Binding and uptake of asialothyroglobulin by isolated rat hepatocytes

Binding and uptake of asialothyroglobulin by freshly isolated rat hepatocytes were studied in dependence on temperature as well as on the concentrations of the ligand and of calcium ions. The endocytosis of native thyroglobulin occurred as a nonspecific process. The number of galactose-specific receptors on the hepatocyte surface was estimated to be 47,400 per cell. The cells exhibited only a single class of binding sites with an association constant for asialothyroglobulin of Kass = 0.18.10(8)M-1. The internalization of surface-bound asialothyroglobulin appeared to be a first-order process. The rate constant of internalization ki, was estimated to be 0.6.10(-3) s-1. The obtained data provide an explanation of the extreme half-life of asialothyroglobulin in the circulation.