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

The purification, properties and distribution of the formate activating enzyme, formyltetrahydrofolate synthetase, from plants were described in the preceding paper (5). Studies of the activation of the enzyme by magnesium and univalent cations are described in this paper. The mechanism of action of magnesium in biological catalysis is reasonably well understood. Most transphosphorylation reactions involving ATP require magnesium ions and it is generally assumed that the reactive species is a magnesium complex of ATP. Knowledge of the mechanism of action of potassium in metabolism, however, is limited. In recent years a number of enzyme catalyzed reactions have been shown to require K+, NH4+ or Rb+ for activity (4). Evans and coworkers (6, 11) have demonstrated a requirement for K+, NH4+ or Rb+ by pyruvate kinase and acetic thiokinase from tissues of higher plants. Optimal activity of these enzymes was produced by a K+ concentration of approximately 0.04 M which is near the normal level of K+ concentration in plant tissues. Bertino et al. (1) reported that optimal activity of formyltetrahydrofolate synthetase from erythrocytes required the presence of K+ or NH4+. A detailed study of the K+ requirement was not made. The purpose of the experiments reported in this paper was to study the activation of the enzyme from plant tissues by potassium ions and to determine the effect of potassium deficiency in plants on the rate of the reaction catalyzed by the enzyme.