Further characterization of membrane proteins involved in the transport of organic anions in hepatocytes. Comparison of two different affinity labels: 4,4'-diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid and brominated taurodehydrocholic acid.

4,4'-Diisothiocyano-1,2-diphenylethane-2,2'-disulfonic acid (H2DIDS) known as an irreversible inhibitor of the anion transport in red blood cells (Cabantchik, Z.I. and Rothstein, A. (1972) J. Membrane Biol. 10, 311-330) blocks also the uptake of bile acids and of some foreign substrates in isolated hepatocytes (Petzinger, E. and Frimmer, M. (1980) Arch. Toxicol. 44, 127-135). [3H]H2DIDS was used for labeling of membrane proteins probably involved in anion transport of rat liver cells. The membrane proteins modified in vitro by [3H]H2DIDS were compared with those labeled by brominated taurodehydrocholic acid. The latter is one of a series of suitable taurocholate derivatives, all able to bind to defined membrane proteins of hepatocytes and also known to block the uptake of bile acids as well as of phallotoxins and of cholecystographic agents (Ziegler, K., Frimmer, M., Möller, W. and Fasold, H. (1982) Naunyn-Schmiedeberg's Arch. Pharmacol. 319, 254-261). The radiolabeled proteins were compared after SDS-electrophoresis with and without reducing agent present, solubilization by detergents, two-dimensional electrophoresis and after separation of integral and peripheral proteins. Our results suggest that the anion transport system of liver cells cannot distinguish between bile acids and the anionic stilbene derivative (DIDS). The labeling pattern for both kinds of affinity labels was very similar. Various combinations of separation techniques gave evidence that the radiolabeled membrane proteins are not subunits of a single native channel protein.