Purificationa and Properties of a Fructose-1,6-diphosphate-activated Lactate Dehydrogenase from Streptococcus Faecalis

An l-(+)-lactate dehydrogenase was purified approximately 35-fold from crude extracts of Streptococcus faecalis. The purified enzyme had an absolute and specific requirement for fructose-1,6-diphosphate (FDP) for catalytic activity. The concentration of FDP required for 50% maximal activity was about 0.045 mm. The activator was bound to the enzyme more effectively at pH 5.8 than it was at a neutral or alkaline pH. Activation appeared to involve a conformational change in the enzyme which made the substrate and coenzyme sites more accessible to the respective reactants. Among the evidence supporting this hypothesis was the fact that FDP lowered significantly the apparent K(m) for both pyruvate and reduced nicotinamide adenine dinucleotide. Moreover, the enzyme, which was quite heat stable in the absence of any of the reactants, was rendered heat labile by FDP.