Bile acid n-acetylglucosaminidation: In vivo and in vitro evidence for a selective conjugation reaction of 7β-hydroxylated bile acids in humans

Abstract

The aim of this study was to define whether N-acetylglucosaminidation is a selective conjugation pathway of structurally related bile acids in humans. The following bile acids released enzymatically from N-acetylglucosaminides were identified: 3α,7β-dihydroxy-5β-cholanoic (ursodeoxycholic), 3β,7β-dihydroxy-5β-cholanoic (isoursodeoxycholic), 3β,7β-dihydroxy-5α-cholanoic (alloisoursodeoxycholic), 3β,7β-dihydroxy-5-cholenoic, 3α,7β,12α-trihydroxy-5β-cholanoic, and 3α,6alpha;,7βtrihydroxy-5β-cholanoic acids. The selectivity of conjugation was studied by administration of 0.5 g ursodeoxycholic (UDCA) or hyodeoxycholic (HDCA) acids, labeled with 13C, to patients with extrahepatic cholestasis, and of 0.5 g of 13C-labeled chenodeoxycholic acid (CDCA) to patients with extra- or intrahepatic cholestasis. After administration of [24-13C]-CDCA, labeled glucosides, and the glucuronide of CDCA were excreted in similar amounts. Labeled N-acetylglucosaminides of UDCA and isoUDCA were also formed. When [24-13C]-UDCA was given, 13C-label was detected in the N-acetylglucosaminide, the glucosides, and the glucuronide of UDCA, and in the N-acetylglucosaminide of isoUDCA. In the patient studied, 32% of the total UDCA excreted in urine was conjugated with N-acetylglucosamine. In contrast, 96% of the excreted amount of [24-13C)HDCA was glucuronidated, and 13C-labeled glucosides but no N-acetylglucosaminide were detected. The selectivity of N-acetylglucosaminidation towards bile acids containing a 7β-hydroxyl group was confirmed in vitro using human liver and kidney microsomes and uridine diphosphate glucose (UDP)-N-acetylglucosamine. These studies show that A-acetylglucosaminidation is a selective conjugation pathway for 7β-hydroxylated bile acids.