Genetical evidence for synthesis of transferrin in the foetus

Studies on the transferrins of adult serum, cord serum, and cerebrospinal fluid. The effect of neuraminidase

Nine of the twelve known variants of human transferrin have been resolved by the action of neuraminidase into stepwise patterns of four additional slower moving components whose relative intensities depended upon the concentration of enzyme. These components appeared to represent the stepwise removal of the four sialic acid residues from the transferrin molecule, and at large enzyme concentrations, almost all of the transferrin was reduced to the position of the slowest moving component. In contrast, the electrophoretic mobilities of haptoglobin, ceruloplasmin, and α₂-macroglobulin showed a gradual decrease with increasing neuraminidase concentration. The transferrins of chimpanzees, rhesus and cynomolgus monkeys, and cattle were resolved by neuraminidase into two slower moving components. These experiments suggested that the primate and cattle transferrins contained only two sialic acid residues accessible to the enzyme. Transferrins C, B₂, and D₁ and a cynomolgus monkey transferrin were purified from serum by starch block electrophoresis and cellulose chromatography. Ultracentrifugal analysis could detect no difference in sedimentation rate between transferrin C, the primate transferrin, and neuraminidase-treated transferrin C. The human transferrins showed no variation in amino acid composition, but the cynomolgus transferrin was approximately 20 per cent higher in serine content and 50 per cent lower in glucosamine than human transferrin C. Reactions of antigenic identity were obtained among five human transferrin variants but a reaction of only partial identity was obtained between transferrin C and the cynomolgus transferrin. The transferrin pattern of cord blood showed a prominent band in the position of transferrin C, accompanied by four faint slower moving bands which coincided with the four transferrin components produced by the action of neuraminidase on transferrin C. The transferrin pattern of cerebrospinal fluid in individuals homozygous for serum transferrin C showed two principal components, one of which appeared to contain no sialic acid. Haptoglobin, ceruloplasmin, and α₂-macroglobulin were also present in cerebrospinal fluid.

Localization of transferrin on the surface of the human placenta by electron microscopic immunocytochemistry

During the course of gestation, a large amount of iron is transferred rapidly and unidirectionally from mother to fetus across the placenta. It has been postulated that one of the first steps involved in placental iron transfer involves binding of the maternal transferrin-iron complex to the surface of the placenta and the subsequent removal of iron and release of transferrin back into the maternal circulation. To determine if transferrin is present on the surface of human placental villi, two different immunocytochemical methods have been used: (1) an unlabeled antibody, peroxidase-antiperoxidase (PAP) method utilizing rabbit antiserum to human transferrin, goat anti-rabbit IgG and rabbit peroxidase-antiperoxidase complex or; (2) a peroxidase-labeled antibody method utilizing goat antiserum to human transferrin and peroxidase-conjugated rabbit anti-goat IgG. The peroxide was then localized by incubation in a diaminobenzidine-hydrogen peroxide medium. Examination of the tissue in the electron microscope revealed the reaction product deposited as discrete patches or particles on the microvillous surface of human syncytial trophoblast. Controls using non-immune serum or an antiserum adsorbed with purified human transferrin showed no reaction product on the surface. The results provide morphological confirmation for the presence of transferrin on the surface of human syncytial trophoblast lining the maternal blood spaces.