Characterization and quantification of soy isoflavone metabolites in serum of renal transplanted patients by high-performance liquid chromatography/electrospray ionization mass spectrometry

During a dietary intervention study on 16 renal transplanted patients, in which 25 g/day of animal proteins were replaced with 25 g of soy proteins, the metabolic profile of soy isoflavones in serum was characterized. This paper describes a reliable and fast liq. chromatog./electrospray ionization mass spectrometry (LC/ESI-MS) method, in neg. ion mode, allowing the characterization and simultaneous quantification of several soy isoflavone metabolites. Six metabolites were identified and quantified: daidzein ([M-H]- at m/z 252.8), dihydrodaidzein (DHD, [M-H]- at m/z 254.8), equol ([M-H]- at m/z 240.9), O-desmethylangolensin (O-DMA, [M-H]- at m/z 256.8), genistein ([M-H]- at m/z 268.8), and dihydrogenistein (DHG, ([M-H+]- at m/z 270.8). Quantification was assessed using two deuterated internal stds., D3-daidzein and D4-genistein. This method permitted a limit of quantification (LOQ, S/N = 10) and a limit of detection (LOD, S/N = 3) of 0.05 ?M and 0.005 ?M for all analytes, except for genisein, where the LOQ and LOD were 0.005 ?M and 0.001 ?M, resp. The linearity ranges were from 0.005 to 1.5 ?M for genistein, from 0.05 to 1.5 ?M for DHG, and from 0.05 to 0.7 ?M for the other metabolites. The relative std. deviations (RSDs) were between 0.19% and 13.9% at the LOQ concn. for all metabolites, and between 0.6% and 4.8% at the max. concn. On the basis of the results obtained in the dietary intervention study, it was possible to split the patients into five groups characterized by different metabolic pathways.