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Membranes and Bioenergetics

Involvement of Na⁺ in Active Uptake of Pyruvate in Mesophyll Chloroplasts of Some C₄ Plants ¹

Na⁺/Pyruvate Cotransport

Department of Biochemistry, Faculty of Science, Saitama University, Urawa 338, Japan, Institut für Biochemie der Pflanze, Universität Göttingen, Untere Karspüle 2, 3400 Göttingen, Federal Republic of Germany

An artificial Na⁺ gradient across the envelope (Na⁺ jump) enhanced pyruvate uptake in the dark into mesophyll chloroplasts of a C₄ plant, Panicum miliaceum (NAD-malic enzyme type) (J Ohnishi, R Kanai [1987] FEBS Lett 219:347). In the present study, ²²Na⁺ and pyruvate uptake were examined in mesophyll chloroplasts of several species of C₄ plants. Enhancement of pyruvate uptake by a Na⁺ jump in the dark was also seen in mesophyll chloroplasts of Urochloa panicoides and Panicum maximum (phosphoenolpyruvate carboxykinase types) but not in Zea mays or Sorghum bicolor (NADP-malic enzyme types). In mesophyll chloroplasts of P. miliaceum and P. maximum, pyruvate in turn enhanced Na⁺ uptake in the dark when added together with Na⁺. When flux of endogenous Na⁺ was measured in these mesophyll chloroplasts preincubated with ²²Na⁺, pyruvate addition induced Na⁺ influx, and the extent of the pyruvate-induced Na⁺ influx positively correlated with that of pyruvate uptake. A Na⁺/H⁺ exchange ionophore, monensin, nullified all the above mutual effects of Na⁺ and pyruvate in mesophyll chloroplasts of P. miliaceum, while it accelerated Na⁺ uptake and increased equilibrium level of chloroplast ²²Na⁺. Measurements of initial uptake rates of pyruvate and Na⁺ gave a stoichiometry close to 1:1. These results point to Na⁺/pyruvate cotransport into mesophyll chloroplasts of some C₄ plants.

¹ Supported by a Grant-in-Aid for Scientific Research from the Ministry of Education, Science and Culture of Japan, and also by grants from the Ministry of Agriculture, Forestry and Fishery, from Itoh Science Foundation, Japan, and from Deutsche Forschungsgemeinschaft, FRG.

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