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In this communication, we show that the plant uncoupling mitochondrial protein (PUMP) present in potato tuber mitochondria is induced by aging at 28³C and that this induction is strongly stimulated when the potato tubers are stored at low temperature (4³C). PUMP activity was detected by the degree of linoleic acid (LA)-induced ATP-sensitive mitochondrial uncoupling measured as a function of the decrease in membrane potential (v v8 8). The PUMP content was evaluated by immunoblot analysis using polyclonal antibodies raised against potato PUMP that specifically detected a 32 kDa band. In agreement with the effect of LA on v v8 8, the content of the 32 kDa band increased during storage and was stimulated by low temperature. These results support the proposed role of PUMP in plant thermogenesis and possibly in fruit ripening and senescence.z 1999 Federation of European Biochemical Societies.

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Background. We have previously shown that chronic metabolic acidosis, induced in rats by NH 4 Cl feeding, leads to nephron hypertrophy and to a decreased water-salt reabsorption by the kidneys. Since mitochondria are the main source of metabolic energy that drives ion transport in kidney tubules, we examined energy-linked functions (respiration, electrochemical membrane potential and coupling between respiration and ADP phosphorylation) in mitochondria isolated from rat kidney and liver at 48 h after metabolic acidosis induced by NH 4 Cl. Methods. Mitochondria isolated from the kidneys and liver of metabolic acidotic rats, induced by NH 4 Cl, was used to study of the oxygen consumption by Clarktype electrode, mitochondrial electrical transmembrane potential estimated by the safranine O method and the variations in free medium Ca 2+ concentrations examined by absorbance spectrum of Arsenazo III set at the 675-685 nm wavelength pair. Results. Whole kidney and liver mitochondria isolated from 48 h acidotic rats presented higher resting respiration, lower respiratory control and a lower ADP/O ratio than controls. These differences in mitochondrial coupling, between respiration and oxidative phosphorylation (ATP synthesis), were totally corrected when experiments were carried out in the presence of carboxyatractyloside, GDP and BSA, indicating that mitochondrial uncoupling proteins are more active in acidotic rat kidneys. Interestingly, determination of Ca 2þ transport demonstrated a faster rate of initial Ca 2þ uptake by acidotic kidney mitochondria, which resulted in a lower concentration of extra-mitochondrial Ca 2þ under steady-state conditions (Ca 2þ set point) when compared with control mitochondria.In contrast, there were no significant differences in the rates of Na þ or ruthenium red induced Ca 2þ efflux. Conclusions. We suggest that the mild uncoupling and higher Ca 2þ accumulation represents an adaptation of the mitochondria to cope with conditions of oxidative stress and high cytosolic Ca 2þ , which are associated with a decreased efficiency of oxidative phosphorylation that may explain, at least in part, the striking natriuresis observed under chronic acidosis. Finally, there were no changes in Ca 2þ transport or coupling in liver mitochondria isolated from the acidotic rats.