Inhibition Mechanism Of Transition Metals-substituted Polyoxometalates On Tyrosinase

Enzymatic reactions can lead to color changes in fruits and vegetables that significantly reduce the visual appeal of consumers.De Castro has reported that there is a close interconnection between fruit browning and activity of PPO.PPO,also known as tyrosinase?EC1.14.18.1?,is a multifunctional copper-containing enzyme.Tyrosinase can oxidase phenolic compounds to quinones that are highly reactive compounds and can spontaneously polymerise to form dark macromolecular pigments,resulting in fruit browning.It can also combine with other compounds,such as amines,amino acids,peptides and proteins,to cause the nutrition loss in fruits and vegetables.Moreover,tyrosinase may play a role in the melanoma and Parkinson's disease.Thus,tyrosinase inhibitors are of great concern to scientist in food preservation as well as in clinical medicine.POMs have a wide range of applications as a result of their incomparable structural versatility and interesting properties.There have been many reports confirming their significant effects such as anti-NTPDase,anti-protein kinase,and anti-ATPase.Moreover,a lot of Researchers considered that POMs can be potentially useful in enzyme inhibition.Based on these results,some transition metals-substituted POMs were synthesized and characterized,and their inhibitory effect on mushroom tyrosinase were investigated.This paper aimed at extending the application of POMs and discovering safety and effective POMs compounds as food additives or food preservatives.The main contents of this paper are as follows:?1?Three vanadate replacement-type polyoxometalates,namely?-SiW₉V₃,?-SiW₁₀V₂,?-SiW₁₁V were synthesized and evaluated as tyrosinase inhibitors using the enzymological kinetic method.The results demonstrated that?-Si W₉V₃ can inhibit tyrosinase strongly.The?-SiW₉V₃concentration leading to 50%activity lost(IC₅₀)was estimated as 0.68 mmol·L^-1 for mushroom tyrosinase.0.70 mmol·L^-1 of?-Si W₉V₃ resulted in the lag time of the monophenolase increasing2.77 times.The inhibition mechanism of?-Si W₉V₃ on monophenolase of mushroom tyrosinase behavesasreversibleprocess,inhibitiontypefound?-SiW₉V₃tobea competitive-uncompetitive mixed-II type inhibitor for the oxidation of L-tyrosine,and the inhibition constants(K_I and K_IS)were determined to be 4.22 mmol·L^-1 and 2.39 mmol·L^-1,respectively.?2?Two transition metals-substituted(Co²⁺,Ni²⁺)phosphomolybdate(PMo₁₁Co,PMo₁₁Ni,respectively)were synthesized and then characterized,their inhibitory effects and mechanisms on mushroom tyrosinase,as well as PMo₁₂,were evaluated by Native-PAGE electrophoresis method and spectrophotometry.The results showed that Tyrosinase are encoded by a multigene family,which are all active.The inhibitory effects of these compounds indicated that they were potent inhibitors of three molecular weight kinds of tyrosinase and exhibited effective inhibitory activities as reversible inhibitors.Both PMo₁₂ and PMo₁₁Co were suggested to be competitive inhibitor of tyrosinase with K_I were 9.03 and 1.71 mmol·L^-1,respectively,while PMo₁₁Ni was found to be a mixed-type inhibitor,with K_I=0.83 mmol·L^-1 and K_IS=1.03 mmol·L^-1.The inhibitory effects on mushroom tyrosinase were evaluated by using IC₅₀ and Inhibition constants as parameters.?3?Si W₁₁Co was synthesized and characterized,while its inhibitory effects and mechanisms on mushroom tyrosinase were evaluated.The Native-PAGE result showed that when the concentration of SiW₁₁Co was 4.0 mmol·L^-1,enzyme specific bands disappeared,moreover,the spectrophotometry assay showed that it was an irreversible and competitive inhibitor(IC₅₀=239?mol·L^-1,K_I=81.5?mol·L^-1).Both the results suggested that SiW₁₁Co had a significant inhibitory effect on tyrosinase dose-dependently.In addition,the assay on the browning of apple section indicated that Si W₁₁Co could dramatically decrease the browning process of apples and significantly inhibit the activity of polyphenol oxidase.