Oxidative in vitro metabolism of the soy phytoestrogens daidzein and genistein

The oxidative metabolism of the major soy isoflavones daidzein and genistei n was investigated using Liver microsomes from Aroclor-treated male Wistar rats. Both daidzein and genistein were extensively metabolized and are ther efore excellent substrates for cytochrome P450 enzymes. The identity of the metabolites was elucidated using high-performance liquid chromatography (H PLC) with diode array detection, gas chromatography-mass spectrometry (GC/M S), and HPLC/atmospheric pressure ionization electrospray mass spectrometry (API-ES MS) as well as reference substances. Daidzein was converted to nin e metabolites, comprising four monohydroxylated, four dihydroxylated, and o ne trihydroxylated metabolite. Genistein was metabolized to four monohydrox ylated and two dihydroxylated products. With both isoflavones the additiona l hydroxy groups are exclusively introduced into the ortho positions of exi sting phenolic hydroxy groups. The major metabolites of daidzein were ident ified as 6,7,4'-trihydroxyisoflavone, 6,7,3',4'-tetrahydroxyisoflavone, 7,8 ,4'-trihydroxyisoflavone, and 5,6,7,4'-tetrahydroxyisoflavone. The main mic rosomal metabolites of genistein were 5,6,7,4'-tetrahydroxyisoflavone and 5 ,7,8,4'-tetrahydroxyisoflavone. Furthermore, the GC/MS and HPLC/API-ES MS a nalysis support the conclusion that one monohydroxylated metabolite of daid zein and genistein is hydroxylated at the aliphatic position C-2 of the C-r ing. The UV-vis, GC/MS, and HPLC/MS data of all detected metabolites as wel l as the derived chemical structure of the metabolites are presented. Most metabolites are reported in this paper for the first time: On the basis of these findings it is suggested that hydroxylation reactions may also play a n important role in the in vivo metabolism of the soy isoflavones daidzein and genistein.