The behavior in vivo of transferrin in loading and unloading iron from its two sites was examined in rats. Radioiron entering the plasma from the gastrointestinal tract in iron-deficient, normal and iron-loaded rats did not differ in its subsequent tissue distribution between erythroid marrow and liver of normal recipients from a second isotope added to the same plasma in vitro. Loading studies in vitro were then carried out employing a reticulocyte incubation model designed to place one isotope predominantly on one site of transferrin, more available to the erythron, and the second isotope on the other site, more available to the liver. In 15 groups of animals in which 3 different iron salts were employed to load transferrin with iron, the mean isotope ratio in the erythron was 1.03 (+/-0.06 SD) and the mean liver ratio was 0.75 (+/-0.21 SD). It was found that the incubation of plasma with reticulocytes resulted in contamination of the plasma by radioactive hemoglobin. After allowance was made for hepatic uptake of radiohemoglobin in the 13 groups in which proper correction could be made, the isotope ratio in the liver became 0.97 (+/-0.17 SD). It is concluded that iron atoms from the two sites of transferrin have similar tissue distributions in vivo in the experimental situations examined.

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