Bile Acid Synthesis in Isolated Rat Liver Cells

Bile acid synthesis in isolated rat hepatocytes was measured using radioimmunoassays for conjugated cholic and chenodeoxycholic acids. Synthesis of both bile acids was increased in hepatocytes from rats fed a diet containing the bile‐acid‐sequestering agent, cholestyramine, as compared to cells from rats given a pellet diet. When the cholestyramine‐fed rats were subjected to a reversed light cycle a further increase in bile acid synthesis was observed in the hepatocytes. Addition of 7α‐hydroxycholesterol at a concentration of 0.068 mM to the incubation medium of the hepatocytes stimulated 2–3‐fold the synthesis of both conjugated cholic and chenodeoxycholic acids. Conjugated chenodeoxycholic acid synthesis was also increased at concentrations of 0.14 and 0.68 mM 7α‐hydroxycholesterol, but conjugated cholic acid synthesis was inhibited at these levels. This suggests that high concentrations of 7α‐hydroxycholesterol may inhibit the 12α‐hydroxylation step in bile acid synthesis. Other oxygenated derivatives of cholesterol such as the 12α‐hydroxy, 25‐hydroxy, 26‐hydroxy and 7‐oxo derivatives did not stimulate bile acid synthesis in the liver cells. It is concluded that the increase in bile acid synthesis found in isolated hepatocytes on addition of 7α‐hydroxycholesterol to the incubation medium is specific for this compound, indicating that cholesterol‐7α‐hydroxylase is the rate‐limiting enzyme in the biosynthetic pathway from cholesterol to bile acids.