10 mM Mg2+) converted the high affinity binding to low affinity, suggesting that receptor and G-protein are dissociated by the guanine nucleotide. The solubilized receptor was more easily inactivated by exposure to the reducing agent dithiothreitol (IC50 = 3 mM) than in membranes (IC50 = 220 mM), suggesting increased accessibility of structurally essential disulfide bridges.