Table III. Ligand binding properties of wild type and S277A human A_2a receptors.

Table shows mean ± S.E. of two or three independent experiments, each performed in duplicate. Agonist and antagonist binding affinities (K_i values) were determined in [³H]XAC (2 nm) competition binding studies using membrane homogenates prepared from transiently transfected COS-7 cells, as described under “Experimental Procedures.” K_i values were calculated from IC₅₀ value using the GraphPad program. About 15 µg of membrane protein/tube were used.

Ligand	Ki		Ki (S277)/Ki (wt)
	WT	S277A
	nm		-fold
Agonists
CGS 21680	40.7 ± 9.5	43600 ± 1280	1070
CADO	208 ± 66.4	81300 ± 21400	390
DPMA	373 ± 66.0	40000 ± 8240	107
IB-MECA	1450 ± 70.0	96000 ± 5700	66
NECA	72.5 ± 10.0	29100 ± 6240	400
R-PIA	792 ± 49.0	33800 ± 3710	43
Antagonists
CGS 15943	20.7 ± 0.8	17.5 ± 4.1	0.8
CPX	663 ± 164	219 ± 31.5	0.3