Uptake of palmitate by hepatocyte suspensions: facilitation by albumin?

Abstract

Albumin-dependent uptake of unbound [3H]palmitic acid by hepatocytes isolated from female rat livers was studied and the experimental results compared with the predictions of a noncompartmental diffusion-reaction theory for the cellular uptake of protein-bound ligands. The outright theoretical predictions involve values for the parameters of the system, some newly measured (hepatocyte radii and the rate constant for the dissociation of palmitate-albumin complex) and some taken from the literature (diffusion coefficients and the equilibrium association constant for the palmitate-albumin complex). The measured unbound clearance of [3H]palmitic acid, defined as the initial uptake velocity divided by the unbound [3H]palmitic acid concentration in the medium, was enhanced 6.6-fold as the concentration of human serum albumin was increased from approximately 5 to 480 microM. This enhancement factor was predicted by the theory, according to which the enhancement reflects codiffusion of bound ligand across the unstirred layer adjacent to the cell membrane and, therefore, an increased delivery of unbound ligand to the cell surface. In contrast, the absolute magnitude of the unbound clearance was consistent with the theory only for the lowest published value for the equilibrium association constant, 15 microM-1. For higher published values (62 and 94 microM-1), the magnitude of the unbound clearance observed experimentally was severalfold higher than that predicted by the theory. If in fact the association constant exceeds 30 microM-1, the data would imply that an albumin-dependent facilitation mechanism exists which enhances the availability of palmitate to the cell over and above the enhancement predicted by the diffusion-reaction theory.

Effect of Albumin on Phenytoin and Tolbutamide Metabolism in Human Liver Microsomes: An Impact More Than Protein Binding

“…Pond et al (1992) reported that facilitation mechanism was involved in uptake of palmitate by hepatocyte suspensions. They found that the measured unbound clearance of [ 3 H]palmitic acid, defined as the initial uptake velocity divided by the unbound [ 3 H]palmitic acid concentration in the medium, was enhanced 6.6-fold as the concentration of human serum albumin was increased from ϳ5 to 480 M. In fact, in many cases, the uptake rate appears to be determined more by the bound than by the unbound ligand concentration (Weisiger et…”

Reduced hepatic extraction of palmitate in steatosis correlated to lower level of liver fatty acid binding protein

“…1B). In this work, permeability has not been adjusted for fraction unbound in perfusate as described previously (12), because there have been suggestions that perfusate albumin facilitates palmitate uptake (2,24).…”

Transport of organic anions in the liver. An update on bile acid, fatty acid, monocarboxylate, anionic amino acid, cholephilic organic anion, and anionic drug transport

“…It was argued that in the intact organ, albumin facilitation would be predicted to be present when the dissociation of the protein-ligand complex is slow and rate limiting for the observed uptake. The hypothesis was questioned by Bass and Pond who introduced an alternative hypothesis, the idea that phenomena occurring in a stagnant layer surrounding the cells can explain albumin facilitation (Bass and Pond 1988;Pond et al 1992). Albumin simply helps the organic anion to diffuse across this layer, which means that the albumin effect is caused by events inside this stagnant layer and should be independent of the nature of the membrane.…”

“…However, although the studies with artificial membranes helped to abandon the albumin-receptor hypothesis, they could not provide insight into the manner in which hepatocytes take up fatty acids (Schwab and Goresky 1991). Pond et al (1992) measured [3H]palmitate uptake in isolated rat hepatocytes in the presence of 5-80 gM human serum albumin (HSA) and calculated the clearance of the fatty acid at each albumin concentration. Clearance was determined by dividing the measured uptake rate by the total fatty acid concentration in the extracellular fluid.…”