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Uptake of iron from transferrin by isolated rat-liver mitochondria mediated by phosphate compounds.

1. Isolated rat-liver mitochondria accumulate iron from transferrin at neutral pH by a mechanism which is markedly stimulated by small-molecular-weight polyphosphate compounds. The efficiency of the phosphate compounds decreases in the order: pyrophosphate > ATP > GTP > 2,3-bis-(phospho)glycerate > phosphate. 2. The uptake has a very low energy dependence, and it does not depend on the hydrolysis of ATP or the saturation of transferrin, but it increases in parallel to the concentration of iron to reach a saturation level of 800-1200 pmol iron/mg protein. 3. Following a chase with unlabelled transferrin (125I)-labelled transferrin bound to the mitochondria remains constant, whereas the progressive uptake of 59Fe levels off. 4. During reincubation of iron-loaded mitochondria up to 30% of the iron is mobilized in the presence of ascorbate and ATP (or apotransferrin). 5. The results suggest that iron is mobilized from transferrin by the polyphosphate compounds outside the mitochondria in a subsequent reaction.