Mechanism of Cl⁻ transport at the plasma membrane ofChara corallina: II. Transinhibition and the determination of H⁺/Cl⁻ binding order from a reaction kinetic model

Summary

Internal Cl⁻ and low internal pH are strong inhibitors of Cl⁻ influx at the plasma membrane ofChara. The present investigation seeks to understand the mechanism by which this is achieved. Since both Cl⁻ and H⁺ are transported by the same system, one possible mechanism is simply through a change in the electrochemical gradients of these ions. However, it is found that transport is more sensitive to theinternal concentrations of the two ions than to their respective gradients. It is demonstrated that Cl⁻ influx, which shows Michaelis-Menten kinetics with respect to external concentration, is affected only in itsV _max by internal Cl⁻ and pH; the apparentK _m of the transport system for external Cl⁻ is unchanged. In addition, it is found that there is an apparent interaction between internal Cl⁻ and pH in their effects on Cl⁻ influx, both in intact cells and those that have been perfused internally. A kinetic model is proposed which can account quantitatively for all these observations simply through the effects of substrate concentration on the apparent rate constants of a recycling carrier. The model predicts (i) strictly ordered binding of Cl⁻ and H⁺ to the carrier at both internal and external surfaces, with Cl⁻ first on and first off (ii) movement of charge through the membrane on the loaded, rather than the unloaded, carrier. The present model is expected to account for similar kinetic observations from a variety of other cotransport systems.