title: Inhibition by green tea catechins of metabolic activation of procarcinogens by human cytochrome P450.
abstract: Catechins, major polyphenol constituents of green tea, are potent chemopreventive agents against cancers caused by chemical carcinogens in rodents. The effects of four epicatechin derivatives, epigallocatechin gallate (EGCG), epicatechin gallate (ECG), epigallocatechin (EGC) and epicatechin (EC), on the metabolic activation of benzo[a]pyrene (B[a]P), 2-amino-1-methyl-6-phenylimidazo-[4,5-b]pyridine (PhIP) and aflatoxin B(1) (AFB(1)) by human cytochrome P450 (CYP) were examined. B[a]P, PhIP and AFB(1) were activated by respective human CYP1A1, CYP1A2 and CYP3A4 expressed in the membrane fraction of genetically engineered Salmonella typhimurium (S. typhimurium) TA1538 cells harboring the human CYP and human NADPH-CYP reductase (OR), when the membrane fraction was added to S. typhimurium TA98. Galloylated catechins, ECG and EGCG inhibited the mutagenic activation potently, while EGC and EC showed relatively weak inhibitory effects. Catechins also inhibited the oxidations of typical substrates catalyzed by human CYPs, namely ethoxycoumarin O-deethylation by CYP1A1, ethoxyresorufin O-deethylation by CYP1A2 and midazolam 1'-hydroxylation by CYP3A4. The IC(50) values of catechins for the inhibition of human CYP were roughly the same as those seen in the mutagenic activation. EGCG inhibited other forms of human CYP such as CYP2A6, CYP2C19 and CYP2E1, indicating the non-specific inhibitory effects of EGCG toward human CYPs. Furthermore, EGCG inhibited human NADPH-cytochrome CYP reductase (OR) with a K(i) value of 2.5 microM. These results suggest that the inhibition of the enzyme activity of CYP is accounted for partially by the inhibition of OR.
authors.affiliation: Laboratory of Drug Metabolism, Graduate School of Pharmaceutical Sciences, Hokkaido University, N12W6, Kita-ku, 060-0812, Hokkaido, Sapporo, Japan.
authors.forename: S
authors.initials: S
authors.lastname: Muto
authors.forename: K
authors.initials: K
authors.lastname: Fujita
authors.forename: Y
authors.initials: Y
authors.lastname: Yamazaki
authors.forename: T
authors.initials: T
authors.lastname: Kamataki
mesh_terms: D016604:Aflatoxin B1 / Q000378:metabolism; D016588:Anticarcinogenic Agents / Q000494:pharmacology / Q000627:therapeutic use; D000975:Antioxidants / Q000494:pharmacology; D001564:Benzo(a)pyrene / Q000494:pharmacology; D001628:Beverages; D002273:Carcinogens; D002392:Catechin / Q000031:analogs & derivatives* / Q000494:pharmacology* / Q000627:therapeutic use*; D002462:Cell Membrane / Q000378:metabolism; D019363:Cytochrome P-450 CYP1A1 / Q000378:metabolism; D019388:Cytochrome P-450 CYP1A2 / Q000378:metabolism; D051544:Cytochrome P-450 CYP3A; D003577:Cytochrome P-450 Enzyme System / Q000378:metabolism*; D004305:Dose-Response Relationship, Drug; D004789:Enzyme Activation; D004791:Enzyme Inhibitors / Q000494:pharmacology; D006801:Humans; D007093:Imidazoles / Q000494:pharmacology; D007700:Kinetics; D006899:Mixed Function Oxygenases / Q000378:metabolism; D016296:Mutagenesis; D009251:NADPH-Ferrihemoprotein Reductase / Q000378:metabolism; D011485:Protein Binding; D012486:Salmonella typhimurium / Q000235:genetics / Q000378:metabolism
chemical_list: D016588:Anticarcinogenic Agents; D000975:Antioxidants; D002273:Carcinogens; D004791:Enzyme Inhibitors; D007093:Imidazoles; D001564:Benzo(a)pyrene; D002392:Catechin; D003577:Cytochrome P-450 Enzyme System; C049584:2-amino-1-methyl-6-phenylimidazo(4,5-b)pyridine; C062669:epicatechin gallate; D016604:Aflatoxin B1; C045651:epigallocatechin gallate; D006899:Mixed Function Oxygenases; C104464:CYP3A protein, human; D019363:Cytochrome P-450 CYP1A1; D019388:Cytochrome P-450 CYP1A2; D051544:Cytochrome P-450 CYP3A; C510163:CYP3A4 protein, human; D009251:NADPH-Ferrihemoprotein Reductase; C057580:gallocatechol