Isoflavones derived from many edible plants have been reported to possess significant
antioxidant, estrogenic and
tyrosine kinase inhibitory activity.
Genistein has been found previously to provide protection from oxidative damage induced by UV radiation both in vitro and following dietary administration. We have therefore examined the potential of a number of
isoflavones from red clover (Trifolium pratense) and some metabolically related compounds to offer protection from UV irradiation in hairless mice by topical application after UV exposure. We show that whereas the primary
isoflavones,
daidzein,
biochanin A and
formononetin, were inactive, 20 microM lotions of
genistein and the metabolites
equol, isoequol and the related derivative
dehydroequol had powerful potential to reduce the inflammatory
edema reaction and the suppression of
contact hypersensitivity induced by moderate doses of solar-simulated UV radiation. For
equol the protection was concentration dependent and 5 microM
equol markedly reduced the UV-induced
inflammation but abrogated the UV-induced immunosuppression.
Equol protected similarly from immunosuppression induced by the putative epidermal mediator, cis-
urocanic acid (UCA), indicating a potential mechanism of action involving inactivation of this UV-photoproduct. Since immunosuppression induced by both UV radiation and by cis-UCA appears to be an
oxidant-dependent response our observations support the actions of these topically applied
isoflavones and their metabolites as
antioxidants. They also indicate that lotions containing
equol, unlike topical UV
sunscreens, more readily protect the immune system from photosuppression than from the
inflammation of the
sunburn reaction, even when applied after exposure, and thus such compounds may have a future role as sun-protective cosmetic ingredients.