Three types of human asialo-transferrin and their interactions with the rat liver

Three types of asialo-transferrin were obtained from immunologically pure human transferrin by chromatography on DEAE-cellulose, followed by desialylation and affinity chromatography on a column of the immobilized asialo-glycoprotein-binding hepatic lectin from rabbit liver. Of the asialo-transferrins, type 1 was derived from the principal DEAE-cellulose chromatographic component of transferrin, i.e. the one that contains two biantennary glycans. The two other asialo-transferrins (types 2 and 3) were derived from a minor DEAE-chromatographic transferrin component, which is assumed to possess one biantennary and one triantennary glycan. The three asialo-transferrin types were indistinguishable by electrophoretic mobility, but they were readily distinguished on the basis of their binding strengths to the hepatic lectin in intact rats. Glycan structures responsible for the difference in binding strengths between asialo-transferrin types 2 and 3 are not known. Metabolic studies in rats showed that none of the individual asialo-transferrin types was capable of generating a signal for endocytosis at low doses (<1mug/100g body wt.) and, consequently, most of the injected protein was recoverable with the plasma and the liver 35min after injection. However, endocytosis and catabolism of each asialo-transferrin type was readily induced by injecting a larger dose (50-250mug/100g body wt.) of unlabelled asialo-transferrin of the same type or of a different type a short interval after the labelled dose. These findings support the view that the dose-dependent uptake of human asialo-transferrin by the hepatocyte, as established in an earlier study with asialo-transferrin made from whole transferrin [Regoeczi, Taylor, Hatton, Wong & Koj (1978) Biochem. J.174, 171-178], also holds for these asialo-transferrin subfractions. Furthermore, the present studies indicate that asialo-transferrins of different carbohydrate compositions are capable of synergistically promoting endocytosis of each other.