Kinetic characterization of binding and internalization of fractionated [3H]heparin in rat liver parenchymal cells in primary culture

Abstract

The binding and internalization of fractionated [3H]heparin (FH) was kinetically analyzed in rat liver parenchymal cells to clarify its cellular uptake mechanism. The binding of FH to the cell surface was saturable with the dissociation constant (K(d)) of 53.5 nM and a maximum binding capacity (B(max) of 19.9pmol/mg protein. The binding of FH to the cell surface was competitively inhibited not only by heparin sulfate, a polyanion analogous to heparin, but also by rose bengal, an organic anion, suggesting the binding is based on an electric interaction requiring an anionic charge for substrates and consistent with the earlier suggestion of the involvement of the scavenger-like receptor. According to kinetic model analysis, the rate constants of association (k(on)), dissociation (k(off)), and internalization (k(int.app)) were estimated to be 0.0005 nM-1 min-2, 0.0112 min-1 and 0.0056 min-1, respectively. Although both K(d) and B(max) were larger than those reported in Kupffer cells, suggesting lower affinity and higher capacity in liver parenchymal cells, the apparent internalization rate constant was similar to that in Kupffer cells. We thus provided additional evidence suggesting that a scavenger-like receptor exists in rat liver parenchymal cells, and then kinetically characterized the surface binding and internalization of fractionated heparin by this receptor.