Partial resialylation of human asialotransferrin type 3 in the rat

After the injection of a small dose (1 micrograms/100 g of body weight) of 125I-labeled human asialotransferrin type 3 in rats, the radioactivity became rapidly associated with the liver. However, during the ensuing 12 hr a significant fraction of the dose returned to the circulation as protein-bound 125I. The protein released by the liver was indistinguishable by gel filtration from the original preparation and was precipitable by an antiserum to human transferrin. Nevertheless, it no longer bound to the immobilized Gal/GalN-specific lectin from rabbit liver. However, binding could be restored to a large extent by treatment with neuraminidase, indicating that the loss of binding was due to resialylation. Changes in the electrophoretic mobility of asialotransferrin released by the liver showed that resialylation was partial--i.e., it involved the attachment of two or three sialyl residues. From analysis by deconvolution of the plasma curve of partially resialylated asialotransferrin it was calculated that the liver "repaired" this way approximately one asialotransferrin molecule out of four. Plasma clearance of partially resialylated asialotransferrin was similar to that of nondesialylated transferrin.