Screening of four Cyclopia (honeybush) species for putative phyto-oestrogenic activity by oestrogen receptor binding assays
Phyto-oestrogens mediate an oestrogenic effect through binding to the oestrogen receptor (ER) subtypes, ERα and ERβ. Four commercially available Cyclopia (honeybush) species - C. intermedia, C. subternata, C. genistoides and C. sessiliflora - together with nine commercially obtainable polyphenols present in some or all of the species, were screened for phyto-oestrogenic activity, using a competitive whole-cell ER binding assay. Only naringenin, formononetin and luteolin were able significantly to displace 3H-E2 from hERα, whereas luteolin, naringenin, formononetin, eriodictyol, narirutin and eriocitrin displaced 3H-E2 from hERβ. Mangiferin, hesperidin and hesperetin did not bind to either receptor subtype. To our knowledge, this is the first time that binding of eriodictyol, eriocitrin and narirutin to the hERβ has been shown. Furthermore, both aqueous and methanol extracts from three independent harvestings of each Cyclopia species were screened. The results suggest that C. genistoides and C. subternata display significant phyto-oestrogenic activity and that methanol extracts from 'unfermented' (unoxidized) plant material generally display greater activity. Great variation exists within a species, however, with one C. genistoides harvesting displacing 3H-E2 from both ER subtypes, while another harvesting displaced 3H-E2 from only hERβ, and a third did not displace 3H-E2 from either receptor subtype.