Quantification of Soy Isoflavones, Genistein and Daidzein, and Conjugates in Rat Blood Using LC/ES-MS

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Abstract
Genistein is the principal soy isoflavone to which the putative beneficial effects of soy consumption have been attributed; however, the possibility of adverse biological effects (e.g., estrogenic, antithyroid) has also been raised. This paper describes development and validation of a simple and sensitive analytical method for the determination of genistein in the blood of rats receiving dietary genistein (90%), and only small amounts of the sulfate conjugate and the aglycone were observed at all dose levels. No evidence for additional metabolites was obtained. The 7- and 4‘-glucuronide conjugates of genistein were identified using electrospray mass spectrometry and 1H NMR. Total blood genistein ranged from <15 nM in animals fed soy-free control diet to as high as 8.9 μM in male rats fed 1250 μg of genistein/g of chow and encompasses blood isoflavone levels observed in humans consuming a typical Asian diet and nutritional supplements (0.1−1 μM) and infants consuming soy formulas (2−7 μM). Keywords: Genistein; soy; isoflavone; electrospray mass spectrometry