Stromal Free Calcium Concentration and Light-Mediated Activation of Chloroplast Fructose-1,6-Bisphosphatase

Light-mediated activation of fructose-1,6-bisphosphatase (EC 3.1.3.11) in intact spinach chloroplasts (Spinacia oleracea L.) is enhanced in the presence of 10(-5) molar external free Ca(2+). The most pronounced effect is observed during the first minutes of illumination. Ruthenium red, an inhibitor of light-induced Ca(2+) influx, inhibits this Ca(2+) stimulated activation. In isolated stromal preparations, the activation of fructose-1,6-bisphosphatase is already enhanced by 2 minutes of exposure to elevated Ca(2+) concentrations in the presence of physiological concentrations of Mg(2+) and fructose-1,6-bisphosphate. Maximal activation of the enzyme is achieved between 0.34 and 0.51 millimolar Ca(2+). The Ca(2+) mediated activation decreases with increasing fructose-1,6-bisphosphate concentration and with increasing pH. The data are consistent with the proposal that the illumination of chloroplasts leads to a transient increase of free stromal Ca(2+). In dark-kept chloroplasts the steady-state concentration of free stromal Ca(2+) is 2.4 to 6.3 micromolar as determined by null point titration. These observations support our previous proposal that light-induced Ca(2+) influx into chloroplasts does not only influence the cytosolic concentration of free Ca(2+) but also regulates enzymatic processes inside the chloroplast.