ELECTROGENIC L-MALATE TRANSPORT BY LACTOBACILLUS-PLANTARUM - A BASIS FOR ENERGY DERIVATION FROM MALOLACTIC FERMENTATION

L-Malate transport in Lactobacillus plantarum was inducible, and the pH optimum was 4.5. Malate uptake could be driven by an artificial proton gradient (DELTA-pH) or an electroneutral lactate efflux. Because L-lactate efflux was unable to drive L-malate transport in the absence of a DELTA-pH, it did not appear that the carrier was a malate-lactate exchanger. The kinetics of malate transport were, however, biphasic, suggesting that the external malate concentration was also serving as a driving force for low-affinity malate uptake. Because the electrical potential (DELTA-PSI, inside negative) inhibited malate transport, it appeared that the malate transport-lactate efflux couple was electrogenic (net negative) at high concentrations of malate. De-energized cells that were provided with malate only generated a large proton motive force (> 100 mV) when the malate concentration was greater than 5 mM, and malate only caused an increase in cell yield (glucose-limited chemostats) when malate accumulated in the culture vessel. The use of the malate gradient to drive malate transport (facilitated diffusion) explains how L. plantarum derives energy from malolactic fermentation, a process which does not involve substrate-level phosphorylation.