Study The Inhibition Activity And Inhibition Mechanism Of Five Anthraquinones From Rhubarb On Tyrosinase

Tyrosinase is a key enzyme in the production of melanin which determines the color of skin and hair in the human body.However,excessive accumulation of melanin,due to the overproduction of tyrosinase,can cause diseases in skin such as age spots,freckles,melasma and malignant melanoma.Inhibition of the activity of tyrosinase can reduce the production of melanin and achieve the effect to lighten our skin.Recently,a lot of tyrosinase inhibitors have been developed but only a few of them are used today because of associated side-effects such as photosensitive effect,low stability,skin cancer and so on.Therefore it is necessary to search for new candidates that show effective inhibition of tyrosinase activity but are devoid of side effects when they are used in cosmetics industry.Rhubarb is a traditional Chinese herb which has various pharmacological activities.It has been reported that its water extract has inhibitory activity of tyrosinase while the material basis of the inhibition is not yet clear.Anthraquinones are the main ingredients of rhubarb.Therefore in our research work we chose five anthraquinone compounds(emodin,physcion,aloe-emodin,chrysophanol and rhein)extracted from rhubarb as targets to study their inhibitory activity and mechanism.We detected their binding ability to tyrosinase by the methods of enzymatic kinetics,spectroscopy and molecular simulation.The results showed that five anthraquinone compounds have significant inhibitory activity of tyrosinase compared with kojic acid.Inhibitory activities of five anthraquinones were physcion>emodin>aloe-emodin>chrysophanol>rhein.The inhibitory type of five anthraquinone compounds on tyrosinase was typically competitive inhibition.The results of spectroscopy and molecular simulation indicated their binding to amino acid residues around the active site of tyrosinase through hydrophobic interaction and electrostatic force.The interaction induced the change of conformation of tyrosinase and quenching of intrinsic fluorescence of tyrosinase.Thus it could be concluded that the oxidation of substrate L-DOPA by tyrosinase were inhibited by five anthraquinones and the catalytic center of the enzyme was occupyed when they inserted into the active site of tyrosinase to induce conformational changes of tyrosinase and avoid entrance of the substrate,which may be the main mechanism of inhibition of tyrosinase.In the last of our study,we used the method of quantitative structure-activity relationship(QSAR)to generate a 2D-QSAR model which had good predictability.It would provide useful guidance for further design of novel and efficient tyrosinase inhibitors.