High-field phosphorus NMR studies of the stoichiometry of the lactate/proton carrier in <em>Streptococcus faecalis</em>.

Highfield ³¹P-NMR studies of whole cells of Streptococcus fecaelis 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 approx; 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 Michel's et ala. ((1979) FEMS Microbion, Letts. 5, 357–364). [ABSTRACT FROM AUTHOR]