Interaction of rat asialotransferrin with adult rat hepatocytes: Its relevance for iron uptake and protein degradation

Comparative studies with rat transferrin ( r Tf) and asialotransferrin (asialo‐ r Tf) were performed on suspended adult rat hepatocytes with the aim of elucidating the mechanism of enhanced hepatic Fe acquisition from asialo‐ r Tf observed previously in vivo. At low ligand concentrations (0.05–20 μ/ml), the cells bound more asialo‐ r Tf than r Tf. However, the excess binding was abolished by incubation either in the presence of 1.55 mg/ml of diferric r Tf or of 1 mg/ml of asialomucin. Following either treatment, asialo‐ r Tf and r Tf were bound to comparable extents. These findings indicate that both transferrin receptors and the hepatic galactose recognition system (lectin) are essential for preferential binding of asialo‐ r Tf by hepatocytes. The possibility is considered that the lectin facilitates capture of asialo‐ r Tf by the same binding sites that are normally available for r Tf rather than that it functions as an alternative pathway. In agreement with this view, asialo‐ r Tf could not be channeled into the lectin‐mediated degradative pathway by blocking Tf receptors with human Tf. Enhanced Fe uptake from asialo‐ r Tf was fully prevented by asialomucin and partially prevented by human Tf. The incomplete efficacy of human Tf in this regard supports reports in the literature about Fe uptake by the liver in a manner that is independent of Tf receptors. Rhesus asialo‐Tf was deployed to show that no recognition mechanism exists for heterologous asialo‐Tf in rat hepatocytes. The importance of using undenatured labeled proteins for studies with cells is demonstrated.