title: Lactate efflux-induced electrical potential in membrane vesicles of Streptococcus cremoris.
abstract: We developed a procedure for isolating membrane vesicles from the homolactic fermentative bacterium Streptococcus cremoris. The membrane vesicles were shown to have a right-side-out orientation by freeze-etch electron microscopy and to be free of cytoplasmic constituents. The membrane vesicles retained their functional properties and accumulated the amino acids L-leucine, L-histidine, and L-alanine in response to a valinomycin-induced potassium diffusion gradient. Studies with these membrane vesicles strongly supported the possibility that there was a proton motive force-generating mechanism by end product efflux (Michels et al., FEMS Lett. 5:357-364, 1979). Lactate efflux from membrane vesicles which were loaded with L-lactate and diluted in a lactate-free medium led to the generation of an electrical potential across the membrane. The results indicate that lactate efflux is an electrogenic process by which L-lactate is translocated with more than one proton.
mesh_terms: D000596:Amino Acids / Q000378:metabolism; D002458:Cell Fractionation; D002462:Cell Membrane / Q000378:metabolism / Q000648:ultrastructure; D007773:Lactates / Q000378:metabolism*; D019344:Lactic Acid; D008564:Membrane Potentials; D009861:Onium Compounds / Q000378:metabolism; D009943:Organophosphorus Compounds; D011188:Potassium / Q000378:metabolism; D013291:Streptococcus / Q000378:metabolism* / Q000648:ultrastructure
chemical_list: D000596:Amino Acids; D007773:Lactates; D009861:Onium Compounds; D009943:Organophosphorus Compounds; D019344:Lactic Acid; D011188:Potassium; C013289:tetraphenylphosphonium