Binding of a Phosphorylated Inhibitor to Ribulose Bisphosphate Carboxylase/Oxygenase during the Night

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Abstract
The activity of ribulose-1,5-bisphosphate carboxylase/oxygenase was measured at various times during the purification of the enzyme from leaves of Nicotiana tabacum which were collected either 1 hour before the start of the photoperiod (predawn) or in the middle of the photoperiod (midday). The activity of the enzyme in extracts of the predawn leaves (0.8 units/mg enzyme) was consistently about 2-fold lower than that measured in extracts of midday leaves (1.7 units/mg enzyme). The activity of the predawn enzyme was increased to that of the midday enzyme following removal of CO2 and Mg2+ (deactivation). (NH4)2SO4 precipitation, or incubation in SO42- (18 millimolar required for one-half maximal increase). Following purification to > 95% homogeneity, the predawn enzyme was found to have .apprx. 0.5 moles of bound organic phosphate per mole of enzyme active sites, while the midday enzyme had only .apprx. 0.08 moles of bound organic phosphate per mole of enzyme active sites. Deactivation of the predawn enzyme or treatment with 0.2 molar SO42- resulted in the removal of most of the bound organic phosphate. These findings support the hypothesis that following the night period about 50% of the enzyme is catalytically inactive because of the tight-binding of a small molecular weight, phosphorylated inhibitor at the active site.