Double labeling of transferrin: tritium labeling of sialic acid and 125I or 59Fe labeling of the protein moiety.

A method is described in which the glycoprotein transferrin was double labeled. Its sialic acid residues were labeled with 3H through a consecutive oxidation-reduction technique utilizing tritiated NaBH4. Its protein moiety was labeled with either 125I or 59Fe. Incubation of this double-labeled molecule at 4 degrees C with K562 cells gave overlapping curves, indicating identical patterns of binding for all labels. At 37 degrees C, 3H and 125I demonstrated identical patterns while 59Fe was cummulatively retained. This method can be used to follow the fate of other glycoproteins and their possible desialation in vivo.