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Photosynthetic Metabolism of Aspartate in Mesophyll and Bundle Sheath Cells Isolated from Digitaria sanguinalis (L.) Scop., a NADP⁺-Malic Enzyme C₄ Plant

Department of Biochemistry, University of Georgia, Athens, Georgia 30602, Richard B. Russell Agricultural Research Center, United States Department of Agriculture, Science and Education Administration, Agricultural Research, Athens, Georgia 30613

Mesophyll cells and bundle sheath strands isolated from leaves of the C₄ plant Digitaria sanguinalis (L.) Scop. are capable of utilizing aspartate as a Hill oxidant. The resulting O₂ evolution upon illumination depends on the presence of 2-oxoglutarate, is inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, and is stimulated by methylamine. The rate of aspartate-dependent O₂ evolution with mesophyll cells was similar to those with phosphoenolpyruvate + CO₂ or with oxalacetate. Amino-oxyacetate, an inhibitor of aspartate aminotransferase, inhibited the aspartate-dependent O₂ evolution. Aspartate aminotransferase and NADP⁺ -malate dehydrogenase are located in the mesophyll chloroplasts. These data suggest that aspartate is converted to oxalacetate via aspartate aminotransferase in the chloroplasts of mesophyll cells and that oxalacetate is subsequently reduced to malate, which is coupled to the photochemical evolution of O₂. This suggestion is further verified by the inhibition of phosphoenolpyruvate-dependent ¹⁴CO₂ fixation by aspartate + 2-oxoglutarate, which presumably acts as oxalacetate and competes with phosphoenolpyruvate + CO₂ for NADPH. DL-Glyceraldehyde inhibited aspartate-dependent O₂ evolution in the bundle sheath strands but not in the mesophyll cells. The data indicate that aspartate may be converted to malate in both mesophyll and bundle sheath cells. In NADP⁺ -malic enzyme species, aspartate may exist as a C₄-dicarboxylic acid reservoir which can contribute to the C₄ cycle through its conversion to malate.

² Present address: Botany Department, Washington State University, Pullman, WA 99164.