The influence of diet composition on phase I and II biotransformation enzyme induction.

The expression of phase I and II biotransformation enzymes was examined with respect to experimental diet composition and with the addition of the bi-functional inducer flavone. Enzymatic activity and mRNA levels of cytochrome P450 monooxygenase (CYP) isoforms (CYP1A1, CYP1A2, CYP2B1/2) and glutathione-S-transferase (GST) isoforms (GSTA, GSTM, and GSTP) were used as indices for the changes in expression. An amino acid based (AA) diet and a semi-purified egg white (EW) diet were designed to include similar levels of nutrients and were compared to a standard laboratory chow (SC) diet. Rats (Sprague-Dawley) and mice (C57BL/6) were used as animal models. Animals were fed one of the three diets for 7 days prior to incorporation of flavone (2%, wt/wt). Diets with or without flavone were next fed for an additional 3 days. Enzymatic activities of the CYPs in mice and GSTs in both mice and rats were determined. In mice, the relative mRNA levels for each of the CYP and GST isoforms were also measured. The increase in phase I and II enzyme expression observed in response to flavone was most dynamic when the AA-based diet was used (often >20-fold for given isoform enzymatic activities and >200-fold for specific mRNAs), followed by the EW diet (10 to 20-fold and 100 to 200-fold, respectively). The SC diet resulted in a higher level of background expression of CYP and GST isoforms and as a consequence the observed fold increases in CYP and GST isoforms (enzymatic and mRNA levels) were substantially less (1 to 10-fold and 1 to 150-fold. respectively), when the SC diet fed group with or without flavone was compared.

Influence of dietary quercetin on glutathione redox status in mice.

Flavonoids such as quercetin have been shown to serve as a protective defense against oxidative damage in vivo. However, the bioavailability of quercetin depends on the food source and type of glycosidic moiety linked to the molecule. In this study, mice were fed 1 mg/day quercetin in the form of quercetin aglycone, rutin, apple, or onion, and reduced glutathione (GSH), oxidized glutathione (GSSG), and protein-GSH mixed disulfides were determined to investigate the influence of dietary quercetin on the GSH redox status in metabolically active tissues, mitochondria, and plasma of mice. All quercetin treatment groups produced increases in the GSH:GSSG ratio and decreases in mixed disulfide levels in hepatic tissue. Cardiac tissue did not change in response to dietary quercetin; however, cardiac mitochondria demonstrated a reduction in the GSH:GSSG ratio and an increase in protein mixed disulfide levels. No significant changes were observed in the plasma GSH:GSSG ratio, but mixed disulfide levels were decreased for all of the diets. The changes in plasma redox status did not parallel the changes in the tissues. Onion fed mice demonstrated the greatest increases in GSH:GSSG ratios and the greatest decreases in protein mixed disulfide levels of all diets compared. For all treatment groups, increases in the GSH:GSSG ratios corresponded with decreases in protein mixed disulfide levels. The results of this study indicate that quercetin influences GSH:GSSG ratios and protein thiolation in a tissue-specific manner and that these effects are dependent on food source and bioavailability.

Isozyme- and gender-specific induction of glutathione S-transferases by flavonoids.

Most dietary flavonoids have antioxidant activity in vitro however, secondary mechanisms such as the ability to influence gene expression with the consequent modulation of specific enzymatic activities involved in the intracellular response against oxidative stress, are being realized. In the following study, we examined the ability of the flavonoids: flavone, morin, naringenin, (+)-catechin, and quercetin to modulate the activity of glutathione S-transferases (GSTs) mGSTA, mGSTP and mGSTM in hepatic tissues of male and female Swiss Webster mice. Subchronic dietary exposure to morin, naringenin, (+)-catechin, and quercetin (2,500 mg/kg diet for 20 days) did not produce statistically significant changes in GST activity. Conversely, gender-, and isozyme-specific induction of mGSTs were observed in animals fed flavone. A sevenfold increase in total mGST activity was observed in female animals whereas a fourfold increase was observed in male animals. Enzyme specific assays indicate that there were greater increases of both mGSTM (eightfold) and mGSTP (fourfold) activities in females as compared to males (sixfold and twofold, respectively). As testosterone is involved in the regulation of GSTs in mice, castrated males were fed flavone for 5 days (2,500 mg/kg diet). In this case, dietary flavone resulted in similar fourfold increases in total GST activity in inact and castrated animals. Isozyme specific studies indicate that increases could be attributed to an induction of mGSTM and mGSTP.