GROWTH AND ENERGY GENERATION BY LACTOCOCCUS-LACTIS SUBSP LACTIS BIOVAR DIACETYLACTIS DURING CITRATE METABOLISM

Growth of Lactococcus lactis subsp. lactis biovar diacetylactis was ob served on media with citrate as the only energy source. At pH 5.6, ste ady state was achieved in a chemostat on a citrate-containing medium i n the absence of a carbohydrate. Under these conditions, pyruvate, ace tate, and some acetoin and butanediol were the main fermentation produ cts. This indicated that energy was conserved in L. lactis subsp. lact is biovar diacetylactis during citrate metabolism and presumably durin g the conversion of citrate into pyruvate. The presumed energy-conserv ing step, decarboxylation of oxaloacetate, was studied in detail. Oxal oacetate decarboxylase was purified to homogeneity and characterized. The enzyme has a native molecular mass of approximately 300 kDa and co nsists of three subunits of 52, 34, and 12 kDa. The enzyme is apparent ly not sodium dependent and does not contain a biotin moiety, and it s eems to be different from the energy-generating oxaloacetate decarboxy lase from Klebsiella pneumoniae. Energy-depleted L. lactis subsp. lact is biovar diacetylactis cells generated a membrane potential and a pH gradient immediately upon addition of citrate, whereas ATP formation w as slow and limited. In contrast, lactose energization resulted in rap id ATP formation and gradual generation of a proton motive force. Thes e data were confirmed during studies on amino acid uptake. Alpha-Amino isobutyrate uptake was rapid but glutamate uptake was slow in citrate- energized cells, whereas lactose-energized cells showed the reverse te ndency. These data suggest that, in L. lactis subsp. lactis bv. diacet ylactis, a proton motive force could be generated during citrate metab olism as a result of electrogenic citrate uptake or citrate/product ex change together with proton consumption bv the intracellular oxaloacet ate decarboxylase.