Lactic acid excretion via carrier-mediated facilitated diffusion in Lactobacillus helveticus

Summary During growth on a complex medium containing 2% (w/v) lactose, Lactobacillus helveticus produced about 180 mm lactate. Due to the acidification, the external pH decreased to 3.7. The pH remained constant at a level of 0.5–0.7 units (40 mV), and µ_Lac decreased gradually from –60 to 0 mV. The mechanism of lactate extrusion was studied with resting cells. Upon dilution of lactate-loaded cells in a buffer containing [¹⁴C]-lactate, a typical counterflow was observed, suggesting that a carrier system was employed in lactate excretion. Influx of lactate could not be driven by an artificial membrane potential, indicating that lactate was electroneutrally transported. By examining efflux under various lactate anion and lactic acid concentrations, the undissociated form of the acid was shown to influence the velocity of the transport process. A pH-dependent apparent K_m value of the carrier system was observed in efflux experiments with increasing internal lactate concentrations. It was concluded that the mode of end-product excretion can be defined as a carrier-mediated facilitated diffusion with the undissociated lactic acid or the lactate anion in symport with one proton, respectively, as the object of transport.Abbreviations L_tota total lactate - L_undissb free lactic acid - L_dissc lactate anion - pH_ed external pH - pH_ie internal pH - pH transmembrane H⁺ gradient - µ_Lacf transmembrane gradient of total lactate - µ_HLg transmembrane gradient of the free lactic acid - µ_Lh transmembrane gradient of the lactate anion - V_Effii efflux velocityOffprint requests to: G. Gottschalk