Table 4. Phytoestrogens, their potential target enzymes and subsequent effect on estrogen levels. Enzymes involved in estrogen synthesis and metabolism are considered potential target enzymes when in vitro effect concentrations (Table 3) are equal to or lower than reported plasma concentrations for a specific phytoestrogen (Table 2). Enzymes for which whole cell data are available are printed in bold. Potential effect on estrogen levels are hypothetical effects based on the physiological roles of the enzymes in estrogen homeostasis (see Fig. 1).

Phytoestrogen	Target enzymes involved in estrogen synthesis and metabolism (IC50 < Cmax)	Potential effect on estrogen levels
8-PN	Aromatase	Lower estrogen production
Biochanin A	3β-HSD2, CYP1B1	Not clear a
Daidzein	3β-HSD, SULT1A1	Not clear
Genistein	3β-HSD, 17β-HSD, SULT1E1, SULT1A1, CYP1B1	Increased intracellular E1, lower circulating E1/E2S
Naringenin	Aromatase, STS, CYP1B1	Altered estrogen production in ovary (↓) and breast (↑)
Resveratrol	CYP17, SULT, CYP1A1	Decreased androgen production, lower circulating E1/E2S
Quercetin	Aromatase, 17β-HSD, SULT, CYP1A1, CYP1B1, COMT	Increased estrogen (ovary)

^a In vivo, effects of biochanin A might be similar to genistein due to metabolic conversion.