Simulations of radioactive Arg ⁺ efflux induced by lowering the Arg ⁺ concentration in the bathing solution and the variation of fractional uptake of radioactive Arg ⁺ with a symmetric change in pH. (A and B) The time courses of fractional uptake of radioactive Arg⁺ (mean ± σ) were simulated using our model with the parameters determined from the polarization measurements under the symmetric pH conditions. As the initial substrate conditions, the vesicles contained 5 mM Arg⁺, whereas the bathing solution contained 50 μM radioactive Arg⁺. The ratio of volumes of the bathing solution and vesicles was 50. (A) 30 min into the simulation, the calculation was continued either without any intervention (closed red circles) or with the Arg⁺ concentration of the bathing solution being raised to 5 mM (open black circles), the latter of which markedly lowered the fractional uptake in a time-dependent manner or induced an efflux of radioactive Arg⁺. (B) The simulations were performed with our model (of kinetic parameters detemined in the polarization study at the corresponding pH) under three conditions where pH was varied symmetrically on both sides of the membrane. For comparison, the simulations under the same conditions with the model whose apo path for conformational transitions was eliminated were shown in the inset. Note that Tsai et al. (2012) showed that mutant AdiC with native cysteines removed had slower kinetics than wild-type AdiC. As expected, the kinetics of the simulated time courses using our model with the kinetic parameters obtained from an AdiC mutant with native cysteines removed are slower than those of wild-type AdiC reported by Fang et al. (2007) but are comparable with those of the mutant AdiC reported by Tsai et al. (2012).