High‐field phosphorus NMR studies of the stoichiometry of the lactate/proton carrier in Streptococcus faecalis

High-field 31P-NMR studies of whole cells of Streptococcus faecalis have shown that delta pH can be formed by ATP hydrolysis and also by lactate transport. We have used 31P-NMR to measure the pH dependence of the variable stoichiometry of the proton/lactate carrier. At low external pH (pH approximately equal to 6.5) the influx stoichiometry was 1.1 H+/lactate, while at high pH (7.5) the ratio was almost 2; the apparent midpoint pH of this variable stoichiometry is 7. delta psi measurements support the electrogenic nature of lactate transport at high pH; the variable rate of membrane depolarization caused by lactate transport also had a midpoint near pH 7.0. The data is consistent with a symmetrical carrier operating with variable stoichiometry as proposed by Michels et al.