Light-dependent active uptake of pyruvate was reported in mesophyll chloroplasts of a C4plant,Panicum miliaceumOhnishi and Kanai (1987)Plant Cell Physiol. 28: 1. The present study tried to clarify the energy source of this active uptake. Preillumination of the mesophyll chloroplasts increased over tenfold their pyruvate uptake in the light and dark. This indicates that light itself is not essential for the enhancement. The pyruvate uptake capacity (the initial uptake rate) of the mesophyll chloroplasts increased on illumination and reached a steady-state level after a few minutes; this rise was faster under higher light intensities. When the chloroplasts were returned to darkness, the uptake capacity decayed with a half-life of about 1 min; this was independent of the light intensity of preillumination. Illumination of the chloroplasts also increased the stromal pH from about 7 to 8 and the stromal ATP level from about 5 to 15–25 nmol. (mg chl)−1. The change of the former during dark-to-light and light-to-dark transitions occurred within 2 to 5 min, while the change of the latter took place much faster within 1 min. The steady-state levels of the pyruvate uptake capacity and stromal pH were saturated at a light intensity of 3μE.m−2.s−1, while the ATP level increased with a further increase in the light intensity. The former two parameters also showed similar sensitivity to the inhibition by carbonylcyanide-m-chlorophenylhydrazone, while a higher concentration of the inhibitor was needed to reduce the ATP level. Nitrite at 4 mminhibited the light-dependent pyruvate uptake and stromal alkalization but had little effect on the stromal ATP level, while 2 mmarsenate decreased the stromal ATP without significant effects on pyruvate uptake and stromal pH. The good correlation of pyruvate uptake and stromal pH suggests that the active pyruvate uptake by the mesophyll chloroplasts is primarily driven by the pH gradient across the
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