Figure 9.

Independence of MTSEA modification on the introduced cysteines. (A) Reaction schemes for MTSEA modification. When the modification rates are smaller than the slow-gating transition rates, the schemes can be reduced to linear models. (B) Slow-gate open probability contributes to the apparent MTSEA-modification rate. (Left) Calculated current-induction curves of the homodimer () when P _D1/P _D2 are 0, 0.5, and 1, respectively. The normalized curve of the heterodimer () is the same as that of the homodimer when P _D1/P _D2 = 1. (Right) Ratios of the time constants derived from single-exponential curve fittings plotted against P _D1/P _D2. (C) Experimental comparison of the apparent modification rates between hetero- and homodimeric channels. (Left) Current induction in the heterodimer (□, n = 6) and homodimer (○, n = 3) by 3 μM MTSEA. All current amplitudes, after subtraction by the current before applying MTSEA, were normalized to the maximal current induced by MTSEA. One error bar plotted in every 10 points. Time constants were 49.3 and 80.7 s for the hetero- and homodimer, respectively. (Right) Current-induction rates in the homodimeric (○) and heterodimeric channels (□) at various concentrations of MTSEA. The slopes and y intercepts of the solid lines were: (heterodimer) 7.2 × 10³ M⁻¹s⁻¹ and 0.0037 s⁻¹; (homodimer) 4.6 × 10³ M⁻¹s⁻¹ and 0.0032 s⁻¹. (Inset) Ratios of the induction time constants of the homodimer to those of the heterodimer.