lOOOOOg-Supernatant Preparations from the Rat Liver

1. The binding of cholic acid to 10000Og supernatants from rat livers was investigated by equilibrium dialysis and gel-exclusion chromatography. 2. Supernatants were found to contain at least two classes of binding site for cholic acid. 3. These receptor molecules are probably proteins since incubation with proteolytic enzymes resulted in complete loss of cholic acid binding. 4. Supernatants were added to columns of Sephadex G-75, and two groups of fractions were shown to bind cholic acid. One of these contained lowaffinity binding sites and the other contained both lowand high-affinity binding sites. 5. Feeding cholestyramine had no effect on cholic acid binding. 6. Increased cholic acid binding occurred after injection of phenobarbitone. There was an increase in the amount of the low-affinity component but no change in the high-affinity component. 7. The dissociation constants of the binding of cholic acid suggest that the binding proteins may be involved in bile acid transport.