Abstract
The amino acid sequences of the human UDP-glucuronosyltransferases (UGTs) 1A9 and 1A10 are 93% identical, yet there are large differences in their activity and substrate selectivity. For example, the regioselectivity in propranolol glucuronidation, the regioselectivity in dobutamine glucuronidation, and the glucuronidation rate of α- and β-estradiol differ greatly between UGT1A9 and UGT1A10. To identify the residue responsible for the activity differences, we divided the N-terminal half of the two UGTs into five comparable segments by inserting four unique restriction sites at identical positions in both genes and constructing chimeras in which segments of UGT1A9 were individually replaced by the corresponding segments from UGT1A10. Activity analyses of the resulting mutants, 910A [1A10(1–83)/1A9(84–285)], 910B [1A9(1–83)/1A10(84–147)/1A9(148–285)], 910C [1A9(1–147)/1A10(148–181)/1A9(182–285)], 910D [1A9(1–181)/1A10(182–235)/1A9(236–285)], and 910E [1A9(1–235)/1A10(236–285)] indicated that more than one residue is responsible for the differences between UGT1A9 and UGT1A10. We next prepared four double chimeras, in which two of the above UGT1A9 segments were replaced simultaneously by the corresponding UGT1A10 segments. However, none of the double chimeras glucuronidated either estradiol, propranolol, or dobutamine at rates that resembled those of UGT1A10. On the other hand, studying the kinetics of 1-naphthol glucuronidation yielded more focused results, indicating that residues within segment B (84–147) contribute directly to the Km value for this substrate. Further mutagenesis and activity assays suggested that Phe117 of UGT1A9 participates in 1-naphthol binding. In addition, it appears that residues within segment C of the N-terminal domain, mainly at positions 152 and 169, contribute to the higher glucuronidation rates of UGT1A10.
Footnotes
This study was supported by funding from the Sigrid Juselius Foundation (to M.F.); and the Magnus Ehrnrooth Foundation (to L.L.).
Article, publication date, and citation information can be found at http://dmd.aspetjournals.org.
doi:10.1124/dmd.109.031229.
↵
The online version of this article (available at http://dmd.aspetjournals.org) contains supplemental material.
-
- UGT
- UDP-glucuronosyltransferase
- UDPGA
- UDP-glucuronic acid
- HPLC
- high-performance liquid chromatography
- DMSO
- dimethyl sulfoxide.
- Received November 17, 2009.
- Accepted January 20, 2010.
- Copyright © 2010 by The American Society for Pharmacology and Experimental Therapeutics
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|