Bilitranslocase and sulfobromophthalein/bilirubin-binding protein are both involved in the hepatic uptake of organic anions.

Abstract

The hepatic uptake of cholephiic organic anions is a carrier-mediated process. Three distinct proteins [bilitranslocase (BTL), sulfobromophthalein (BSP)/bilirubinbinding protein (BBBP), and organic anion-binding protein] have been isolated from the basolateral plasma-membrane domain of the hepatocyte. To investigate the relative role of the first two of them in accounting for the hepatic uptake oforganic anions, we measured the initial rates of uptake of 35S-labeled BSP into rat liver plasma-membrane vesicles. Because transport by BTL is electrogenic but transport by BBBP is electroneutral, studies were done either with or without a positiveinside membrane potential produced by adding valinomycin in the presence of an inwardly directed K+ gradient (outside K+ > inside K+). Both electrogenic and electroneutral transport systems followed saturation kinetics. Electroneutral uptake showed an apparent Km of 20 ± 3 ,uM (mean ± SD) and a V.m of 1.0 ± 0.13 nmol(mg of prot)'1 15 sec'1, whereas the electrogenic portion of BSP uptake exhibited a Km of 5.2 ± 0.8 ,uM and a V. of 1.1 ± 0.1 nmol-(mg of prot)"1-5 sec'1. In this case, an overshoot was observed 15 sec after valinomycin addition. Electroneutral BSP uptake was inhibited by incubation with anti-BBBP antibody, whereas anti-BTL antibody did not show any inhibitory effect. Conversely, the electrogenic uptake was inhibited by anti-BTL antibody at a BSP concentration of 5 FM; no inhibition was seen either at 20 IAM BSP or upon addition of anti-BBBP antibody. From these data we conclude that the hepatic uptake of organic ions occurs via two immunologically distinct carrier proteins (BTL and BBBP) operating in parallel. BTL is a higher affinity electrogenic transporting system of organic ions, whereas BBBP is a lower affinity electroneutral transporter.The hepatoceilular uptake of the cholephilic organic anions, such as bilirubin and phtaleins, is a carrier-mediated process (1, 2). Studies on the hepatic uptake of sulfobromophthalein (BSP), a ligand that follows a transport process similar to that of bilirubin (1), indicate the existence of specific binding/ transporting proteins at the level of the sinusoidal plasma membrane.

Protein-Mediated Facilitated Uptake Processes for Fatty Acids, Bilirubin, and Other Amphipathic Compounds

“…In a comparative study, Claudio Tiribelli's group has shown that uptake of bilirubin by bilitranslocase is electrogenic whereas such uptake by the BSP/ bilirubin-binding protein is electroneutral. These authors concluded that the two proteins operate in parallel, where the relative role of each on the overall transport depends on substrate concentration (95).…”

Expression and Function of Renal Organic Anion Transporters in Cholestasis

“…BSP/Bilirubin binding protein (BBBP) is a protein isolated from rat liver located on the sinusoidal membrane of the liver and described as a transport protein involved in the well known sodium-independent hepatic uptake of BSP and bilirubin (Stremmel & Berk, 1986;Stremmel et al, 1986;Torres et al, 1993;Torres, 1997). The transport of this protein is electroneutral as indicated by experiments in liver plasma membrane vesicles.…”

Substrate specificity of sinusoidal bile acid and organic anion uptake systems in rat and human liver

“…92 Despite these uncertainties about the involved carrier proteins, the observations of low and high affinity bile salts and the nonbile salt organic anion BSP. 7,8,12,15,83 Furthermore, competitive interactions with hepatocellular BSP uptake systems 34,89,90,[93][94][95][96][97][98][99][100] (Table 1) with different potential dependencies, 89 distinct cellular distribution, 101 dif-Na / -independent bile salt uptake has also been observed for cardiac glycosides and other neutral steroids, 7,8,11,13,84,85 vari-ferential modulation by sex steroids, 98 and selective inducibility by phenobarbital 6,90 clearly indicate that more than ous linear and cyclic peptides, 7,8,10,45,66,86,87 certain organic cations, 13,14 and numerous other drugs and toxins. 7,8,16,17,88 one sinusoidal BSP/bilirubin uptake systems are present in rat hepatocytes.…”