Lactose transport in Escherichia coli cells. Dependence of kinetic parameters on the transmembrane electrical potential difference.

We determine the kinetic parameters V and KT of lactose transport in Escherichia coli cells as a function of the electrical potential difference (delta psi) at pH 7.3 and delta pH = 0. We report that transport occurs simultaneously via two components: a component which exhibits a high KT (larger than 10 mM) and whose contribution is independent of delta psi, a component which exhibits a low KT independent of delta psi (0.5 mM) but whose V increases drastically with increasing delta psi. We associate these components of lactose transport with facilitated diffusion and active transport, respectively. We analyze the dependence upon delta psi of KT and V of the active transport component in terms of a mathematical kinetic model developed by Geck and Heinz (Geck, P. and Heinz, E. (1976) Biochim. Biophys. Acta 443, 49-63). We show that within the framework of this model, the analysis of our data indicates that active transport of lactose takes place with a H+/lactose stoichiometry greater than 1, and that the lac carrier in the absence of bound solutes (lactose and proton(s) is electrically neutral. On the other hand, our data relative to facilitated diffusion tend to indicate that lactose transport via this mechanism is accompanied by a H+/lactose stoichiometry smaller than that of active transport. We discuss various implications which result from the existence of H+/lactose stoichiometry different for active transport and facilitated diffusion.