Diacytosis of human asialotransferrin type 3 by isolated rat hepatocytes

In suspensions of freshly isolated hepatocytes, asialotransferrin type 3 became rapidly bound by the asialoglycoprotein-binding hepatic lectin. Suspended hepatocytes, just as the liver of intact animals, catabolized asialotransferrin in a concentration-dependent manner. At low asialotransferrin concentrations (0.4-1 nM), the fraction of labeled protein degraded was much smaller than found with comparable concentrations of asialofetuin in an earlier study (Tolleshaug, H., Berg, T., Nilsson, M., and Norum, K. R. (1977) Biochim. Biophys. Acta 499, 73-84). The fraction of endocytosed asialotransferrin that was degraded, could, however, be substantially increased by raising the concentration of asialotransferrin the medium. Release studies using a chelating agent or competitive inhibitors of the binding reaction showed that at low asialotransferrin concentrations, hepatocytes exocytose the preponderance of the intracellular asialotransferrin with a half-life of approximately 20 min. This novel observation raises the possibility that lysosomal homing of an endosome transporting asialoglycoprotein requires an intracellular target signal.