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

1. 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.
2. The clearance rate of [³H]-palmitate in the presence of α₁-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.
3. 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⁻¹/10⁶ cells, 4.8 μl s⁻¹/10⁶ cells and 5.5 μl s⁻¹/10⁶ cells for α₁-acid-glycoprotein, albumin and lysozyme, respectively; whereas the measured hepatocyte palmitate clearance rates were 1.2±0.1 μl s⁻¹/10⁶ cells, 2.3±0.3 μl s⁻¹/10⁶ cells and 7.1±0.7 μl s⁻¹/10⁶, respectively.
4. Hepatocyte palmitate clearance was significantly faster (P<0.01) in the presence of lysozyme than albumin which was significantly faster than α₁-acid-glycoprotein (P<0.01). The marked difference in clearance rates could not be explained by considering differences in solution viscosity.
5. 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.