Regulation of fructose-1,6-bisphosphatase activity in leaves

Fructose-1,6-bisphosphatase (EC 3.1.3.11) activity increased markedly (greater than 10-fold) upon illumination of wheat leaves. Darkening caused a relatively slow but complete reversal of light activation. The effects of O₂ and CO₂ concentration and light intensity on fructose-bisphosphatase activation were measured. In ratelimiting light, 2% O₂ stimulated enzyme activity, whereas varying the CO₂ concentration had little effect. In saturating light, lowering the oxygen tension had no effect, but CO₂ at near-saturating concentrations for photosynthesis inhibited enzyme activity. Dark inactivation of the enzyme was completely prevented by incubation of leaves in N₂, but was facilitated by O₂, indicating that O₂ is the major oxidant in darkened leaves. It is argued that while fructose bisphosphatase is redox-regulated in leaves, modulation of enzyme activity by this mechanism is unlikely to contribute to the regulation of CO₂ fixation in leaves.