5'-N-Ethylcarboxamide[³H]Adenosine Binding Sites of Mouse Mastocytoma P815 Cell Membranes: Characterization and Solubilization

Abstract

Mouse mastocytoma P815 cell membranes were found to possess adenosine binding sites as assessed by using the adenosine agonist [³H]5'-N-ethylcarboxamideadenosine (NECA). The K_d and B_max for the [³H]NECA binding at 0° were 380nM and 17pmol/mg of protein, respectively. The rank order of potency for inhibition of [³H]NECA binding was NECA>5' -N-cyclopropylcarboxamideadenosine>2-chloroadenosine>2', 5'-dideoxyadenosine>isobutylmethylxanthine>theophylline>N⁶-[(R)-1-methyl-2-phenylethyl] adenosine=N⁶-[(S)-1-methyl-2-phenylethyl] adenosine. Thermodynamic analyses of the adenosine receptor agonist and antagonist binding showed that all such ligands displayed negative values of both enthalpy and entropy which suggested that the driving force for the binding was enthalpic. [³H]NECA binding sites of P815 cell membranes were solubilized with sodium cholate and retaining the same ligand-binding characteristics as those of the membrane-bound form. By gel filtration on a Sepharose CL-6B column, the adenosine binding site was estimated to have a Stokes radius of approximately 6.7nm.