Intracellular pH and the role of D-lactate dehydrogenase in the production of metabolic end products by Leuconostoe Laois

the kinetics of lactate dehydrogenase from Leuconostoc lactis NCW1 were studied. The pH optimum for the enzyme depended on the concentration of pyruvate used in the assay and the enzyme displayed an ordered mechanism with respect to substrate binding. The K_mfor pyruvate and NADH and the V_max of the enzyme decreased 20-, 30– and 6-fold respectively as the pH decreased from 8.0 to 5.0. No activators were found and none of the intermediates of the phosphoketolase pathway tested inhibited the enzyme. ATP, ADP, GTP and NAD⁺ were inhibitory. The intracellular volume (Vol_in) and intracellular pH (pH_in) decreased as the extracellular pH (pH_ex) decreased. Co-metabolism of citrate and glucose affected the Vol_in but did not affect the pH_in) which decreased by 0.6 units per unit change in pH_ex; at pH 7.0, the pH_in and pH_ex were equal. The results suggest that pH_in may play a role in determining the production of diacetyl and acetoin at low pH by Leuconostoc.