Interconverts simultaneously and stereospecifically pyruvate and lactate with concomitant interconversion of NADH and NAD(+). (S)-lactate + NAD(+) = H(+) + NADH + pyruvate Fermentation; pyruvate fermentation to lactate; (S)-lactate from pyruvate: step 1/1. Homotetramer. Isolates from various regions of the US and Canada show some variations. PubMed:8105474 has sequenced LDH-B isolates called Florida 1A, 1B, 2A and 2B; Georgia 2A and 2B; Maine 2A; New Jersey 121A, 121B, 137A, 137B and 197B; Nova Scotia 1A, 1B, 2A and 2B. Belongs to the LDH/MDH superfamily. LDH family.