Validation of [22,23-3H]cholic acid as a stable tracer through conversion to deoxycholic acid in human subjects

Bile acids labeled with 3H on the sterol nucleus lose a substantial fraction of label during enterohepatic cycling and conversion to secondary bile acids. We tested the isotopic stability of a side-chain 3H label, [22,23-3H]cholic acid in humans. The 3H-labeled compound was administered simultaneously with [24-14C]cholic acid to four healthy volunteers. Duodenal bile was collected daily for 5 days after isotope administration to determine the ratio of 3H/14C in bile acids. Urine was collected to determine loss of radioactivity by this route. Cholic acid and deoxycholic acid were isolated from biliary bile acids by thin-layer chromatography after deconjugation with cholylglycine hydrolase. The ratio of 3H/14C in cholic acid and deoxycholic acid remained constant and identical to that of the administered mixture in all subjects, indicating stability of the 3H label during enterohepatic cycling. Cumulative loss of 3H in urine averaged only 1.2% of administered dose and was identical to loss of 14C (average 1.3%) indicating little if any transfer of 3H from bile acid to body water. Deconjugation of biliary bile acids by alkaline hydrolysis resulted in 15-20% loss of 3H label, consistent with known base-catalyzed exchange of alpha-carbon protons on carboxylic acids. We conclude that [22,23-3H]cholic acid is a biologically stable, and therefore reliable, isotopic tracer of cholic acid in humans during enterohepatic cycling including conversion to deoxycholic acid, provided deconjugation is performed enzymatically. Because the 22,23-3H label can be inserted into most C24 bile acids, it appears the best way to tag 3H-labeled bile acids for metabolic studies.