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

AUTOR(ES)

Shieh, Yuh-Jang

RESUMO

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.

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