Evidence for receptor-mediated hepatic uptake of pullulan in rats

Fluorescein-labeled pullulan (FP-60; MW 58,200) was prepared by reaction with FITC according to the method of de Belder and Granath. The hepatic distribution of FP-60 was examined using a specific high-performance size-exclusion chromatography. Intravenously administered FP-60 was rapidly eliminated from the blood circulation followed by an appreciable distribution to the liver. A marked dose-dependency was seen in the hepatic uptake of FP-60 which was markedly reduced by the coadministration of both asialofetuin and arabinogalactan. Measurement of the hepatocellular localization demonstrated the overwhelming distribution of FP-60 in the parenchymal liver cell fraction. Furthermore, microscopic examination revealed that FP-60 was effectively endocytosed by the parenchymal liver cells. Radiolabeled pullulan ([¹²⁵I]P-60) was prepared by ¹²⁵I-labeling the tyramine derivative of pullulan which was synthesized by the cyano-transfer method. [¹²⁵I]P-60 was predominantly accumulated in sliced rat liver tissue at 37°C, which was drastically inhibited by the addition of both asialofetuin and arabinogalactan. The kinetic parameters of the specific binding of [¹²⁵I]P-60 to monolayered hepatocytes at 0°C were almost identical to those for asialofetuin. The binding of [¹²⁵I]P-60 to isolated parenchymal cells was significantly inhibited by arabinogalactan and asialofetuin, however dextran, the same glucan as pullulan, did not affect the binding of [¹²⁵I]P-60. It was found that pullulan, which is bound to the asialoglycoprotein receptor with high affinity, is subsequently internalized to the hepatocyte via receptor-mediated endocytosis.