Synthesis And Antibrowning Mechanisms Of Polyoxometalates On Tyrosinase

Tyrosinase（EC 1.14.18.1）, also called polyphenol oxidase（PPO）, is a copper-containing enzyme, which is derived from the embryonic neural stem cell. It is the key enzyme of melanin biosynthesis. Its abnormal expression is responsible for various dermatological disorders in people. In addition, excessive level of melanin can also cause enzymatic browning in fruits and vegetables, which may influence the appearance, flavor and market value of plant- derived foods. Therefore, the design and synthesis of novel efficient tyrosinase inhibitors is of great importance on food preservation, prevention and treatment of diseases caused by excessive melanin.Taking Dawson-typed, Keggin-typed and glycine heteropolytungstates with Keggin structure polyoxometalates（POMs） as effectors, the inhibitory effects, inhibitory efficiency and mechanism of them on the activity of mushroom tyrosinase were studied. The results showed that these compounds could inhibit the activity of the enzyme in different degrees. Research of inhibition of the novel tyrosinase inhibitors on tyrosinase was done, which provided theory and practical foundation of POMs used as tyrosinase inhibitors, as well as extending the application of POMs. The contents and results of the research are as follows:（1） Dawson-type phosphotungstic polyoxometalate α/β-K₆P₂W-9180O₆₂?10H₂O(P₂W₁₈) was synthesized and its inhibitory effect on the mushroom tyrosinase has been investigated. Results showed that P₂W₁₈ could inhibit the monophenolase and diphenplase activities of mushroom tyrosinase. When the concentration of P₂W₁₈ reached 0.067 mmol/L, the monophenolase activity was dropped by 74.5%. The diphenolase activity was dropped by 52% with the concentration of P₂W₁₈ at 0.67 mmol/L. It could inhibit diphenolase activity of mushroom tyrosinase as an irreversible inhibitor. When the concentration of the enzyme reached 0.0176 mg?mL-1, the P₂W₁₈ concentration leading to 50% activity lost（IC50） were 0.05 mmol/L for monophenolase and 0.64 mmol/L for diphenolase.（2） Taking H₃PW₁₂O₄₀ and H₄SiW₁₂O₄₀ as effectors, inhibitory kinetics of them on diphenolase activity of mushroom tyrosinase have been investigated. The results showed that the two polyoxometalates could inhibit diphenolase activity of the enzyme effectively as reversible inhibitors. The IC50 values of them were 1.55 mmol/L and 1.39 mmol/L, respectively. The inhibitory kinetics analyzed from Lineweaver-Burk plots showed that H₃PW₁₂O₄₀ was a mixed Ι type inhibitor, while H₄ Si W₁₂O₄₀ belonged to be a competitive inhibitor. The tyrosinase inhibitory ability of the two compounds was evaluated by using IC50 and KI as parameters.（3） Glycine heteropolytungstates with Keggin structure POMs,（HGly）₃PW₁₂O₄₀ and（HGly）4SiW₁₂O₄₀, were synthesized and their inhibitory effects on diphenolase activity of mushroom tyrosinase have been investigated. Results showed that the two compounds could lead to reversible inhibition on the enzyme. The IC50 values of them were estimated to be 1.57 mmol/L and 2.31 mmol/L, respectively. The kinetic analyses demonstrated that（HGly）₃PW₁₂O₄₀ was an uncompetitive inhibitor, while（HGly）4SiW₁₂O₄₀ belonged to be a noncompetitive inhibitor.