Factors Affecting the Activity of the Lactate Dehydrogenase of Streptococcus cremoris

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Abstract
Studies with partially purified extracts of the nicotinamide adenine dinucleotide-linked l (+)-lactate dehydrogenase of Streptococcus cremoris US3 showed that fructose-1,6-diphosphate (FDP) was essential for the catalytic reduction of pyruvate in the p H range 5.0 to 7.0, outside of which the organism does not grow. In the absence of FDP, enzyme activity was observed only in the region of p H 8.0. The optimal p H for the oxidation of lactate was approximately 8.0 in the presence and absence of FDP. The FDP-activated enzyme was markedly inhibited by inorganic phosphate. The enzyme lost activity on standing at 5 C in alkaline triethanolamine, was quite stable at p H 6.0 to 6.5, and underwent irreversible denaturation below p H 5.0. Inorganic phosphate or FDP increased the stability of the enzyme in alkaline buffers. Some distinguishing properties of individual lactate dehydrogenases, activated by FDP, are discussed.