Uptake of sulfobromophthalein-glutathione conjugate by isolated hepatocytes

Uptake of sulfobromophthalein-glutathione conjugate (BSP-GS) was studied using isolated hepatocytes and was compared with previous results with the parent BSP (Schwenk et al., Eur. J. Biochem. 64: 189-197, 1976). Adsorption of BSP-GS to the liver cell membrane has two sites with different affinities and binding capacities. Uptake of the conjugate is decreased by two metabolic inhibitors and appears to be saturable up to 12.5 microM with an apparent Km of 4 microM (7 for BSP) and a Vmax of 0.16 nmol x mg protein-1 x min-1 (2.6 for BSP). Similar to the parent BSP at higher concentrations a second mechanism of uptake is observed indicated by the nonlinear Hanes plot. SP-GS transport is competitively inhibited by BSP with a Ki of about 1 microM, and, similar to BSP, replacement of sodium ions in the medium had little effect on the rates of uptake. These results suggest a common transport mechanism for BSP and its glutathione conjugate. Furthermore, the effect of organic anions, bases, and steroids on uptake of BSP-GS has been tested.