Involvement of cytochrome P450 2C9, 2E1 and 3A4 in trimethadione N-demethylation in human microsomes.
OBJECTIVE
Trimethadione (TMO), an antiepileptic drug, may be used as a candidate for estimating hepatic drug-oxidizing activity. While TMO metabolism is mainly catalysed by CYP2C9, CYP2E1 and CYP3A4 the contribution of the different isoforms is unclear. In this study, we determined the percentage contribution of the three CYPs (CYP2C9, CYP2E1 and CYP3A4) to TMO N-demethylation.


METHODS
We used human liver microsomes and human recombinant CYPs expressed in human B-lymphoblast cells and baculovirus-infected insect cells.


RESULTS
The mean Km, Vmax and Vmax/Km values of TMO N-demethylation in human microsomes were 3.66 (mm), 503 (pmol/min/mg) and 2.61 (mL/h/mg), respectively. In the microsomes from human B-lymphoblast cells or baculovirus-infected insect cells, CYP 2C9, CYP 2E1 and CYP3A4 exhibited similar Km and higher Vmax in baculovirus-infected insect cells than B-lymphoblast cells. In baculovirus-infected insect cells, CYP2C9, CYP2E1 and CYP3A4 exhibited activities of 32, 286 and 77 pmol/min/pmol CYP, respectively. No CYP activity catalysed by CYP1A2 and 2D6 were detected in the two human cDNA expressed CYP isoforms.


CONCLUSIONS
TMO is metabolized not only by CYP2E1 but also CYP3A4 and CYP2C9. The order of this metabolism is as follows: CYP2E1 >> CYP3A4 > CYP2C9.
Tanaka
E
E
Institute of Community Medicine, University of Tsukuba, Tsukuba-shi, Ibaraki-ken, Japan. einosuke@md.tsukuba.ac.jp
Kurata
N
N
Yasuhara
H
H
D000927:Anticonvulsants / Q000378:metabolism*; D001189:Aryl Hydrocarbon Hydroxylases / Q000494:pharmacology*; D016367:Baculoviridae / Q000187:drug effects / Q000378:metabolism; D065729:Cytochrome P-450 CYP2C9; D019392:Cytochrome P-450 CYP2E1 / Q000494:pharmacology*; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000494:pharmacology*; D006801:Humans; D008862:Microsomes, Liver / Q000187:drug effects / Q000201:enzymology / Q000378:metabolism*; D014293:Trimethadione / Q000378:metabolism*
D000927:Anticonvulsants; D003577:Cytochrome P-450 Enzyme System; C450260:CYP2C9 protein, human; D065729:Cytochrome P-450 CYP2C9; D019392:Cytochrome P-450 CYP2E1; D001189:Aryl Hydrocarbon Hydroxylases; C104464:CYP3A protein, human; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D014293:Trimethadione