Molecular structure influence in the recognition of phthaleins by the electrogenic organic anion carrier at the sinusoidal plasma membrane level in the liver

The hepatocytic uptake of cholephilic organic anions occurs by carrier-mediated mechanisms. Electrogenic and electroneutral transport systems have been described. The aim of this study was to determine the dissociation constant (Kd) of the Electrogenic Carrier System(s) (ECS) for tetrabromosulfophthalein (BSP), dibromosulfophthalein (DBSP), tetrabromophthalein (TBP), tetrabromosulfonephthalein (TBS) and thymol blue (TB). Kd (uM) values for ECS-organic anion complexes were: ECS-BSP = 3.61 ± 0.18; ECS-DBSP = 11.61 ± 1.32; ECS-TBP = 0.51 ± 0.08; ECS-TBS = 1.31 ± 0.25; ECS-TB = 9.44 ± 1.80.From these data, it is possible to conclude that molecular characteristics of the organic anions are important factors in determining the dissociation constant for the electrogenic hepatic carrier(s). In this sense, the addition of two sulphonic groups on the phenolic ring; the presence of a sulphonic on the benzenic ring and the absence of two or four bromines on the molecule confers a lower affinity for ECS.