A(1) ADENOSINE RECEPTORS - 2 AMINO-ACIDS ARE RESPONSIBLE FOR SPECIES-DIFFERENCES IN LIGAND RECOGNITION

Species differences in ligand binding to A, adenosine receptors were l ocalized to the seventh transmembrane (TM7) region based on the bindin g of [8-S-3]cyclopentyl-1,3-dipropylxanthine and three other ligands t o wild type and six bovine/canine interspecies receptor chimeras expre ssed in COS-1 cells. Subsequent site-directed mutagenesis experiments identified amino acid 270 (isoleucine/methionine, bovine/canine) as be ing primarily responsible for species differences in the binding of N- 6-adenine-substituted compounds, R-N-6-phenylisopropyladenosine (R-PIA ) and (S)-N-6-endonorbornan-2-yl-9-methyladenine, and the C-8-substitu ted xanthine, [H-3]cyclopentyl-1,3-dipropylxanthine. These data are co nsistent with the hypothesis that the N-6 region of adenines and the C -8-region of xanthines bind to the same region of the receptor. A seco nd TM7 amino acid, 277 (serine/threonine, bovine/canine), selectively influences the binding of the ribose-substituted adenosine analog, 5'- N-ethylcarboxamidoadenosine to a variable extent, depending on the nat ure of amino acid 270. We hypothesize that amino acid 270 of the A, re ceptor interacts with the N-6 region of adenosine, while amino acid 27 7 is important, especially in the absence of an N-6 substitution, for interactions with a distinct nucleoside region, possibly on the ribose .