Abstract
5-(4′-Hydroxyphenyl)-5-phenylhydantoin (4′-HPPH), a major metabolite of phenytoin in human, is exclusively metabolized to a glucuronide. 4′-HPPH has a chiral center. (S)-4′-HPPH is a predominant form produced from phenytoin in humans, and (R)-4′-HPPH is an extremely toxic form with respect to gingival hyperplasia. In the present study, we investigated stereoselective 4′-HPPH O-glucuronide formation in human liver microsomes. Human liver microsomes predominantly formed (S)-4′-HPPH O-glucuronide rather than (R)-4′-HPPH O-glucuronide from racemic 4′-HPPH. Among human UDP-glucuronosyltransferase (UGT) enzymes, UGT1A1, UGT1A9, and UGT2B15 showed 4′-HPPH O-glucuronide formation. Interestingly, UGT1A1 stereoselectively formed (R)-4′-HPPH O-glucuronide, whereas UGT1A9 and UGT2B15 stereoselectively formed (S)-4′-HPPH O-glucuronide from racemic 4′-HPPH. By using UGT1A double-expression systems in HEK293 cells that we previously established, the effects of UGT-UGT interactions on 4′-HPPH O-glucuronide formation were investigated. It was demonstrated that coexpression of UGT1A4 increased the Vmax values of (S)- and (R)-4′-HPPH O-glucuronide formation catalyzed by UGT1A1 but decreased the Vmax values of (S)- and (R)-4′-HPPH O-glucuronide formation catalyzed by UGT1A9. Coexpression of UGT1A6 increased the S50 values and decreased the Vmax values of (S)- and (R)-4′-HPPH glucuronide formation catalyzed by UGT1A1 and UGT1A9. However, the interaction did not alter the stereoselectivity. In conclusion, we found that 4′-HPPH O-glucuronide formation in human liver microsomes is catalyzed by UGT1A1, UGT1A9, and UGT2B15 in a stereoselective manner, being modulated by interaction with other UGT1A isoforms.
Footnotes
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H.Y. is supported as a Research Fellow of the Japan Society for the Promotion of Science.
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doi:10.1124/dmd.107.015909.
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ABBREVIATIONS: 4′-HPPH, 5-(4′-hydroxyphenyl)-5-phenylhydantoin; UGT, UDP-glucuronosyltransferase; HPLC, high-performance liquid chromatography; UDPGA, UDP-glucuronic acid.
- Received March 26, 2007.
- Accepted June 13, 2007.
- The American Society for Pharmacology and Experimental Therapeutics
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