Table 3.

Kinetic constants for association (k_on) and dissociation (k_off) and calculated equilibrium constants (K_d) for SPX and GYM with Aplysia (A−) and Lymnaea (L−) AChBPs

Receptor Ligand	koff, min−1	kon, 109 M−1 min−1	Kd, nM
A−AChBP
SPX	0.041 ± 0.023	2.2 ± 0.7	0.019
GYM	0.038 ± 0.013	8.1 ± 1.3	0.0047
L−AChBP
SPX	0.82 ± 0.25	0.69 ± 0.07	1.2*
GYM	0.0037 ± 0.0013	2.8 ± 0.9	0.0013

Values are means ± SD from 3–6 measurements.

*A K_d of 1.26 ± 0.11 nM was alternatively determined by equilibrium titration using scintillation proximity assay for this lower affinity complex.