| Abstract: | When treated with formaldehyde, serum albumin is known to be taken up and degraded by sinusoidal liver cells via adsorptive endocytosis. The present study aimed at characterization and identification of the membrane-associated receptor on rat sinusoidal liver cells. Kinetic studies of binding of 125I-labeled formaldehyde-treated serum albumin (125I-f-Alb) with the membranes of sinusoidal liver cells demonstrated the presence of specific, high-affinity, saturable membrane-bound receptors with an apparent Kd = 8 micrograms of f-Alb/ml and the optimal pH at around 8.0. The 125I-f-Alb binding to the membranes was not inhibited by either native albumin, asialofetuin, methylamine-treated alpha 2-macroglobulin, mannan, or immune complexes. The binding process exhibited independence of calcium and susceptibility both to heat treatment and to destruction by proteases. The binding was inhibited by concanavalin A and the inhibition was effectively reversed by the presence of alpha-methyl-D-glucoside, a haptenic inhibitor for this lectin, indicating the glycoprotein nature of the receptor. The binding protein was extracted from the membrane preparations with octyl beta-D-glucopyranoside and immunoprecipitated by anti-ligand antibody as a complex with the ligand. Sodium dodecyl sulfate-gel electrophoresis of the immunoprecipitate revealed two polypeptide chains with molecular weights of approximately 53,000 and 30,000, respectively. |
