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

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Abstract
Several species of the sulfobromophthalein(BSP)-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 dodecyl sulfate electrophoresis. They are basic proteins of .apprx. 46,000 daltons, composed of 2 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 proteins forms 1:1 primary complex with BSP, with the binding constant of about 5 .times. 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-naphthenesulfonate. These proteins show broad binding specificity for various organic anions which are all known to be rapidly transported into the liver. The binding of BSP by the Y-fraction is probably not due to a single protein like ligandin.