Glucuronidation of monohydroxylated short chain bile acids by human liver microsomes and purified human liver UDP-glucuronosyltransferases

Abstract

The glucuronidation of monohydroxylated bile acids and their analogs with a shortened side chain (short-chain bile acids) by human liver microsomes and by two UDP-glucuronosyltransferases purified therefrom has been studied in vitro. In microsomes, all 18 substrates tested underwent glucuronidation; the rate of reaction and the site of attachment of glucuronic acid (hydroxyl group in position 3, side chain carboxyl group, or various ratios of both products) were strongly dependent on the length of the side chain, the configuration of the 3-hydroxyl group, and the configuration of the A/B ring junction (5 alpha-H/5 beta-H). Two UDP-glucuronosyltransferases (UDPGTs) purified from human microsomes, designated "pl 7.4" and "pl 6.2" according to their behavior in chromatofocusing, accounted for the formation of hydroxyl-linked glucuronides of a different pair of bile acids each. The pl 7.4 human liver UDPGT catalyzed the glucuronidation of C20 and C22 with the 3 alpha-OH, 5 beta-H configuration, while the pl 6.2 human liver UDPGT catalyzed the glucuronidation of either 3-OH epimer of C21 and C24 acids with the 5 alpha-H configuration. The enzymes displayed a relatively high selectivity in that they did not accept any of the remaining 14 bile acids as substrates; none of the enzymes led to the formation of a carboxyl-linked glucuronide. In addition, purified human liver UDPGT did not catalyze the glucuronidation of cholate, deoxycholate, chenodeoxycholate, ursodeoxycholate, lithocholate, hyocholate, hyodeoxycholate, bilirubin, morphine, or 4-hydroxybiphenyl. The above results suggest that several bile-acid UDP-glucuronosyltransferases of high specificity exist in human liver.