Are lysosomes directly involved in the iron uptake by reticulocytes?

The mechanism by which iron is transferred from the plasma protein transferrin into erythroid precursors for incorporation in heme is not completely understood. To show a direct functional role of lysosomes in the process of iron uptake we tried to isolate lysosomes from reticulocytes, which have been incubated with 125ITf59Fe. However, with various cell fractionating techniques described for liver cells no pure lysosomes from reticulocytes could be obtained. Fluorescence and electron microscopy showed that reticulocytes hardly contain well-defined lysosomes. There are several indications that in reticulocytes acid vacuoles instead of lysosomes are involved in the removal of iron from endocytosed transferrin. The presence of apoTf, monoferric TfFe(A), monoferric TfFe(B) in the medium after incubation of reticulocytes with diferric transferrin, together with the fact that both iron binding sites of transferrin release their iron at pH present in acid vacuoles, suggests a second mechanism of iron uptake by reticulocytes, in which acid vacuoles are not involved.