Reaction rates of MTSES modification at various membrane potentials. (A and B) Macroscopic recordings of cysless/V345C and cysless/M348C showing modification by 1 mM MTSES when the membrane potential is held at −50 mV (left) and −100 mV (right). Red lines are the results of exponential fitting. Note that the time scales of the left and the right traces are the same. (C) Summary of the second-order reaction rate constants at three different membrane potentials. n = 4–7 for each data point.