Dd. Axe et Je. Bailey, TRANSPORT OF LACTATE AND ACETATE THROUGH THE ENERGIZED CYTOPLASMIC MEMBRANE OF ESCHERICHIA-COLI, Biotechnology and bioengineering, 47(1), 1995, pp. 8-19
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.