Regulation of C4 Photosynthesis : Stability and Other Characteristics of the ADP-ATP-Dependent Inactivation of Pyruvate,P_I Dikinase in Maize Chloroplast Extracts

Abstract

Recovery of the capacity for the ADP plus ATP-dependent inactivation of pyruvate,P_I dikinase in maize mesophyll chloroplast extracts was highly variable and the remaining enzyme inactivating activity was rapidly lost during subsequent storage at both 0°C and 25°C. Usually no more than 20% of the original activity remained after about 45 min at 25°C or 120 min at 0°C. These losses were largely or completely prevented by the addition of 0.4 mM orthophosphate or 0.1 mM ATP and probably also ADP. AMP or phosphoenolpyruvate at 0.1 mM had little or no effect. Following these studies an analysis of mesophyll cell extracts showed the protein catalysing inactivation of pyruvate,P_I dikinase to be exclusively located in chloroplasts and an assay procedure was developed to allow the monitoring of this activity during fractionation.