Catalyzes the conversion of L-lactate to pyruvate. Seems to be a primary dehydrogenase in the respiratory chain. To a lesser extent, can also oxidize DL-alpha-hydroxybutyrate, but not D-lactate. (S)-lactate + A = AH2 + pyruvate Is activated by 2-fold in the presence of high concentrations of sodium and potassium ions. Is almost completely inhibited by a high concentration of pyruvate (10 mM). Optimum pH is 8-9. Forms homooligomers. Is induced by aerobic growth on L-lactate, but not by aerobic growth on D-lactate or glycerol or by anaerobic growth on glucose. Cells lacking this gene lose the ability to grow on L-lactate as the sole source of carbon and energy, but can still utilize D-lactate. Belongs to the FMN-dependent alpha-hydroxy acid dehydrogenase family.