Uptake of iron from N-terminal half-transferrin by isolated rat hepatocytes. Evidence of transferrin-receptor-independent iron uptake.

The aim of the present study was to determine if human N-terminal half-transferrin (N- fragment), prepared by thermolysin cleavage of diferric transferrin, would bind to the rat hepatocyte transferrin receptor and donate iron to the cell. Competition experiments between 125I-labelled N-fragment and diferric transferrin revealed no receptor binding of the half-transferrin. Still, the N-fragment delivered iron to the cells in amounts approximately 30-fold above what could be accounted for by uptake of the fragment itself. The rate of cellular iron uptake from the fragment was comparable to what is seen with the intact transferrin. The uptake of 125I-labelled N-fragment was not inhibited by excess non-radioactive diferric transferrin. By comparison, the uptake of 59Fe from the N-fragment was inhibited 70% by excess nonradioactive diferric transferrin. This suggests that iron derived from diferric transferrin competes with the iron derived from the N-fragment for a common transport pathway. Although some cellular degradation of the N-fragment occurred, the extent of degradation was too low to explain the amount of iron accumulated by the cells. The results show that the hepatocyte has an effective transferrin-receptor-independent mechanism for accumulation of iron from transferrin.