Polymethoxy-flavonoids are naturally occurring compounds found abundantly in the juice and peel of citrus fruits such as Citrus nobilis, Citrus aurantium, Citrus hassaku and Citrus depressa HAYATA (Rutaceae). They include nobiletin (5,6,7,8,3Ј,4Ј-hexamethoxyflavone), tangeretin (5,6,7,8,4Ј-pentamethoxyflavone), sinensetin (5,6,7,3Ј,4Ј-pentamethoxyflavone) and natsudaidain (3-hydroxy-5,6,7,8,3Ј,4Ј-hexamethoxyflavone). [1][2][3][4][5] Studies have demonstrated that nobiletin possesses various biological properties such as anti-proliferative, 6,7) anti-carcinogenic, [8][9][10] and anti-inflammatoric [11][12][13] along with potent cell differencing activity. 14-16) For example, nobiletin suppresses inflammation through the inhibition of cyclooxygenase (COX)-2, a causal enzyme for skin inflammation and decreases tumor invasion through the suppression of matrix metalloproteinase 9, a collagenase that hydrolyzes components of the cell matrix such as collagen and gelatin. It has also been reported that nobiletin inhibits the growth of tumor cells more potently than other polyhydroxyflavonoids including quercetin and taxifolin.7) It is thought that nobiletin can enter into cells more easily than polyhydroxy-flavonoids due to its high lipophilicity.Cytochrome P450 (P450) is a key enzyme catalyzing the oxidative metabolism of a large number of exogenous and endogenous compounds 17) and is reported to be involved in the metabolism of flavonoids.18) Nielsen et al. 18,19) used liver microsomes of Aroclor 1254 (a commercial PCB preparation)-pretreated rats to show that the main metabolic pathway of polyhydroxy-flavonoids such as apigenin, naringenin and kaempferol is a hydroxylation in the ring A or B of flavone molecule. They also showed that the main pathways of tangeretin are demethylation in ring A or B and hydroxylation at 3Ј-position of ring B.18,19) Recent studies using recombinant human P450s have demonstrated that genistein, an isoflavone abundant in soy beans, is metabolized predominantly to 3Ј-OH-genistein by CYP1A1, CYP1A2, CYP1B1 and CYP2E1 and to two other metabolites by CYP3A4. 20) As well, the tangeretin metabolism has been shown to be catalyzed by CYP1A2, CYP3A4, CYP2D6 and CYP2C9. 21) On the other hand, there are not many reports on the biotransformation of nobiletin in animals. [22][23][24][25] Yasuda et al. found a few demethylated metabolites in 24 h-urine of rats orally administered nobiletin and determined that a major metabolite is 4Ј-demethylated (4Ј-OH) nobiletin. 24) However, the mechanism which the biological activities mentioned above are exerted by a parent compound or its metabolites remains obscure. For a better understanding of this point, it is very important to compare the metabolic profile of nobiletin in various animals and to elucidate which P450s are involved in the metabolism of nobiletin. Therefore, we investigated the in vitro metabolism of nobiletin using liver microsomes of rats, hamsters and guinea pigs, and ten recombinant rat P450s. We also examined the inductive effects of pro...
