DIETARY ANTIOXIDANTS, AGING, AND OXIDATIVE STRESS STATUS
Location: Human Nutrition Research Center on Aging
Title: The kinetic basis for age-associated changes in quercetin and genistein glucuronidation by rat liver microsomes
| Bolling, Bradley - TUFTS UNIVERSITY |
| Court, Michael - TUFTS UNIVERSITY |
| Blumberg, Jeffrey |
| Chen, C-Y Oliver - JM USDA HNRCA @ TUFTS |
Submitted to: Journal of Nutritional Biochemistry
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: February 9, 2009
Publication Date: May 14, 2009
Citation: Bolling, B.W., Court, M.H., Blumberg, J., Chen, C. 2009. The kinetic basis for age-associated changes in quercetin and genistein glucuronidation by rat liver microsomes. Journal of Nutritional Biochemistry. DOI: 10.1016/j.jnutbio.2009.02.006.
Interpretive Summary: High intakes of dietary flavonoids are thought be protective against some cancers, stroke, and cardiovascular disease. Upon consumption, flavonoids are metabolized to more easily excretable (water-soluble) forms. The UDP-glucuronosyltransferases (UGTs) is a major enzyme class, responsible for this conversion. The stability of UGTs to flavonoid metabolism with age is not known. Prior evidence indicates that metabolism of flavonoids may change with age. Therefore, we examined the ability of hepatic UGTs to metabolize the flavonoid quercetin at different ages of adult rats. We found distinct changes in UGT enzyme kinetics at different ages. Quercetin can be metabolized (conjugated with a sugar molecule) at three different positions. While the overall capacity of the UGT enzyme was preserved with age, the profile of the metabolites was different from 4, 18, and 28 month rats. Further, we found that age-related changes to UGT activity was different using the isoflavone genistein, another flavonoid. Therefore, age-related changes to UGT activity depend on the type of flavonoid.
The dietary bioavailability of the isoflavone genistein is decreased in older rats compared to young adults. Since flavonoids are metabolized extensively by the UDP-glucuronosyltransferases (UGTs), we hypothesized that UGT flavonoid conjugating activity changes with age. The effect of age on flavonoid glucuronidation was determined using hepatic microsomes from male F344 rats. Kinetic models of UGT activity toward the flavonol quercetin and the isoflavone genistein were established using pooled hepatic microsomal fractions of rats at different ages, and glucuronidation rates determined using individual samples. Intrinsic clearance (Vmax/Km) values in 4, 18, and 28 mo old rats were 0.100, 0.078, and 0.087 mL/min/mg for quercetin-7-O-glucuronide, 0.138, 0.133, and 0.088 for quercetin-3'-O-glucuronide, and 0.075, 0.077, and 0.057 for quercetin-4'-O-glucuronide, respectively. While there were no differences in formation rates of total quercetin glucuronides in individual samples, the production of the primary metabolite, quercetin-7-O-glucuronide, at 30 uM quercetin concentration was increased from 3.4 and 3.1 nmol/min/mg at 4 and 18 mo to 3.8 nmol/min/mg at 28 mo, while quercetin-3'-O-glucuronide formation at 28 mo declined by a similar degree (P </=0.05). At 30 and 300 uM quercetin concentration, the rate of quercetin-4'-O-glucuronide formation peaked at 18 mo at 0.9 nmol/min/mg. Intrinsic clearance values of genistein 7-O-glucuronide increased with age. Thus, the capacity for flavonoid glucuronidation by rat liver microsomes is dependent on age, UGT isoenzymes, and flavonoid structure.