Reversible Light-Activation of Ribulose Bisphosphate Carboxylase/Oxygenase in Isolated Barley Protoplasts and Chloroplasts

Abstract

The enzyme ribulose-1,5-bisphosphate carboxylase/oxygenase displayed near-maximal activity in isolated, intact barley (H. vulgare L. cv. Pennrad) mesophyll protoplasts. The carboxylase deactivated 40-50% in situ when protoplasts were dark-incubated 20 min in air-equilibrated solutions. Enzyme activity was fully restored after 1-2 min of light. Addition of 5 mM NaHCO3 to the incubation medium prevented dark-inactivation of the carboxylase. There was no permanent CO2-dependent activation of the protoplast carboxylase either in light or dark. Activation of the carboxylase from ruptured protoplasts was not increased significantly by in vitro preincubation with CO2 and Mg2+. In contrast to the enzyme in protoplasts, the carboxylase in intact barley chloroplasts was not fully reactivated by light at atmospheric CO2 levels. The lag phase in C assimilation was not lengthened by dark-adapting protoplasts to low CO2 demonstrating that light-activation of the carboxylase was not involved in photosynthetic induction. Irradiance response curves for reactivation of the carboxylase and for CO2 fixation by isolated barley protoplasts were similar. The above results show that there was a fully reversible light-activation of the carboxylase in isolated barley protoplasts at physiologically significant CO2 levels.