Role of F1F0-ATP synthase and uncoupling protein in mitochondrial respiratory uncoupling of ripened avocado fruits

The present study was carried out to determine the contribution of the F1F0-ATP synthase, the ATP/ADP carrier (AAC) and a putative plant uncoupling mitochondrial protein (PUMP) to the increased respiratory activity of ripened avocado fruits. We have compared the β-subunit content of F1 and AAC, determined by photoaffinity labelling with 8-azido-[α-32P]ATP, and the transmembrane electrical potential difference (ΔΨ), established by substrate oxidation, in mitochondria isolated from pre- and postclimacteric avocado fruits. The β-subunit content of F1 decreased, while that of AAC did not change, with ripening. The ΔΨ established by succinate oxidation of postclimacteric avocado mitochondria was very low and could be substantially increased by oligomycin, which inhibits H+ transport through F0, and by the purine nucleotides GTP or ATP, which are known to inhibit H+ transport through the uncoupling protein present in brown adipose tissue and potato tuber mitochondria. Oligomycin and GTP had no effect on the ΔΨ of preclimacteric avocado mitochondria. Carboxyatractyloside and bongkrekic acid, specific inhibitors of AAC, had no effect on the ΔΨ of pre- or postclimacteric avocado mitochondria. These results suggest that a PUMP-like protein may exist in mitochondria of postclimacteric avocado fruits, and that this protein and the F0 from β-detached ATP synthase, but not the AAC, could provide pathways for energy dissipation in ripened avocado fruit mitochondria.