Uptake and endocytic pathway of transferrin and iron in perfused rat liver

Uptake and distribution of transferrin and iron in perfused rat liver are dependent on perfusion temperature, time and uptake affinity. Transferrin passes at least two different compartments on its receptor-mediated recycling pathway, which are separable by centrifugation in a shallow Nycodenz gradient. Perfusion at lowered temperature (16 degrees C) is sufficient for internalization of transferrin and iron. Passage of radiolabelled iron to other than endosomal compartments as well as recycling of labelled transferrin are largely suppressed at this perfusion temperature, as much less is released by further perfusion with unlabelled transferrin than at 4 degrees C where the ligand is largely washed off the surface, or 37 degrees C, where the recycling pathway is operating. But also at lowered temperature only a part of the iron in endosomal fractions can be assigned to transferrin. A considerable part of the total uptake of transferrin and iron can be attributed to low-affinity mechanisms even at very low transferrin concentrations. Transferrin receptors are concentrated in endosomal fractions in comparison to fractions representing different plasma membrane domains of the liver. Endosomal fractions specifically display detergent-activated NADH-acceptor oxidoreductase which may be part of the iron uptake system.