Stimulatory effect of chloride ions on NADP malic enzyme from leaves of two C₄ species of cyperus

NADP malic enzyme (EC 1.1.1.40) from leaves of two C₄ species of Cyperus (C. rotundus and C. brevifolius var leiolepis) exihibited a low level of activity in an assay mixture that contained low concentrations of Cl^-. This low level of activity was markedly enhanced by increases in the concentration of NaCl up to 200 mM. Since the activity of NADP malic enzyme was inhibited by Na₂SO₄ and stimulated by relatively high concentration of Tris-HCl (50-100 mM, pH 7-8), the activation of the enzyme by NaCl appears to be due to Cl^-. Variations in the concentration of Mg²⁺ affected the K_A (the concentration of activator giving half-maximal activation) for Cl^-, which decreased from 500 mM to 80 mM with increasing concentrations of Mg²⁺ from 0.5 mM to 7 mM. The K_m for Mg²⁺ was decreased from 7.7 mM to 1.3 mM with increases in the concentration of NaCl from zero to 200 mM, although the increase of V_max was not remarkable. NADP malic enzyme from Cyperus, being similar to that from other C₄ species, was able to utilize Mn²⁺. The K_m for Mn²⁺ was 5 mM, a value similar to that for Mg²⁺. The addition of 91 mM NaCl markedly decreased the K_m for Mn²⁺ to 20 +M. NADP malic enzyme from Setaria glauca, which contains rather less Cl^- than other C₄ species, was inactivated by concentrations of NaCl above 20 mM, although slight activation of the enzyme was observed at low concentrations of NaCl at pH7.6.