Transport and metabolism of pyruvate in Lactobacillus plantarum

Pyruvate is the substrate for diacetyl and acetoin synthesis. In Lactobacillus plantarum ATCC 8014, the addition of exogenous pyruvate stimulated acetoin production by several fold when glucose was present as an energy source. Acetoin production increased as pyruvate utilization increased. The conversion efficiency was high at low external pH. The energy derived from glucose via glycolysis generates a constant protonmotive force (PMF) and maintains internal pH homeostasis. PMF, especially {dollar}Delta{dollar}pH component, was important for pyruvate transport. In the whole cells, protonophores inhibited pyruvate transport, whereas valinomycin had no effect on pyruvate uptake. In membrane vesicles pyruvate was accumulated against a concentration gradient when the membrane vesicles an artificial pH gradient was imposed. The transport rate and the degree of accumulation increased as the proton gradient increased. A membrane potential generated by potassium-efflux ({dollar}-{dollar}61 mV) drove the active transport of leucine but not pyruvate. Carboxylic acids that are commonly encountered by lactic acid bacteria had no effect on pyruvate uptake. The results suggested that pyruvate was transported by a proton symport system specific for pyruvate. The transporter had an optimal pH of 3.0, with an apparent Km of 35 {dollar}mu{dollar}M.; High intracellular pyruvate concentration, which was achieved by transport of external pyruvate, was necessary for acetoin synthesis. At low pH, energized cells rapidly transported and accumulated pyruvate. When large amounts of pyruvate were transported and accumulated internally, the co-transported protons rapidly lowered the internal pH. The decrease of internal pH increased the activities of {dollar}alpha{dollar}-acetolactate synthase and acetoin dehydrogenase, resulting in enhanced acetoin production. The conversion of pyruvate to acetoin instead of other acids as end products contributed to maintenance of pH homeostasis. Therefore, the amount of acetoin formed was a combined function of the pyruvate transport rate, the internal pyruvate concentration, the internal pH, and enzyme activities.