Purification and assay of rubisco activase from leaves

Ribulose 1,5-bisphosphate carboxylase/oxygenase (rubisco) activase protein was purified from spinach leaves by ammonium sulfate precipitation and ion exchange fast protein liquid chromatography. This resulted in 48-fold purification with 70% recovery of activity and yielded up to 18 milligrams of rubisco activase protein from 100 grams of leaves. Based on these figures, the protein comprised approximately 2% by weight of soluble protein in spinach (Spinacia oleracea L.) leaves. The preparations were at least 95% pure and were stable when frozen in liquid nitrogen. Addition of ATP during purification and storage was necessary to maintain activity. Assay of rubisco activase was based on its ability to promote activation of rubisco in the presence of ribulose-1,5-bisphosphate. There was an absolute requirement for ATP which could not be replaced by other nucleoside phosphates. The initial rate of increase of rubisco activity and the final rubisco specific activity achieved were both dependent on the concentration of rubisco activase. The initial rate was directly proportional to the rubisco activase concentration and was used as the basis of activity. The rate of activation of rubisco was also dependent on the rubisco concentration, suggesting that the activation process is a second order reaction dependent on the concentrations of both rubisco and rubisco activase. It is suggested that deactivation of rubisco occurs simultaneously with rubisco activase-mediated activation, and that rubisco activation state represents a dynamic equilibrium between these two processes.