Cooperative Protection of Glucose-6-Phosphate Dehydrogenase by Ligands in Extracts from Wheat Grains

Protein stabilities can be influenced by interactions with low molecular weight compounds. Using glucose-6-phosphate dehydrogenase as a model protein in this context, we found that it was stabilized by low concentrations (below 5 mM) of inorganic phosphate or sulfate. The pyridine nucleotide NADP (one of the substrates in the enzyme reaction) already protected glucose-6phosphate dehydrogenase activity at concentrations below 0.2 mM, whereas NAD was not effective. An excellent stabilization of glucose-6-phosphate dehydrogenase was achieved by the simultaneous addition of NADP and inorganic phosphate. Interactions of various ligands with different binding sites of the same enzyme can affect the susceptibility to proteolysis in a complex manner. Metabolic changes might influence selectively the catabolism of proteins in vivo by altering such interactions.