by Shyam Gupta, Ph.D.
Skin depigmentation to lighten skin color and even skin tone has been practiced since ancient times. While darker skinned people are seeking a fairer tone, Caucasian consumers utilize skin-whitening compositions to brighten their skin from the dulling and blotching effect that becomes pronounced upon aging. Age spots and localized skin darkening, such as circles around the eyes, are a significant concern to men and women.
The use of nature-based treatments to whiten or brighten dark colored skin has long been popular among Asian, African, Middle Eastern and South American cultures. Skin-brightening ingredients have now become an important arsenal for skin anti-aging cosmetics among Caucasians to address age-related dark spots, skin pigmentation, freckles and other skin pigmentation disorders.
Hydroquinone has been the Western ingredient of choice for skin whitening products; however, serious skin safety concerns have caused many countries to ban or propose a ban on its use in cosmetic formulations. Nature-based skin-whitening agents, such as kojic acid and arbutin, which are chemically similar to hydroquinone, have been popular; but, safety concerns such as skin irritation have caused reduced application in cosmetic formulations. Topical application of ascorbic acid and its esters has also claimed to have skin-lightening properties; however, ascorbic acid-based formulations tend to discolor in air and light. These issues have created a worldwide need for new, safe and effective skin-whitening and skin-brightening ingredients from plant sources.
The Basis of Skin Pigmentation
The color of human skin is differentiated by the chemical nature and quantity of the natural pigment melanin present in the epidermal layers of the skin. Melanin is produced from the amino acid phenylalanine, involving the enzyme tyrosinase. Phenylalanine is first oxidized to tyrosine by phenylalanine hydroxylase. Tyrosinase converts tyrosine into dihydroxyphenylalanine (l-dopa), and then into dopaquinone. Dopaquinone is then converted into eumelanin (black, white and Asian skin types: skin color dependent on the quantity of eumelanin in skin) or phaeomelanin (red-haired skin types).
Melanin Biosynthesis
The development of skin depigmentation compositions depends on two principal pathways: the inhibition of melanin biosynthesis and/or conversion of melanin and its colored precursors (dopaquinones) into colorless entities. The inhibition of melanin biosynthesis is achievable by two skin depigmentation technologies: the inactivation of enzyme tyrosinase, or the competitive replacement of tyrosinase substrates and tyrosine or l-dopa with other chemically related ingredients. In practice, the inhibition of tyrosinase, the competitive replacement of the substrates for tyrosinase, the inhibition of the transport of pigment molecules and the conversion of colored biochemical species into colorless or less colored materials constitute the three rational approaches to skin-whitening cosmetics.The incorporation of more than one ingredient in a cosmetic formulation is more beneficial than a single ingredient in producing the desired skin lightening effect. These ingredients need to be carefully selected from a chemically non-reactive combination of a tyrosinase inhibitor, a tyrosine competitor (to block enzyme tyrosinase); and a reducing agent (to convert melanin and its pigmented precursors into colorless, or less colored, species).
