Transferrin-receptor interaction and iron uptake by reticulocytes of vertebrate animals--a comparative study

Transferrin-receptor interactions and iron uptake were studied in eleven different species of vertebrate animals (3 eutherian mammals, 3 marsupials, 2 reptiles and 1 bird, amphibian and bony fish). In the initial experiments it was shown that the uptake of transferrin-bound iron by immature erythroid cells from marsupial and reptilian species occurs by receptor-mediated endocytosis as in other vertebrate animals. Reticulocytes were incubated with 125I-59Fe-labelled transferrins from heterologous species and the results for iron and transferrin uptake compared with those obtained with the homologous protein. Cells from eutherian mammals were able to take up transferrin and iron from other eutherians and from the bob-tailed lizard but not from marsupials and other submammalian species. With marsupials and reptiles a similar specificity was observed, and the marsupial cells could also utilize chicken transferrin but not vice versa. The results were extended by performing competition experiments in which the cells were incubated with radiolabelled homologous transferrin in the presence of increasing concentrations of non-radioactive heterologous transferrins. From the ability of the heterologous proteins to inhibit uptake of the homologous protein relative association constants (Ka1) for the transferrin-receptor interactions could be calculated. These Ka1 values reflected the patterns observed in the first series of experiments. These studies demonstrate that, although specificity exists in transferrin-receptor interactions throughout the range of vertebrate animals, in several instances reactivity between widely divergent species is also observed. Hence, structural similarities have been maintained throughout evolution. Nevertheless, no evidence of interaction between transferrin and its receptor from the two divisions of the Mammalia, the eutherians and the marsupials, was observed.