Subcellular distribution of bile acids, bile salts, and taurocholate binding sites in rat liver

We have quantitated bile acids and their conjugates in rat liver using high-pressure liquid chromatography. Over 95% of the hepatic bile acid pool in rat liver homogenates is present as taurocholate and tauromuricholate. Although over 60% of the bile acid pool is recovered in the supernatant, evidence is presented suggesting that taurocholate redistributes among the subcellular fractions during their isolation. Taurocholate (TC) binding to purified subcellular fractions from rat liver was determined by using equilibrium dialysis in a TC concentration range from 0.1 to 100 microM. This is well below the critical micellar concentration of taurocholate (3 mM). All of the fractions investigated exhibited low-affinity binding with dissociation constants from 80 to 240 microM as did membrane lipid vesicles. Therefore, low-affinity binding appears referable to taurocholate nonspecifically partitioning into the lipid bilayer. High-affinity binding is present in plasma membranes, Golgi, and cell supernatant. The high-affinity binding sites in Golgi have a mean dissociation constant (A1) of 1.0 microM and bind 0.15 nmol of TC/mg of protein. Similarly, the high-affinity binding sites of plasma membrane have an A1 of 1.3 microM and bind 0.15 nmol of TC/mg of protein. For cell supernatant, the A1 was 4.8 microM, and 0.35 nmol of TC was bound per mg of protein. Mitochondria, smooth and rough microsomes, and Golgi liposomes showed no detectable amounts of high-affinity binding. These results are compatible with a role for the Golgi complex, cytoplasmic component(s), and plasma membranes in transhepatic bile acid transport.