BILE-ACID N-ACETYLGLUCOSAMINIDATION - INVIVO AND INVITRO EVIDENCE FOR A SELECTIVE CONJUGATION REACTION OF 7-BETA-HYDROXYLATED BILE-ACIDS IN HUMANS

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: 3-alpha,7-beta-dihydroxy-5-beta-cholanoic (ursodeoxycholic), 3-beta,7-beta-dihydroxy-5-beta-cholanoic (isoursodeoxycholic), 3-beta,7-beta-dihydroxy-5-alpha-cholanoic (alloisoursodeoxycholic), 3-beta,7-beta-dihydroxy-5-cholenoic, 3-alpha,7-beta,12-alpha-trihydroxy-5-beta-cholanoic, and 3-alpha,6-alpha,7-beta-trihydroxy-5-beta-cholanoic acids. The selectivity of conjugation was studied by administration of 0.5 g ursodeoxycholic (UDCA) or hyodeoxycholic (HDCA) acids, labeled with C-13, to patients with extrahepatic cholestasis, and of 0.5 g of C-13-labeled chenodeoxycholic acid (CDCA) to patients with extra- or intrahepatic cholestasis. After administration of [24-C-13]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-C-13]UDCA was given, C-13-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-C-13]HDCA was glucuronidated, and C-13-labeled glucosides but no N-acetylglucosaminide were detected. The selectivity of N-acetylglucosaminidation towards bile acids containing a 7-beta-hydroxyl group was confirmed in vitro using human liver and kidney microsomes and uridine diphosphate glucose (UDP)-N-acetylglucosamine. These studies show that N-acetylglucosaminidation is a selective conjugation pathway for 7-beta-hydroxylated bile acids.