The soluble form of the membrane-bound transferrin homologue, melanotransferrin, inefficiently donates iron to cells via nonspecific internalization and degradation of the protein.

Melanotransferrin (MTf) is a membrane-bound transferrin (Tf) homologue found particularly in melanoma cells. Apart from membrane-bound MTf, a soluble form of the molecule (sMTf) has been identified in vitro[Food, M.R., Rothenberger, S., Gabathuler, R., Haidl, I.D., Reid, G. & Jefferies, W.A. (1994) J. Biol. Chem.269, 3034-3040] and in vivo in Alzheimer's disease. However, nothing is known about the function of sMTf or its role in Fe uptake. In this study, sMTf labelled with 59Fe and 125I was used to examine its ability to donate 59Fe to SK-Mel-28 melanoma cells and other cell types. sMTf donated 59Fe to cells at 14% of the rate of Tf. Analysis of sMTf binding showed that unlike Tf, sMTf did not bind to a saturable Tf-binding site. Studies with Chinese hamster ovary cells with and without specific Tf receptors showed that unlike Tf, sMTf did not donate its 59Fe via these pathways. This was confirmed by experiments using lysosomotropic agents that markedly reduced 59Fe uptake from Tf, but had far less effect on 59Fe uptake from sMTf. In addition, an excess of 56Fe-labelled Tf or sMTf had no effect on 125I-labelled sMTf uptake, suggesting a nonspecific interaction of sMTf with cells. Protein-free 125I determinations demonstrated that in contrast with Tf, sMTf was markedly degraded. We suggest that unlike the binding of Tf to specific receptors, sMTf was donating Fe to cells via an inefficient mechanism involving nonspecific internalization and subsequent degradation.