Kinetics of Acetaminophen Glucuronidation by UDP-Glucuronosyltransferases 1A1, 1A6, 1A9 and 2B15. Potential Implications in Acetaminophen-Induced Hepatotoxicity

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• 作者：Abdul E. Mutlib ; Theunis C. Goosen ; Jonathan N. Bauman ; J. Andrew Williams ; Shaila Kulkarni ; and Seva Kostrubsky
• 刊名：Chemical Research in Toxicology
• 出版年：2006
• 出版时间：May 2006
• 年：2006
• 卷：19
• 期：5
• 页码：701 - 709
• 全文大小：339K
• 年卷期：v.19,no.5(May 2006)
• ISSN：1520-5010

文摘

The importance of uridine 5'-diphosphate-glucuronosyltranferases (UGT) 2B15 and other UGT enzymes(1A1, 1A6, and 1A9) in glucuronidating acetaminophen (APAP) is demonstrated. The kinetics andcontributions of various UGTs in glucuronidating APAP are presented using clinically and toxicologicallyrelevant concentrations of the substrate. UGT 1A9 and UGT 2B15 contribute significantly towardglucuronidating APAP when incubations were conducted in either phosphate or Tris-HCl buffers at 0.1and 1.0 mM substrate concentrations. At 10 mM APAP, UGT 1A9 is a significant enzyme responsiblefor metabolizing APAP in either one of the buffers. UGT 1A1 is the next most important enzyme inglucuronidating APAP at this high substrate concentration. The contribution of UGT 1A6 at 10 mMAPAP concentration became obscured by similar relative activities exhibited by UGTs 1A7, 1A8, and2B7. These observations may reflect the differences in kinetic parameters for APAP glucuronidation bythe individual UGTs. UGT 1A1 demonstrated Hill kinetics while UGT 1A9 displayed Michaelis-Mentenkinetics. Substrate inhibition kinetics is observed with UGT 1A6, UGT 2B15, and human liver microsomes.The substrate inhibition is confirmed by employing stable isotope-labeled APAP as the substrate, whileAPAP glucuronide is used to test for inhibition of d₄-APAP glucuronide. The in vitro hepatotoxicitycaused by APAP in combination with phenobarbital or phenytoin is demonstrated in this study. Theinhibition of APAP glucuronidation by phenobarbital leads to an increase in APAP-mediated toxicity inhuman hepatocytes. The toxicity to hepatocytes was further increased by coadministering APAP withphenytoin and phenobarbital. This synergistic increase in toxicity is postulated to be due to inhibition ofUGTs (1A6, 1A9, and 2B15) responsible for detoxifying APAP through the glucuronidation pathway.