TRANSPORT OF LACTATE AND ACETATE THROUGH THE ENERGIZED CYTOPLASMIC MEMBRANE OF ESCHERICHIA-COLI

Escherichia coli produces lactate and acetate in significant amounts d uring both aerobic and anaerobic glycolysis. A model describing the me chanism of protein-mediated lactate transport has previously been prop osed. A simple theoretical analysis here indicates that the proposed m odel would drain cellular energy resources by catalytically dissipatin g the proton-motive force. An experimental analysis of lactate and ace tate transport employs nuclear magnetic resonance (NMR) spectroscopy t o measure the relative concentrations of these end products on the two sides of the cytoplasmic membrane of anaerobically glycolyzing cells. Comparison of measured concentration ratios to those expected at equi librium for various transport modes indicates that acetate is a classi cal uncoupling agent, permeating the membrane at comparable rates in t he dissociated and undissociated forms. The lactate concentration rati o changes markedly after an initial period of sustained glycolysis. Th is change is most readily explained as resulting from a lactate transp ort system that responds to an indicator of glycolytic activity. The d ata further indicate that lactate permeates the membrane in both disso ciated and undissociated forms. Both acids, then, are capable of catal ytically dissipating the proton-motive force. (C) 1995 John Wiley and Sons, Inc.