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Effects of Light and Inhibitors on Glutamate Metabolism in Leaf Discs of Vicia faba L

Sources of ATP for Glutamine Synthesis and Photoregulation of Tricarboxylic Acid Cycle Metabolism

^a Department of Plant Biology, University of Newcastle upon Tyne, Newcastle upon Tyne, NE1 7RU, England, United Kingdom

Metabolism of [¹⁴C]glutamate was studied in leaf discs of Vicia faba L. in light and in darkness. In white light glutamine was the main labeled product. In the dark label was principally in compounds closely associated with tricarboxylic acid cycle metabolism, predominantly aspartate. Entry of label from glutamate into tricarboxylic acid metabolism appeared to be at least partially by decarboxylation of glutamate to -amino butyric acid, followed by conversion to succinate. 3-(3,4-dichlorophenyl)-1, 1-Dimethylurea inhibited light-enhanced synthesis of glutamine and caused reversion toward the dark pattern of metabolism. Methionine sulfoximine severely inhibited glutamine synthesis and caused accumulation of labeled malate.

Monochromatic 650 nanometer light gave similar results to white light. Monochromatic light of 710 nanometers had a much smaller effect on glutamine synthesis but did significantly raise the ratio of labeled malate to aspartate. -Amino [¹⁴C]butyric acid was metabolized entirely via tricarboxylic acid cycle metabolism in light or dark, and in the light the ratio of labeled malate to aspartate was raised.

These results suggest that illuminated leaves metabolize glutamate to glutamine mainly in the chloroplasts. When chloroplastic glutamine synthesis fails to take place, either in darkness or in the presence of inhibitors, glutamate is apparently metabolized outside the chloroplast. Light lowers the NAD⁺ to NADH ratio outside the chloroplast, consequently altering the equilibrium of the malate dehydrogenase reaction. Alteration of the malate to aspartate ratio by 710 nanometer light suggests that ATP generated by photosystem I-dependent cyclic photophosphorylation may affect extrachloroplastic NAD⁺ to NADH ratios.

² Present address: Department of Biochemistry, University College Cardiff, P.O. Box 78, Cardiff, CF1 1XL, Wales, United Kingdom.