Uptake and release of iron from human transferrin.

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Abstract
Purified fractions of human apotransferrin, monoferric transferrins with Fe on the acid-labile binding site and on the acid-stable binding site, and diferric transferrin were prepared. The Fe loading and unloading behavior of these preparations was examined by isoelectric focusing. Fe release from the 2 monoferric transferrin preparations to human reticulocytes was of similar magnitude. In a mixture containing equal amounts of diferric and monoferric Fe, .apprx. 4 times the amount of Fe delivered by the monoferric species was delivered by the diferric species. Fe loading of transferrin in vitro showed a random distribution between monoferric and diferric transferrin. Among the monoferric transferrins, loading of the acid-labile binding sites was greater than that of the acid-stable binding sites. In vivo Fe distribution in normal subjects, as evaluated by in vitro-added 59Fe, gave similar results. Absorption of a large dose of orally administered Fe in Fe-deficient subjects resulted in a somewhat greater amount of diferric transferrin at low saturation and a somewhat smaller amount of diferric transferrin at higher saturations than would have been anticipated by random loading. In the human, Fe loading of transferrin may be essentially random. Unloading from the 2 monoferric transferrin species is of similar magnitude, but far less than that delivered by diferric transferrin.