A fraction of Fe(III) in normal human serum is bound to both low molecular weight as well as protein ligands besides transferrin. Citrate was shown to be the major Fe(III)-binding substance in the low molecular weight fraction. Amino acids, sugars, and organic acids, such as ascorbate, pyruvate, and lactate, showed very little or no binding to Fe(III) in normal serum. Iron(III)-binding proteins other than transferrin were shown to be present in normal serum when the native serum with [59Fe(III)] was fractionated by (NH4)2SO4 and Sephadex G-150. The presence of these proteins was observed when trace amounts of Fe(III) were added to the normal serum and when the iron-binding capacity was saturated with Fe(III) to 50% and 100%. These proteins were eluted in the void volume of Sephadex G-150 and none of them corresponded electrophoretically to transferrin. The results of the gel filtration of a mixture of [131I]-transferrin and the proteins eluted in the void volume of Sephadex G-150 were strongly in favor of the Fe(III)-proteins as being neither transferrin aggregates nor transferrin adducts with other proteins. Immunoelectrophoresis of the Sephadex G-150 void volume proteins on agar gel against the antibody to transferrin revealed the absence of transferrin. The presence of at least six proteins in this fraction was shown by immunoelectrophoresis. Positive precipitin reactions were obtained with the antibodies to α2-macroglobulin, γG-globulin, γA-globulin, and γM-globulin. At least two more proteins in this fraction remained unidentified. When the same fraction containing [59Fe(III)] was treated with the whole antisera and the precipitates were counted for radioactivity, a typical antigen-antibody reaction curve was obtained as the antibody concentration was increased. Similar experiments with this fraction and antibodies to α2-macroglobulin, γG-globulin, γA-globulin, and γM-globulin failed to show any significant radioactivity in the precipitate. Since this fraction did not contain any transferrin, it was concluded that there are proteins besides transferrin which can act as ligands for Fe(III) in normal blood plasma.
Complement Interaction with Trypanosomatid Promastigotes in Normal Human Serum