Bile-acid binding to isolated rat liver plasma membranes. Failure to find a specific binding site

Bile-acid transport across the hepatocyte is an active Na-dependent process and specific putative binding sites on liver plasma membrane have been described. We studied cholic-acid binding to isolated liver plasma membranes over a wide concentration range. We found no saturability of binding over a concentration range of 0.001-2000 microM. We did demonstrate saturability of binding over a concentration range (1-30 nM) at pH 6.0 and 4 degrees C, but the saturability was an artifact caused by the increasing concentration of ethanol necessary to avoid precipitation of cholic acid at pH 6.0, 4 degrees C. We were unable to eliminate specific binding by heating the plasma membrane for 3 h at 37 degrees C, as other authors have. Although we were able to show a structural specificity of the bile-acid uptake site on the isolated liver cells, we were unable to demonstrate a specific saturable bile-acid binding site on isolated liver plasma membranes.