Abstract
Soybean products containing isoflavones are widely consumed in Western and Asian diets for putative health benefits, but adverse effects are also possible. The conjugated forms of isoflavones present in a soy nutritional supplement (predominately acetyl glucosides) and in blood from two human volunteers after consuming the supplement (7- and 4′-glucuronides and sulfates) were identified using liquid chromatography coupled with electrospray/tandem mass spectrometry. Circulating conjugates of genistein and daidzein were quantified using selective enzymatic hydrolysis and deuterated internal standards for liquid chromatography-electrospray/mass spectrometry. The levels of isoflavone glucuronides were much greater than the corresponding sulfates or aglycones. The substrate activities of genistein and daidzein were evaluated with recombinant human UDP glucuronosyl transferase (UGT) and sulfotransferase (SULT) by using enzyme kinetics. The SULTs 1A1∗2, 1E, and 2A1 catalyzed formation of a single genistein sulfate; however, SULTs 1A2∗1 and 1A3 had no observed activity. None of the SULTs showed activity with daidzein. Although several UGTs (1A1, 1A4, 1A6, 1A7, 1A9, and 1A10) catalyzed 7- and 4′-glucuronidation of genistein or daidzein, the UGT 1A10 isoform, which is found in human colon but not liver, was found to be specific for genistein. Glucuronidation of only genistein was observed in human colon microsomes, although nearly equal activity was observed for daidzein in human liver and kidney microsomes. These findings suggest a prominent role for glucuronidation of genistein in the intestine concomitant with absorption, although hepatic glucuronidation of absorbed genistein and daidzein aglycones is also likely.
Footnotes
-
Send reprint requests to: Daniel R. Doerge, Ph.D., Department of Health and Human Services, National Center for Toxicological Research, 3900 NCTR Rd., Jefferson, AR 72079-9502. E-mail: ddoerge{at}nctr.fda.gov
-
This work was supported in part by Interagency Agreement 224-93-0001 between NCTR/FDA and the National Institute for Environmental Health Sciences/National Toxicology Program. H.C.C. acknowledges support of a fellowship from the Oak Ridge Institute for Science and Education, administered through an interagency agreement between the U.S. Department of Energy and the U.S. Food and Drug Administration.
- Abbreviations used are::
- LC
- liquid chromatography
- ES
- electrospray
- MS
- mass spectrometry
- SULT
- sulfotransferase
- MRM
- multiple reaction monitoring
- UDPGA
- uridine-5′-diphosphate-β,d-glucuronic acid ester
- UGT
- UDP glucuronosyl transferase
- Received August 20, 1999.
- Accepted November 1, 1999.
- U.S. Government
DMD articles become freely available 12 months after publication, and remain freely available for 5 years.Non-open access articles that fall outside this five year window are available only to institutional subscribers and current ASPET members, or through the article purchase feature at the bottom of the page.
|