Immunological studies of an organic anion-binding protein isolated from rat liver cell plasma membrane

The mechanism of organic anion uptake by hepatocytes has kinetics that suggest facilitated diffusion, and carrier-mediated membrane transport has been postulated. In previous studies, we purified a 55,000-mol wt organic anion-binding protein (OABP) by affinity chromatography on sulfobromophthalein (BSP)-Sepharose of deoxycholate solubilized liver cell plasma membrane preparations. Using specific goat and rabbit antibodies to OABP, we have now investigated the distribution of this protein in liver fractions and other tissues by an enzyme-linked immunosorbent assay and by the immunoblot (Western blot) procedure. These studies indicated that OABP is present in significant amounts in all tissues examined except for blood. Although OABP has not as yet been isolated from each of these tissues and characterized, OABP in heart retained the ability to bind organic anions, and was purified by affinity chromatography on BSP-sepharose. In liver, OABP was membrane bound and remained so after extraction with 0.9 M NaCl, which suggests that it is an intrinsic membrane protein. OABP did not have a ubiquitous subcellular distribution within the hepatocyte. Preparation of subfractions of liver cell plasma membrane revealed that OABP is present in the sinusoidal and absent from the canalicular membrane. Immunofluorescence studies performed in short-term cultured hepatocytes suggest that OABP is associated with the surface of these cells and does not have a significant intracellular distribution.