Formic Acid Activation in Plants. II. Activation of Formyltetrahydrofolate Synthetase by Magnesium, Potassium, and Other Univalent Cations

Abstract

Formyltetrahydrofolate synthetase from plants was shown to require a univalent cation for activity. The univalent cation requirement was satisfied by K+, NH4 +, Rb+, or Na+, and maximum activity was obtained with 0.2 to 0.3 M concentrations of these cations. Na+ was least effective in activating the enzyme. Low enzyme reaction rates of extracts from K+-deficient plants were restored to near normal levels by adding 0.2 M K+ to the assay mixture. The influence of several anions on activity was determined. Activity in the presence of NO3- was considerably less than activity in the presence of other anions tested. Increasing the K+ concentration of the assay medium decreased the Km with respect to tetrahydrofolate and increasing the concentration of tetrahydrofolate decreased the Ka with respect to K+. These results suggest that K+ may function in the binding of tetrahydrofolate to the enzyme. K+ concentration had little effect on the Km values with respect to ATP and formate. Mg++ was also required by the enzyme. A Mg++ concentration of 1 x 10 -3 to 2 x 10-3 M was required for maximum activity. Mn++ and Ca++ could replace Mg++ but were not as effective as Mg++ in activating the enzyme.