Proteolytic Inactivation of Glutamine Synthetase in Extracts From Wheat Leaves: Effects of pH, Inorganic Ions and Metabolites

Glutamine synthetase in extracts from young wheat (Triticum aestivum L.) leaves was quite stable at pH 7.0-9.0, whereas it was remarkably more labile below and above this range. Added extract from senescing wheat leaves accelerated the inactivation over the whole investigated pH range (6.0-10.5) and was most effective around pH 8.5-9.0. At pH 7.5, glutamine synthetase inactivation by endogenous or other supplied endopeptidases was delayed in the presence of magnesium sulfate, magnesium chloride and L-lysine, while potassium chloride stabilised only slightly. No major effect was caused by the addition of sucrose, L-alanine, L-serine or glycine. These results, and the fact that the stabilities of various enzymes are affected differently by the same solutes, suggest stabilising interactions with the substrate protein (glutamine synthetase) rather than effects on the inactivating endopeptidases. From immunoblots, it became evident that the inactivation of glutamine synthetase was paralleled by the degradation of the intact subunit. A smaller fragment was detected on immunoblots during the catabolism of this enzyme. Stabilising solutes retarded the loss of the intact subunit and the formation of the fragment. Solute concentrations must be considered as factors regulating the catabolism of a particular protein by given proteolytic activities.