Cholic acid binding to isolated rat liver plasma membranes

Cholic acid binding to isolated rat liver plasma membranes was studied using a centrifugal filtration technique which allowed independent determination of free and membrane-bound cholic acid. Binding of cholic acid was very rapid and reversible. Scatchard analysis revealed at least three binding sites with high, medium and low affinity. The high affinity binding a) displayed saturability and isotope replacement, b) was not present in rat liver mitochondria and red blood cell ghosts and c) was temperature dependent. This binding has a very low capacity with a dissociation constant in the physiological range of plasma cholic acid concentration and has an affinity for other common bile acids. Cholic acid binding to the high affinity binding site was not inhibited by estrone, beta-estradiol or cholesterol. These results would suggest that the high affinity binding site represents a specific binding site for cholic acid and may also be specific for other common bile acids. This binding was not dependent on Na and was inhibited by bromosulfophthalein. Cholic acid binding to the high affinity site has some features in common with cholic acid uptake by isolated rat hepatocytes, and this would suggest that the high affinity binding site could be the postulated carrier for hepatic uptake of cholic acid.