Hepatocellular sulfobromophthalein uptake at physiologic albumin concentrations: kinetic evidence for a high affinity/low capacity sinusoidal membrane system

Background/Aims: Controversy exists regarding the nature of the hepatocyte membrane transport system for sulfobromophthalein and other organic anions and its driving forces. Most studies have been performed in the absence of albumin, the plasma sulfobromophthalein binding protein, or using very low albumin concentrations. We have shown that in the latter case uptake kinetics reflect dissociation/diffusion events and not membrane transport. In contrast, without albumin very high sulfobromophthalein concentrations reach the cell surface and may overwhelm a high affinity/low capacity system operating in vivo. The aim of this study was to test the latter hypothesis. Methods: Sulfobromophthalein uptake was measured by rapid filtration in isolated hepatocytes without albumin (up to 15 μM sulfobromphthalein) and with 600 μM albumin (sulfobromophthalein:albumin from 0.03:1 to 1:1), a physiologic setting which greatly reduces the unbound BSP concentration. Unbound sulfobromophthalein concentration was estimated according to a three binding site model. Results: In the absence of albumin, kinetic parameters for sulfobromophthalein uptake were similar to those reported in the literature (Km: 7.1±1.2 μM; Vmax: 452±37 pmol/min/5×104 cells). In the presence of albumin, sulfobromophthalein uptake displayed much greater affinity and much lower capacity (Km:80±11 nM; Vmax: 60±9 pmol/min/5×104 cells). Conclusions: These findings suggest that in the absence of albumin, resulting high sulfobromphthalein concentrations overload (and make undetectable) a high affinity/low capacity system operating at physiologic albumin concentrations (i.e. at low unbound sulfobromophthalein concentrations). Previously characterized transport systems may be operating only under defined conditions. These findings could explain the apparent controversy regarding the nature of the sulfobromophthalein transport system and its driving forces.