SOY ISOFLAVONES
Mechanism of Action of Soy Isoflavones
Soybeans contain many components that are biologically active in the skin, such as antioxidants (isoflavones like genistein) and small proteinase inhibitors including soybean trypsin inhibitor (STI) and Bowman-Birk protease inhibitor (BBI). Topically applied genistein is readily absorbed into the viable human epidermis. Its main role in skin pigmentation is to inhibit tyrosine kinase. In its chemically reduced form it also acts as an antioxidant by scavenging reactive oxygen species. Protease-activated receptor 2 (PAR-2) cell receptors are found on the surface of keratinocytes but not on melanocytes. PAR-2 mediates the transfer of melanosomes from melanocytes to the surrounding keratinocytes making up the epidermal melanin unit. Soybean trypsin inhibitor (which affects proteolysis) and Bowman-Birk inhibitor (which interferes with chymotrypsin), inhibit the activation of PAR-2 cell receptors. This in turn reduces keratinocytes ability to phagocytize melanosomes thus inhibiting melanosome transfer, which leads to a reversible depigmentation. 7,30 PAR-2 receptors are involved in the development and regulation of inflammation, so they could play a role in conditions such as post- inflammatory hyperpigmentation. PAR-2 expression increases in human skin after exposure to UV radiation.