(±)-Equol 4’-sulfate is a gut-mediated phase II metabolite of the isoflavonoid phytoestrogen (±)-equol .[1] It has been used as a standard for the quantification of unconjugated equol in rat plasma.[2]
Reference:
[1]. Lee, D.-H., Kim, M.J., Lee, S.H., et al. Nutrikinetics of isoflavone metabolites after fermented soybean product (Cheonggukjang) ingestion in ovariectomized mice. Mol. Nutr. Food Res. 61(12), (2017).
[2]. Gavina, J.M., Priem, J., Wood, C.M., et al. Determination of isoflavones in rat serum using liquid chromatography-tandem mass spectrometry with a highly efficient core-shell column. Anal. Bioanal. Chem. 405(8), 2643-2651 (2013).