Conditions Required for the Rapid Activation In Vitro of the Chloroplast Fructose-1,6-bisphosphatase.

Conditions required for the reductive activation of purified, spinach chloroplast fructose-1,6-bisphosphatase (EC 3.1.3.11) have been determined in vitro. Full reductive activation was observed only when fructose-1,6-bisphosphate and Mg(2+) were present at the same time as the reducing agent (dithiothreitol). Reduction in the absence either of fructose-1,6-bisphosphate or of Mg(2+) slowly and irreversibly inactivated the enzyme. The concentration of fructose-1,6-bisphosphate that must be present during reduction for maximum activation depends upon the divalent cation present: it is highest with Mg(2+), lower with Ca(2+), and lowest when both Mg(2+) and Ca(2+) are present. A scheme for the reductive activation and inactivation of the enzyme is presented.