Iron Uptake by Rabbit Reticulocytes

S ummary . In keeping with present concepts of internal iron exchange, it is thought that the two iron binding sites of transferrin molecules bind and donate iron in a similar manner. Since recent data, however, have suggested that iron attached at one iron binding site may be more readily available to cells than iron bound to the other site, a further study of in vitro iron exchange between transferrin and reticulocytes has been undertaken. Transferrin samples were first incubated with reticulocytes to reduce the transferrin iron saturation; rates of iron uptake from incubated transferrin samples and appropriate non‐incubated controls with the same iron saturation were then compared. At 24% transferrin iron saturation, iron uptake from incubated transferrin was significantly less than from the control; at 48% transferrin iron saturation, iron uptake was the same from incubated transferrin and controls. Two transferrin samples of equal protein concentration were prepared; each was 50% iron saturated. One sample contained twice the proportion of iron saturated transferrin molecules (2Fe‐transferrin) present in the other sample. Iron uptake by reticulocytes was decreased by 22–25% from the sample with less 2Fe‐transferrin molecules. Thus iron was exchanged very rapidly between 2Fe‐transferrin and reticulocytes. It is proposed that the preliminary incubation of transferrin samples with reticulocytes caused a marked reduction in their 2Fe‐transferrin molecules which, at low levels of transferrin iron saturation, resulted in the incubated samples containing less 2Fe‐transferrin than the controls. In these circumstances, iron uptake from incubated transferrin was decreased when compared with the control.