Heat-stable and fructose 1,6-bisphosphate-activated L-lactate dehydrogenase from an extremely thermophilic bacterium.

Heat-stable L-lactate dehydrogenase [EC 1.1.1.27] was purified from an extremely thermophilic bacterium belonging to the genus Thermus, and it showed an allosteric nature dependent on fructose 1,6-bisphosphate as an effector. The enzyme had a molecular weight of approximately 120,000 with a subunit molecular weight of 31,000. For pyruvate reduction, the optimal pH was found to be 4.5. At neutral pH, which is a more physiological region, little enzyme activity was observed, but marked reaction resulted from the addition of fructose 1,6-bisphosphate. This addition stabilized the enzyme toward heat treatment at up to 95 degrees C. The optimal temperature for the enzyme reaction was approximately 80 degrees C for pyruvate reduction and 95 degrees C for lactate oxidation.