DIFFERENCE IN HEPATIC-UPTAKE OF TETRA-BROMOSULFOPHTHALEIN AND DI-BROMOSULFOPHTHALEIN IN RAT - ROLE OF HYDROPHOBICITY, BINDING TO PLASMA-PROTEINS AND AFFINITY FOR PLASMA-MEMBRANE CARRIER PROTEIN

The relative role of hydrophobicity, binding to plasma proteins and af finity for one of the plasma membrane transport proteins in the hepati c uptake of 3,4,5,6-tetra- (BSP) and 3,6-di- (DBSP) bromosulfophthalei n was investigated in the rat. In terms of physicochemical characteris tics, the two molecules show different pK(a) values and degrees of hyd rophobicity, as determined from the n-octanol:water partition coeffici ent. In the intact animal, the plasma clearance and the plasma removal rate after a dose of 1.5 mumol/kg i.v. were significantly (P < 0.001) faster for BSP than DBSP, while no difference was found in the plasma distribution volume. The dissociation constant (K(d)) of the high aff inity binding sites of plasma proteins also differed for the two anion s, being significantly lower for BSP than DBSP (0.95 +/- 0.02 vs 1.44 +/- 0.14 muM, P < 0.001). [S-35]BSP uptake by liver plasma membrane ve sicles was saturable with an apparent K(m) of 5.20 +/- 0.80 muM, and w as competitively inhibited by DBSP (K(i) 18.2 +/- 1.2 muM) indicating a common uptake system. The K(d) value for binding of the organic anio ns to purified bilitranslocase, a plasma membrane protein involved in the electrogenic transport of pthaleins, was also significantly lower for BSP than DBSP (1.10 +/- 0.12 vs 3.02 +/- 0.27 muM, N = 3, P < 0.00 1), indicating a higher affinity of the former ligand for the carrier protein. No difference was observed in the capacity of the high affini ty binding sites (32 +/- 3 vs 33 +/- 3 nmol/mg protein, BSP and DBSP, respectively). These data indicate that BSP and DBSP are two different cholephilic organic anions which share a common uptake mechanism, at least partly mediated by bilitranslocase. The greater affinity of BSP than DBSP for the carrier protein may account for the faster plasma di sappearance rate of BSP observed in vivo, in spite of the higher plasm a protein binding.