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Macromolecule-macromolecule interaction in drug distribution. V. Effects of plasma proteins on uptake of fractionated [3H]heparin in isolated rat Kupffer cells.

Effects of plasma proteins such as alpha-globulin on the uptake of high molecular weight (HMWFH: 23000 Da) and low molecular weight fractionated [3H]heparin (LMWFH: 10000 Da) were examined in isolated rat Kupffer cells. alpha-Globulin (8 mg/ml) affected neither surface binding nor internalization of LMWFH by Kupffer cells, while it reduced both surface binding and internalization of HMWFH without affecting the fraction internalized, which was a ratio of internalized amount to the total association. The total associations of HMWFH were about four times larger than that predicted assuming only the unbound fraction is available for uptake, suggesting the participation of protein-mediated transport in the uptake of HMWFH in Kupffer cells. Based on the same assumption, the saturable initial uptake of HMWFH versus concentration profile in the presence of alpha-globulin (8 mg/ml) was also analyzed to further examine the suggested protein-mediated transport. The estimated dissociation constant of 487 nM was three times larger than that in in vitro binding experiments (168 nM) and the binding capacity of 0.155 was one third of the value in vitro (0.5), suggesting apparent reductions in both binding affinity and capacity. Thus, we demonstrated the involvement of protein-mediated transport in the uptake of fractionated heparin in Kupffer cells and kinetically characterized it as the apparent enhancement of dissociation.