Rapid preparation of tritium-labeled bile acids by enolic exchange on basic alumina containing tritiated water.

When a 3-keto bile acid methyl ester was chromatographed on basic alumina inactivated with tritiated water, the enolic hydrogen atoms at C-2 and C-4 exchanged with tritium atoms. The (3)H-labeled keto ester was reduced with borohydride, and the resultant mixture of 3alpha- and 3-hydroxy epimers was resolved by preparative thin-layer chromatography to yield a pure 2,4-(3)H-labeled bile acid ester. Lithocholic, chenodeoxycholic, deoxycholic, and cholic acids having a specific activity of 1-10 micro c/ micromole were prepared from their 3-keto derivatives. The tritium label remained intact during alkaline saponification in vitro and enterohepatic cycling in vivo in human subjects.