N-Glucuronidation of Some 4-Arylalkyl-1H-Imidazoles by Rat, Dog, and Human Liver Microsomes

Abstract

This article is available online at http://dmd.aspetjournals.org ABSTRACT:N-Glucuronidation in vitro of six 4-arylalkyl-1H-imidazoles (both enantiomers of medetomidine, detomidine, atipamezole, and two other closely related compounds) by rat, dog, and human liver microsomes and by four expressed human UDP-glucuronosyltransferase isoenzymes was studied. Rat liver microsomes glucuronidated these compounds at a barely detectable rate. Four expressed human UDP-glucuronosyltransferase isoenzymes (UGT1A3, UGT1A4, UGT1A6, and UGT1A9) were studied for 4-arylalkyl-1H-imidazole-conjugating activity. Only UGT1A4 glucuronidated these compounds at an activity of about 5% of that measured for 4-aminobiphenyl. The observed activity of UGT1A4 does not explain the high efficiency of glucuronidation of 4-arylalkyl-1H-imidazoles in human liver microsomes.

Regio- and Stereospecific N-Glucuronidation of Medetomidine: The Differences between UDP Glucuronosyltransferase (UGT) 1A4 and UGT2B10 Account for the Complex Kinetics of Human Liver Microsomes

“…2). The sharp difference in activity between human liver to intestine microsomes is noteworthy, particularly because UGT1A4 that until now was assumed to be the major, if not the sole, human enzyme catalyzing the N-glucuronidation of imidazole-containing drugs (Kaivosaari et al, 2002), is expressed to some extent in the human intestine (Kaivosaari et al, 2007).…”

“…Glucuronide Characterization by LC-MS. To verify that both G1 and G2 are the expected monoglucuronide metabolites, dex-and levomedetomidine were subjected to glucuronidation catalyzed by HLMs and recombinant UGT1A4, the enzyme previously shown to catalyze such activity (Kaivosaari et al, 2002), and the regioisomeric glucuronide peaks were separated and collected by LC-UV and analyzed by MS. The electrospray ionization-MS spectra of all the four metabolites, DG1, DG2, LG1, and LG2, regardless of whether they were formed by HLMs or UGT1A4, showed an ion m/z 377 at positive ion mode, corresponding to protonated glucuronide.…”

“…1; Bhana et al, 2000;Ji et al, 2004). Interestingly, kinetic analyses in human liver microsomes (HLMs) showed that the glucuronidation of the levoenantiomer was considerably more efficient and at much higher affinity than for the dextro-form, apparent K m values of 16 versus 370 M, respectively (Kaivosaari et al, 2002). Dog liver microsomes also formed N-glucuronides from both medetomidine enantiomers but at a lower efficiency than HLMs.…”

“…Our previous investigation of the glucuronidation activity of recombinant human UGTs 1A3, 1A4, 1A6, and 1A9 toward medetomidine enantiomers revealed that none of these isoforms, not even UGT1A4, which exhibited some activity, account for the high efficiency of medetomidine glucuronidation by HLMs (Kaivosaari et al, 2002). We have now resumed the search for the missing UGT, taking advantage of the availability of a nearly complete set of the human UGTs as recombinant proteins in our laboratory (Kurkela et al, 2007).…”

“…Among the human UGTs, UGT1A4 was largely considered as the enzyme "specializing" in N-glucuronidation because it is capable of conjugating different types of amines: primary aromatic amines, secondary and tertiary aliphatic amines, and secondary and tertiary aromatic N-heterocycles (Green and Tephly, 1998;Kaivosaari et al, 2002;Zenser et al, 2002;Kuehl and Murphy, 2003;Rowland et al, 2006). Several other human UGTs, including 1A1, 1A3, 1A7, 1A9, and 2B7, were documented to catalyze different N-glucuronidation reactions (Green and Tephly, 1998;Kaivosaari et al, 2002;Zenser et al, 2002;Staines et al, 2004;Girard et al, 2005;Kaji and Kume, 2005;Borlak et al, 2006;Rowland et al, 2006;Omura et al, 2007). Nevertheless, for all the latter enzymes, N-glucuronidation seems to be a minor reaction, whereas for UGT1A4, it is probably the major type of activity.…”