Conjugation with taurine prevents side‐chain desaturation of ursodeoxycholic and β‐muricholic acids in bile fistula rats

The metabolism of intravenously infused bile salts, tauroursodeoxycholate, tauro‐ β ‐muricholate and their corresponding unconjugated forms in the liver was investigated in bile salt‐depleted bile fistula rats. The biliary bile salt composition was determined by gas chromatography–mass spectrometry using chemical positive ionization and electron‐impact methods. For an infusion rate of 2  μ mol/min/kg, all bile salts were efficiently secreted in bile, inducing similar choleresis. Only tauroconjugated bile salts were recovered; no glucuronide or glyco derivatives were detected. The infusion of free ursodeoxycholate led to the appearance of a metabolite identified as a Δ 22 derivative (12%). A similar biotransformation rate (11%) was observed following free β ‐muricholate infusion. In contrast, no metabolite was observed after infusion of the tauroconjugated form of ursodeoxycholate and β ‐muricholate. The unsaturation process probably depends on the availability of the carboxyl group for the starting step of the β ‐oxidation mechanism. In conclusion, the current in vivo study demonstrates a hepatic origin for Δ 22 bile salts. It also shows that free bile salts were sensitive to Δ 22 formation while conjugation with taurine totally prevented the side‐chain oxidation of the two 7 β ‐hydroxylated bile salts.