Allosteric modulation, thermodynamics and binding to wild-type and mutant (T277A) adenosine A1 receptors of LUF5831, a novel nonadenosine-like agonist

1 The interaction of a new nonribose ligand (LUF5831) with the human adenosine A(1) receptor was investigated in the present study. 2 Radioligand binding experiments were performed in the absence and presence of diverse allosteric modulators on both wild-type (wt) and mutant (T277A) adenosine A(1) receptors. Thermodynamic data were obtained by performing these assays at different temperatures. In addition, cyclic adenosine monophosphate ( cAMP) assays were performed. 3 The presence of allosteric modulators had diverse effects on the affinity of LUF5831, N-6-cyclopentyladenosine (CPA), a full agonist, and 8-cyclopentyl-1,3-dipropylxanthine ( DPCPX), an inverse agonist/ antagonist, for the adenosine A(1) receptor. PD81,723, for example, increased the affinity of CPA, while the affinity of LUF5831 was decreased. However, the affinity of DPCPX was decreased even more. In addition, LUF5831 was shown to have an affinity for the mutant ( T277A) adenosine A(1) receptor ( K-i 122 +/- 22 nM), whereas CPA's affinity was negligible. The results of temperature-dependent binding assays showed that the binding of LUF5831 was entropy driven, in between the behaviour of CPA binding to the high- and low-affinity states of the receptor, respectively. 4 The inhibition of the forskolin-induced production of cAMP through activation of the wt adenosine A(1) receptor showed that LUF5831 had a submaximal effect (37 +/- 1%) in comparison to CPA (66 +/- 5%). On the mutant receptor, however, neither CPA nor LUF5831 inhibited cAMP production. 5 This study indicates that the nonribose ligand, LUF5831, is a partial agonist for the adenosine A(1) receptor.