Endocytosis of asialoglycoprotein-enzyme conjugates by hepatocytes

Desialylated glycoproteins were covalently linked to two cytochemically detectable enzymes, horseradish peroxidase or tyrosinase, and injected intravenously in amounts of approximately 0.5 per cent of total plasma glycoproteins into rats. Comparative studies of the rates of disappearance and distribution of free enzyme and conjugates established that recognition of the conjugates by the plasma membranes of hepatocytes was due to the exposure of the terminal galactose residue of the desialylated glycoproteins. At 1 minute after injection, reaction products of the enzyme markers were seen in coated pits and vesicles, elongated pinocytic channels and pleomorphic vesicles, at or close to the sinusoidal surface of hepatocytes. Vesicles containing reaction products were also observed along the lateral surfaces of hepatocytes. By 10 minutes, reaction products were seen in residual bodies near the biliary poles of hepatocytes. These studies confirm the existence of hepatocellular channels previously seen only with large excess of hemoglobin or following partial hepatectomy. They also indicate that the specific receptor for asialoglycoproteins is not restricted to the sinusoidal surfaces of hepatocytes and that transport to the catabolic sites proceeds via cytoplasmic channels and vesicles.