Lactate efflux-induced electrical potential in membrane vesicles of Streptococcus cremoris

Abstract

The authors developed a procedure for isolating membrane vesicles from the homolactic fermentative bacterium S. 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 protein motive force generating mechanism by end product efflux. 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.