Catalyzes the conversion of lactate to pyruvate. (S)-lactate + NAD(+) = H(+) + NADH + pyruvate Allosterically activated by fructose 1,6-bisphosphate (FBP) alone under acidic conditions, while it requires additional activation factors such as divalent cations (Mn(2+)) under neutral conditions (PubMed:14601, PubMed:7766183). Under acidic conditions, Mn(2+) is an inhibitor in the absence of fructose 1,6-bisphosphate (FBP) (PubMed:14601, PubMed:7766183). In case of L.casei, L-LDH binds four fructose 1,6-bisphosphate (FBP) molecules per tetramer, while usual allosteric L-LDH binds only two fructose 1,6-bisphosphate (FBP) molecules per tetramer (PubMed:14601, PubMed:7766183). Thermostable up to 50 degrees Celsius. Thermostabilized in the presence of both fructose 1,6-bisphosphate (FBP) and Mn(2+) ions. Fermentation; pyruvate fermentation to lactate; (S)-lactate from pyruvate: step 1/1. Homotetramer. Belongs to the LDH/MDH superfamily. LDH family.