Distinction between binding and endocytosis of human asialo-transferrin by the rat liver

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Abstract
The ability of the rat liver to bind and endocytose human asialo-transferrin was investigated in vivo. Asialo-transferrin was separated from incompletely desialylated transferrin and neuraminidase by chromatography before being labeled with 125I. Plasma readioactivity curves and hepatic radioactivity contents measured over a 1270-fold dose range led to the following observation. At the lowest dose (0.4 .mu.g/100 g body wt), the distribution of asialo-transferrin between plasma and liver resembled a reversible reaction reaching equilibrium in about 20 min. After 35 min, 93% of the dose was recovered with the plasma and liver as protein-bound radioactivity. Most of the asialo-transferrin associated with the liver could be displaced by asialo-orosomucoid, indicating that binding of asialo-transferrin to the galactose-specific lectin on the plasma membrane of hepatocytes was not followed by a signal for endocytosis. A range of doses, up to an average of 509.2 .mu.g of asialo-transferrin per 100 g body wt, resulted in progressive increments in asialo-transferrin catabolism, as evidenced by lower dose recoveries and increased concentrations of non-protein-associated radioactivity in the liver and plasma volume. These observations indicate that binding and endocytosis of human asialo-transferrin by the rat hepatocyte are distinct phenomena. Individual asialo-transferrin molecules, although readily bound by the hepatic lectin, lack either the quantity or spacing of terminal galactose residues necessary for triggering endocytosis. Although endocytosis is induced by several asialo transferrin molecules acting synergistically, preliminary experiments with asialo-glycopeptides and other substances have so far failed to provide further insight into the chemical basis of the signal for endocytosis.