Studies of the binding of different iron donors to human serum transferrin and isolation of iron-binding fragments from the N- and C-terminal regions of the protein

Abstract

Trypsin digestion of human serum transferrin partially saturated with Fe(III)-nitrilotriacetate at pH 5.5 or pH 8.5 produces a carbohydrate-containing Fe-binding fragment of MW 43,000. When Fe(III) citrate, FeCl3, Fe(II) ascorbate and (NH4)2SO4,FeSO4 are used as Fe donors to saturate the protein partially at pH 8.5, proteolytic digestion yields a fragment of MW of 36,000 that lacks carbohydrate. The 2 fragments differ in their antigenic structures, amino acid compositions and peptide maps. The fragment with MW 36,000 was assigned to the N-terminal region of the protein and the other to the C-terminal region. The distribution of Fe in human serum transferrin partially saturated with various Fe donors was examined by electrophoresis in urea/polyacrylamide gels and the 2 possible monoferric forms were unequivocally identified. The site designated A on human serum transferrin was assigned to the C-terminal region of the protein and the B site to the N-terminal region. The distribution of Fe on transferrin in human plasma was determined.