In most instances,sun exposure is the main stimulator of skin hyperpigmentation.A direct effect of UV photons results in activation of tyrosinase, the rate-limiting enzyme in melanin synthesis, as well as an increase in cell surface expression of receptors for at least one of the several known keratinocyte-derived melano genic factors,MSH(3).The inherent defense mechanism of skin against UV-B allows the transformation of tyrosine to 3,4-dihydroxyphenylalanine in the presence of UV-activated tyrosinase.This is further oxidized to DOPA quinine and then to dopachrome(4).Reduction of dopachrome yields 5,6-dihydroxyindole-2-carboxylic acid,which is ultimately converted via dihydroxyindole to the yellow-colored indole 5,6-quinone.Further, oxidative coupling of this results in the formation of eumelanin. Under normal circumstances eumelanins and pheomelanins are formed simultaneousil to lead to the so-called mixed-type melanins. This process requires three to six days before the tan is established(4).
There is a myriad of skin whiteners with variable efficacy(5)readily available for consumers. Modulation of melanogenesis in the melanocytes can be achieved using chemicals that share structural homologies with the substrate tyrosine and as thus competitively inhibit the catalytic function of tyrosinase(6). Kojic acid inhibits the catecholase activity of tyrosinase,which is the rate-limiting,essential enzyme in the biosynthesis of the skin pigment melanin. Hydroquinone, a hydroxyphenolic chemical,is a bleaching agent that has been used for decades as a skin-lightening agent(7). It acts by inhibiting the enzyme tyrosinase,thereby reducing the conversion of DOPA to melanin. Some of the other mechanisms of action are the destruction of melanocytes,degradation of melanosomes,and the inhibition of the synthesis of DNA and RNA(8).
Srrong skin lighteners like clobetasol and hydroquinone can cause complications and significant health problems,especially for individuals eith skin of clolr(9-11).Therefore,there is a constant need to investigate new and diverse potential shin lighteners that are safer and do not have side effects.
- 無名氏Lv 510 years agoFavorite Answer