title: Kinetic parameters of lymphocyte microsomal epoxide hydrolase in carbamazepine hypersensitive patients. Assessment by radiometric HPLC.
abstract: Idiosyncratic hypersensitivity reactions with carbamazepine have been postulated to be due to a deficiency of microsomal epoxide hydrolase (HYL1), although this is based on indirect evidence. Using 3H-cis stilbene oxide (0.5 Ci/mmol) as a substrate, we have developed a radiometric HPLC assay sensitive enough to measure the kinetic parameters of HYL1 in lymphocytes. The intra-assay coefficient of variation was 8%. Enzyme activity has been measured in lymphocytes from six carbamazepine hypersensitive patients, six patients on carbamazepine without any adverse effects, and twelve drug-naive healthy volunteers. No significant difference was observed in three kinetic parameters of the enzyme among these three groups. The values for Km, Vmax, and intrinsic clearance ranged from 6.1-89.9 microM, 3.0-23.2 pmoles diol formed/min/mg protein, and 0.147-0.493 microliter/min/mg protein. There was no difference in enzyme activity between patients currently on carbamazepine and healthy volunteers, indicating a lack of induction of lymphocyte HYL1 by carbamazepine. Co-incubation of lymphocytes with 1,1,1-trichloropropene oxide, an inhibitor of hepatic HYL1, resulted in an 82% inhibition of activity, similar to that observed with the hepatic enzyme. The healthy volunteers were genotyped as being either GSTM1 positive (n = 6) or GSTM1 negative (n = 6). This did not affect the kinetic parameters of lymphocyte microsomal epoxide hydrolase. Our results suggest that there is normal HYL1 activity in lymphocytes of hypersensitive patients using cis-stilbene oxide as a substrate.
authors.affiliation: Department of Pharmacology and Therapeutics, University of Liverpool, U.K.
authors.forename: C D
authors.initials: CD
authors.lastname: Davis
authors.forename: M
authors.initials: M
authors.lastname: Pirmohamed
authors.forename: N R
authors.initials: NR
authors.lastname: Kitteringham
authors.forename: R L
authors.initials: RL
authors.lastname: Allott
authors.forename: D
authors.initials: D
authors.lastname: Smith
authors.forename: B K
authors.initials: BK
authors.lastname: Park
mesh_terms: D000293:Adolescent; D000328:Adult; D000368:Aged; D000927:Anticonvulsants / Q000009:adverse effects*; D002220:Carbamazepine / Q000009:adverse effects*; D002851:Chromatography, High Pressure Liquid; D004342:Drug Hypersensitivity / Q000097:blood / Q000201:enzymology* / Q000209:etiology; D004791:Enzyme Inhibitors / Q000494:pharmacology; D004851:Epoxide Hydrolases / Q000037:antagonists & inhibitors / Q000097:blood*; D005260:Female; D005982:Glutathione Transferase / Q000378:metabolism; D006801:Humans; D006868:Hydrolysis; D007527:Isoenzymes / Q000378:metabolism; D007700:Kinetics; D008214:Lymphocytes / Q000201:enzymology*; D008297:Male; D008861:Microsomes / Q000201:enzymology; D008875:Middle Aged; D011874:Radiometry; D013267:Stilbenes / Q000378:metabolism; D014239:Trichloroepoxypropane / Q000494:pharmacology; D014316:Tritium
chemical_list: D000927:Anticonvulsants; D004791:Enzyme Inhibitors; D007527:Isoenzymes; D013267:Stilbenes; D014316:Tritium; D014239:Trichloroepoxypropane; D002220:Carbamazepine; D005982:Glutathione Transferase; D004851:Epoxide Hydrolases; C025906:stilbene oxide