Mechanism of Maltose Uptake and Glucose Excretion in Lactobacillus Sanfrancisco

Lactobacillus sanfrancisco LTH 2581 can use only glucose and maltose as sources of metabolic energy. In maltose-metabolizing cells of L. sanfrancisco, approximately half of the internally generated glucose appears in the medium. The mechanisms of maltose (and glucose) uptake and glucose excretion have been investigated in cells and in membrane vesicles of L. sanfrancisco in which beef heart cytochrome c oxidase had been incorporated as a proton-motive-force-generating system. In the presence of ascorbate, N,N,N',N'-tetramethyl-p-phenylenediamine (TMPD), and cytochrome c, the hybrid membranes facilitated maltose uptake against a concentration gradient, but accumulation of glucose could not be detected. Similarly, in intact cells of L. sanfrancisco, the nonmetabolizable glucose analog alpha-methylglucoside was taken up only to the equilibration level. Selective dissipation of the components of the proton and sodium motive force in the hybrid membranes indicated that maltose is transported by a proton symport mechanism. Internal [14C]maltose could be chased with external unlabeled maltose (homologous exchange), but heterologous maltose/glucose exchange could not be detected. Membrane vesicles of L. sanfrancisco also catalyzed glucose efflux and homologous glucose exchange. These activities could not be detected in membrane vesicles of glucose-grown cells. The results indicate that maltose-grown cells of L. sanfrancisco express a maltose-H+ symport and glucose uniport system. When maltose is the substrate, the formation of intracellular glucose can be more rapid than the subsequent metabolism, which leads to excretion of glucose via the uniport system.