PT  - JOURNAL ARTICLE
AU  - Nakajima, Miki
AU  - Tanaka, Eriko
AU  - Kobayashi, Tomo
AU  - Ohashi, Noriko
AU  - Kume, Toshiyuki
AU  - Yokoi, Tsuyoshi
TI  - Imipramine <em>N</em>-Glucuronidation in Human Liver Microsomes: Biphasic Kinetics and Characterization of UDP-Glucuronosyltransferase Isoforms
AID  - 10.1124/dmd.30.6.636
DP  - 2002 Jun 01
TA  - Drug Metabolism and Disposition
PG  - 636--642
VI  - 30
IP  - 6
4099  - http://dmd.aspetjournals.org/content/30/6/636.short
4100  - http://dmd.aspetjournals.org/content/30/6/636.full
SO  - Drug Metab Dispos2002 Jun 01; 30
AB  - A method for the direct determination of imipramineN-glucuronidation in human liver microsomes by high-performance liquid chromatography with UV detection was developed. Imipramine was incubated with human liver microsomes and UDP-glucuronic acid. The Eadie-Hofstee plots of imipramineN-glucuronidation in human liver microsomes were biphasic. For the high-affinity component, theKm was 97.2 ± 39.4 μM and theVmax was 0.29 ± 0.03 nmol/min/mg of protein. For the low-affinity component, theKm was 0.70 ± 0.29 mM and theVmax was 0.90 ± 0.28 nmol/min/mg of protein. The imipramine N-glucuronosyltransferase activities were not detectable in two samples of human jejunum microsomes. Among recombinant UDP-glucuronosyltransferases (UGTs) in baculovirus-infected insect cells (Supersomes or Bacurosomes) or human B-lymphoblastoid cells tested in the present study (UGT1A1, UGT1A3, UGT1A4, UGT1A6, UGT1A7, UGT1A8, UGT1A9, UGT1A10, UGT2B7, and UGT2B15), only UGT1A4 showed imipramineN-glucuronosyltransferase activity. The activity in UGT1A4 Supersomes was higher than that in recombinant UGT1A4 expressed in human B-lymphoblastoid cells at all imipramine concentration tested. The kinetics of imipramine N-glucuronidation in UGT1A4 Supersomes did not fit the Michaelis-Menten plot, showing aKm of >1 mM. In contrast, in UGT1A4 expressed in human B-lymphoblastoid cells,Km was 0.71 ± 0.36 mM and theVmax was 0.11 ± 0.03 nmol/min/mg of protein. Interindividual differences in the imipramineN-glucuronidation in liver microsomes from 14 humans were at most 2.5-fold. The imipramineN-glucuronosyltransferase activities in 11 human liver microsomes were significantly (r = 0.817,P < 0.005) correlated with the glucuronosyltransferase activities of trifluoperazine, a typical substrate of UGT1A4. This is the first report of the biphasic kinetics of imipramine N-glucuronide in human liver microsomes. The American Society for Pharmacology and Experimental Therapeutics