Regulation of the L lactate dehydrogenase from Lactobacillus casei by fructose 1,6 diphosphate and metal ions

Abstract

The lactate dehydrogenase of L. casei, like that of streptococci, requires fructose 1,6 diphosphate (FDP) for activity. The L. casei enzyme has a much more acidic pH optimum (pH 5.5) than the streptococcal lactate dehydrogenases. This is apparently due to a marked decrease in the affinity of the enzyme for the activator with increasing pH above 5.5; the concentration of FDP required for half maximal velocity increases nearly 1,000 fold from 0.002 mM at pH 5.5 to 1.65 mM at pH 6.6. Manganous ions increase the pH range of activity particularly on the alkaline side of the optimum by increasing the affinity for FDP. This pH dependent metal ion activation is not specific for Mn2+. Other divalent metals, Co2+, Cu2+, Cd2+, Ni2+, Fe2+, and Zn2+ but not Mg2+, will effectively substitute for Mn2+, but the pH dependence of the activation differs with the metal ion used. The enzyme is inhibited by a number of commonly used buffering ions, particularly phosphate, citrate, and tri(hydroxymethyl)aminomethane maleate buffers, even at low buffer concentrations (0.02 M). These buffers inhibit by affecting the binding of FDP.