Properties of synthetic and recombinant SpTx-1. (A and B) Currents of Kir6.2 coexpressed with SUR1, recorded in the absence (black trace) or presence (red trace) of 20 nM synthetic (A) and recombinant (B) SpTx-1. The dotted line indicates the zero-current level. (C) Fractions of currents plotted against the concentration of synthetic or recombinant SpTx-1. The curves superimposed on data correspond to the fits of an equation for a bimolecular reaction. The fitted K_d values are (1.53 ± 0.13) × 10⁻⁸ M (n = 6, synthetic) and (1.48 ± 0.12) × 10⁻⁸ M (n = 6, recombinant). (D) ^appk_on (solid squares) and ^appk_off (solid circles) of current inhibition plotted against the concentration of recombinant SpTx-1 with the extra sequence shown in Fig 3 C. A linear fit yielded k_on of (1.46 ± 0.42) × 10⁷ M⁻¹ s⁻¹ (n = 5), and k_off, calculated from the mean of ^appk_off at various SpTx-1 concentrations, was (1.23 ± 0.19) × 10⁻¹ s⁻¹ (n = 5). Data represent the mean ± SEM.