Comparison of lactose uptake in resting and energized Escherichia coli cells: High rates of respiration inactivate the lac carrier

Abstract

The transport of lactose by E. coli cells was radically different in the absence and in the presence of an exogenous energy source: in the former case, the time of lactose accumulation was monotonous; in the latter case, lactose accumulation reached a maximum and then deceased to a final steady-state level lower than that observed in the absence of an energy source. It is shown that this 'overshoot' is the result of a decrease in the influx rate and of an increase in the rate constant of efflux as lactose accumulates. These phenomena were irreversible. The extent of the overshoot was dependent upon experimental conditions: it was maximal at alkaline pH, for low external potassium concentrations, and for relatively high external lactose concentrations (around or above the K(T) or uptake). The addition of an energy source to resting E. coli cells resulted in an increase in both the electrochemical gradient of protons and in the rate of respiration. It is demonstrated that the overshoot is the result of the latter and unrelated to the former. An irreversible decrease in the membrane potential as lactose accumulated in the presence of an exogenous energy source was observed. The whole of the data are discussed in terms of an irreversible inactivation of the lactose carrier as a result of a possible interaction with the respiratory chain.