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Metabolism and Enzymology

Regulation of Phosphoenolpyruvate Carboxylase Activity in Maize Leaves ¹

Research Institute for Photosynthesis, University of Sheffield, Sheffield, S10 2TN, United Kingdom, Department of Botany, University of Sheffield, Sheffield, S10 2TN, United Kingdom

The aim of this work was to investigate how light regulates the activity of phosphoenolpyruvate carboxylase in vivo in C₄ plants. The properties of phosphoenolpyruvate carboxylase were investigated in extracts which were rapidly prepared (in less than 30 seconds) from darkened and illuminated leaves of Zea mays. Illumination resulted in a significant decrease in the S_0.5(phosphoenolpyruvate) but there was no change in V_max. The form of the enzyme from illuminated leaves was less sensitive to malate inhibition than was the form from darkened leaves. At low concentrations of phosphoenolpyruvate, the activity of the enzyme was strongly stimulated by glucose-6-phosphate, fructose-6-phosphate, triose-phosphate, alanine, serine, and glycine and was inhibited by organic acids. The enzyme was assayed in mixtures of metabolites at concentrations believed to be present in the mesophyll cytosol in the light and in the dark. It displayed low activity in a simulated `dark' cytosol and high activity in a simulated `light' cytosol, but activities were different for the enzyme from darkened compared to illuminated leaves.

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