Fig. 6.

MTS modification of Q120C Na/K pump-channels. (A) Mean steady-state PTX-induced current–voltage plots, normalized to control current at −90 mV; experiment and solutions as in Fig. 4. Relative to control (circles, n = 8), MTSET⁺ decreased current ≈30% (squares, n = 4) and MTSES⁻ increased current ≈25% (triangles, n = 4). Linear fits between −100 and −20 mV gave slopes 0.011 mV⁻¹ before MTS application, 0.008 mV⁻¹ after MTSET⁺, and 0.016 mV⁻¹ after MTSES⁻. (B) Bound ouabain (10 mM) does not protect Q120C Na/K pumps from modification by MTSET⁺; experiment and solutions as in Fig. 5B. After lowering [ouabain] to 100 μM, PTX induced current that was unaffected by reexposure to MTSET⁺, indicating that all Cys had been modified by MTSET⁺ in 10 mM ouabain.