Evidence for the functional heterogeneity of the two sites of transferrin in vitro.

A recently developed crossed immunoelectrophoretic method for displaying and quantitating the four possible molecular species of transferrin has been utilized to assess the relative effectiveness of each site of rabbit and human diferric transferrin in providing iron to rabbit reticulocytes. The site which appears to reside in the N-terminal half of the rabbit protein was found to be at least 5 times more effective than its counterpart. However, both sites may serve as iron donors in monoferric as well as diferric rabbit transferrins. It is also possible that iron may be removed from rabbit transferrin in pairwise as well as sequential fashion. In human diferric transferrin, the site in the C-terminal domain functions as the better iron donor for rabbit reticulocytes.