Multiplicity of sulfobromophthalein-binding proteins in Y-fraction from rat liver.

Abstract

Several species of the sulfobromophthalein-binding proteins were isolated from the Y-fraction of rat liver cytosol. Each of these proteins, designated as Cv, C1, C2 and C3 according to the elution order from CM-Sephadex column, migrated to the same position as a single protein band in dodecylsulphate electrophoresis. They are basic proteins of approximately 46000 daltons, composed of two subunits of apparently equal size, and similar to the glutathione S-transferase AA, A, B (ligandin), C purified by Jakoby et al. Each of the Cv, C1 and C2 protein forms 1 : 1 primary complex with BSP, with the binding constant of about 5×106 M-1 or more. The C1 protein has a somewhat higher binding constant than the others. The binding constant was little influenced by the purification. The binding constants of these proteins for other organic anions were determined by the competitive technique using 1-anilino-8-naphthalenesulfonate. These proteins show broad binding specificity for various organic anions which are all known to be rapidly transported into the liver. From these results, it is concluded that the binding of BSP by the Y-fraction is not due to a single protein like ligandin.