Table 1. Allosteric parameters for sodium at BRIL-δOR(ΔN/ΔC) and WT δ-OR.

Radioligand binding assays (Figs. 3a and 3b, Figs. S5c and S5d) were analyzed by the allosteric model (see ²⁵ for details). Here α defines the effect of allosteric ligand (in this case, sodium) on orthosteric ligand (in this case, DADLE); an α>1 indicates a positive effect thereby increasing binding affinity, while an α<1 indicates a negative effect thereby reducing binding affinity. For the D95A, D95N and N131A mutations no allosteric effect of sodium was observed (see Fig. 3b for representative data with the D95A mutant).

	Na+ pKB ± SEM	Na+ KB (mM)	pα ± SEM	α	Hill Coefficient
δ-OR WT (HEK293)	1.88 ± 0.10	13.3	1.93 ± 0.34	0.012	0.56 ± 0.16
D95A (HEK293)	NAE	NAE	NAE	NAE	NAE
D95N (HEK293)	NAE	NAE	NAE	NAE	NAE
N131A (HEK293)	NAE	NAE	NAE	NAE	NAE
N131V (HEK293)	1.11 ± 0.15	77	1.61 ± 0.86	0.025	0.61 ± 0.02
BRIL-δOR(ΔN/ΔC) (Sf9)	1.79 ± 0.11	15.9	0.86 ± 0.05	0.138	0.79 ± 0.02

NAE=No Allosteric Effect of sodium observed