Metabolism of Apigenin by Rat Liver Phase I and Phase Ii Enzymes and by Isolated Perfused Rat Liver

Abstract

This article is available online at http://dmd.aspetjournals.org ABSTRACT:The metabolism of apigenin, a low estrogenic flavonoid phytochemical, was investigated in rat using liver models both in vitro (subcellular fractions) and ex vivo (isolated perfused liver). In vitro, phase I metabolism led to the formation of three monohydroxylated derivatives: luteolin which was the major metabolite (K m ‫؍‬ 22.5 ؎ 1.5 M; V max ‫؍‬ 5.605 ؎ 0.090 nmol/min/mg protein, means ؎ S.E.M.), scutellarein, and iso-scutellarein. These oxidative pathways were mediated by cytochrome P450 monooxygenases (P450s). The use of P450 inhibitors and inducers showed that CYP1A1, CYP2B, and CYP2E1 are involved. In vitro studies of phase II metabolism indicated that apigenin underwent conjugation giving three monoglucuronoconjugates and one monosulfoconjugate. Luteolin led to the formation of four monoglucuronoconjugates, two sulfoconjugates, and one methylconjugate identified as diosmetin. Ex vivo during the apigenin perfusion of an isolated rat liver, none of the phase I metabolites could be recovered. In contrast, two monoglucuronoconjugates and one of the sulfoconjugates of apigenin already identified in vitro were recovered. Moreover, two new derivatives were isolated and identified as a diglucuronoconjugate and a glucuronosulfoconjugate. This work provides new data about the metabolism of apigenin and shows the interest value of using various experimental models in metabolic studies.

Tissue distribution in mice and metabolism in murine and human liver of apigenin and tricin, flavones with putative cancer chemopreventive properties

“…It remains to be investigated whether hydroxylation contributes indeed to the metabolic clearance of apigenin in vivo. Experiments in the perfused rat liver model suggest that, unlike in liver microsomes in vitro, phase I metabolites of apigenin could not be found in this ex-vivo system [12], intimating the possibility that apigenin hydroxylation may be quantitatively insignificant in vivo. It needs to be stressed, that the difference in systemic levels between apigenin and tricin after dietary intake may also have been caused by processes unrelated to metabolism, for example by differential absorption.…”

“…Figure 6 shows that analysis of extracts of incubates with apigenin furnished a metabolite peak, whilst analysis of extracts of tricin incubates did not reveal significant peaks in addition to that of parent agent. On UV spectroscopic analysis the apigenin metabolite showed a high absorbance maximum of 347.0 nm, above that of the equivalent aborbance maximum of apigenin, consistent with this metabolite being luteolin [12]. Unlike apigenin, tricin does not seem to undergo phase I drug metabolism at a measurable rate in mouse liver micosomes.…”

“…Our analytical method did 16 not allow localization of the exact position of glucuronyl or sulfate attachment. Previously three monoglucuronides of apigenin have been identified in incubations with rat liver microsomes, again without exact definition of positional attachment of the glucuronyl moiety [12]. One of these metabolites was quantitatively prevalent.…”

“…Apigenin has been suggested to undergo NADPH-dependent phase I drug metabolism in rat liver microsomes in vitro to a mono-hydroxylated derivative, luteolin (3',4',5,7-tetrahydroxyflavone) [12]. In order to explore whether tricin also undergoes phase I biotransformation, tricin and apigenin were incubated side-by-side with mouse liver microsomes in the presence of NADPH.…”

Protective Effects of Kaempferol on Isoniazid- and Rifampicin-Induced Hepatotoxicity

“…The excretion of flavonoids or their conjugated metabolites may involve transport by transporters such as multidrug resistance-associated proteins 1 and 2 or breast cancer resistance protein (5,6). Unconjugated flavonoid aglycones may be substrates of the drug efflux transporter P-glycoprotein (multidrug-resistant 1) (7).…”