Effect of binding protein surface charge on palmitate uptake by hepatocyte suspensions

Studies were directed at determining whether hepatocytes, isolated from female Sprague‐Dawley rats, facilitate the uptake of protein‐bound long‐chain fatty acids. We postulated one form of facilitated uptake may occur through an ionic interaction between the protein‐ligand complex and the cell surface. These interactions are expected to supply additional ligand to the cell for uptake. The clearance rate of [ 3 H]‐palmitate in the presence of α 1 ‐acid‐glycoprotein (pI=2.7), albumin (pI=4.9) and lysozyme (pI=11.0) was investigated. Palmitate uptake was determined in the presence of protein concentrations that resulted in similar unbound ligand fractions (=0.03). The experimental clearance rates were compared to the theoretical predictions based upon the diffusion‐reaction model. By use of our experimentally determined equilibrium binding and dissociation rate constants for the various protein‐palmitate complexes, the diffusion‐reaction model predicted clearance rates were 4.9 μl s −1 /10 6 cells, 4.8 μl s −1 /10 6 cells and 5.5 μl s −1 /10 6 cells for α 1 ‐acid‐glycoprotein, albumin and lysozyme, respectively; whereas the measured hepatocyte palmitate clearance rates were 1.2±0.1 μl s −1 /10 6 cells, 2.3±0.3 μl s −1 /10 6 cells and 7.1±0.7 μl s −1 /10 6 , respectively. Hepatocyte palmitate clearance was significantly faster ( P <0.01) in the presence of lysozyme than albumin which was significantly faster than α 1 ‐acid‐glycoprotein ( P <0.01). The marked difference in clearance rates could not be explained by considering differences in solution viscosity. Our results are consistent with the notion that ionic interactions between protein‐ligand complexes and the cell surface facilitate the ligand uptake by decreasing the diffusional distance of the unbound ligand and/or by facilitating the protein‐ligand dissociation rate.British Journal of Pharmacology (1997) 120 , 1215–1220; doi: 10.1038/sj.bjp.0701030