Light activation of fructose bisphosphatase in isolated spinach chloroplasts and deactivation by hydrogen peroxide : A physiological role for the thioredoxin system.

Spinach chloroplast fructose bisphosphatase (EC 3.1.3.11.) exists in both oxidised and reduced forms. Only the latter has the kinetic properties that allow it to function at physiological concentrations of fructose 1,6-bisphosphate and Mg(2+). Illumination of freshly prepared type A chloroplasts causes a conversion of oxidised to reduced enzyme. The rate of this conversion does not limit the rate of CO2 fixation. In the dark the reduced enzyme partially reverts back to the oxidised form. If catalase is omitted from the reaction medium the rate of CO2 fixation by chloroplasts is decreased and seems to be limited by the rate of conversion of the enzyme to the reduced form. The physiological significance of the light dependent generation of dithiol compounds (such as thioredoxin) within chloroplasts is discussed.