The characterization of energized and partially de-energized (respiration-independent) beta-galactoside transport into Escherichia coli

Abstract

Unidirectional fluxes of [14C]lactose by whole cells of Escherichia coli under highly energized and partially de-energized (in the presence of CN-) conditions are analyzed kinetically. When the cells are energized, the value for V influx is 0.45 +/- 0.01 mM internal concentration increment/s and Kt is 0.26 +/- 0.03 mM. At an external concentration of 0.61 mM the steady-state internal concentration is 0.25 M, reached after about 1h. The maximum steady-state concentration ratio is 2-10(3). The efflux process under these conditions is non-saturable, being linearly dependent upon internal concentration over the range 25-250 mM with a first-order rate constant of 8.8 +/- 0.2-10(-4) S-1. The transport in the presence of CN- is active, with a maximum concentration ratio (internal concentration/external concentration) of 104, and the uptake is mimicked by anoxia (less than 70 ppm O2). The effects of CN- are to lower the V for influx and to change the efflux from a non-saturable to a saturable process with a value for Kt (60 mM) intermediate between that for energized efflux (greater than 250 mM) and influx (0.3-0.6 mM), the latter value not changing appreciably. Partial de-energization thus affects both the influx and efflux processes.