Preferential utilization of newly synthesized cholesterol as substrate for bile acid biosynthesis. An in vivo study using 18O2-inhalation technique.

Incorporation of 18O in cholic anc chenodeoxycholic acid was determined after inhalation of 18O2 by rats with biliary fistula. After a 30-min inhalation, the maximal incorporation of 18O in the three hydroxyl groups of cholic acid was about 1.8 atoms, and in the two hydroxyl groups of chenodeoxycholic acid about 1.1 atoms. About 0.4 atom of 18O in the cholic and chenodeoxycholic acid isolated was present at C-3. It was calculated that at least 50% of the biosynthesized bile acids were derived from newly synthesized cholesterol. The time course for the incorporation of 18O at C-3 of chenodeoxycholic acid was slightly different from that of cholic acid, indicating that a small part of chenodeoxycholic acid might have been synthesized from a pool of cholesterol different from that utilized in the biosynthesis of cholic acid. Incorporation of 18O in biliary cholesterol was less than 0.05 atom, indicating that the major part of this cholesterol is derived from a pool different from that utilized in bile acid biosynthesis.