Biological evaluation of Keggin-type polyoxometalates on tyrosinase: Kinetics and molecular modeling.

Abnormal overexpression of tyrosinase activity can lead to the production of hyperpigmentation in human skin and enzymatic browning in fruits and vegetables. Herein, the inhibition and mechanism of the H3 PMo12 O40 and two transition metal-substituted Keggin-type polyoxometalates (Na7 PMo11 CoO40 and Na7 PMo11 ZnO40 ) on tyrosinase were studied by kinetics and molecular modeling. Kinetic studies indicated that all compounds had more potent inhibitory activities than standard arbutin, and H3 PMo12 O40 (IC50  = 0.443 ± 0.006 mm) is ~15-fold stronger inhibition than arbutin. Additionally, all compounds inhibited tyrosinase in a reversible competitive manner. Intriguingly, molecular modeling elucidated that three compounds competitively bind to tyrosinase mainly through more interactions with Cu2+ ions and the amino acid residue capable of forming cation-π and hydrogen bonding, forming a reversible non-covalent complex. Molecular simulation study correlated well with the biological activity of three compounds in vitro. This work provided new insights into design and synthesis of polyoxometalates as tyrosinase inhibitors in the field of medicine, cosmetic, and food.

Biological evaluation of two Keggin-type polyoxometalates containing glycine as mushroom tyrosinase inhibitors.

Two Keggin-type polyoxometalates (POMs) containing glycine, (HGly)3 PW12 O40 and (HGly)4 SiW12 O40 , were synthesized and evaluated as mushroom tyrosinase inhibitors. The spectrophotometric method results showed that both (HGly)3 PW12 O40 and (HGly)4 SiW12 O40 could strongly inhibit the diphenolase activity of the tyrosinase and that their inhibition mechanisms were reversible. Their half-inhibition concentration values were estimated to be 1.55 and 1.39 mmol/L, respectively. The inhibition kinetics analysis by Lineweaver-Burk plots indicated that (HGly)3 PW12 O40 was an uncompetitive inhibitor with KIS  = 0.046 mmol/L, whereas (HGly)4 SiW12 O40 was a noncompetitive inhibitor with KI  = KIS  = 2.17 mmol/L. This study may help to extend the application of POMs in the fields of medicine and food preservation.

Inhibitory effects of Na7PMo11CuO40 on mushroom tyrosinase and melanin formation and its antimicrobial activities.

Keggin-type Cu-substituted phosphomolybdic acid (Na7PMo11CuO40, abbreviated as PMo11Cu) was synthesized and characterized. The inhibitory effects of PMo11Cu on mushroom tyrosinase and melanin formation in B16 melanoma cells were studied. The results showed that PMo11Cu could strongly inhibit the activity of tyrosinase, and it was reversible and competitive inhibitor. The IC50 value was estimated to be 0.48 mM for diphenolase activity. PMo11Cu also exhibited inhibitory effects on cell viability, cellular tyrosinase activity and melanin formation in B16 melanoma cells at concentrations ranging from 0 to 200 µM for 24 h. Furthermore, the antimicrobial activities of PMo11Cu against Sarcina lutea, Staphylococcus aureus, Bacillus subtilis and Escherichia coli were investigated. The results showed that PMo11Cu had an obvious antimicrobial activities, and it was more effective against two kinds of coccus than two kinds of bacillus. This study may provide theoretical basis for designing novel effective mushroom tyrosinase inhibitors and extend the use of polyoxometalates in the fields of food preservation and depigmentation.

Inhibitory effects of α-Na8SiW11CoO40 on tyrosinase and its application in controlling browning of fresh-cut apples.

α-Na8SiW11CoO40 was synthesized and characterized. The inhibitory effects of α-Na8SiW11CoO40 on the activity of mushroom tyrosinase and the effects of α-Na8SiW11CoO40 on the browning of fresh-cut apples were studied. The Native-PAGE result showed that α-Na8SiW11CoO40 had a significant inhibitory effect on tyrosinase. Kinetic analyses showed that α-Na8SiW11CoO40 was an irreversible and competitive inhibitor. The inhibitor concentration leading to a 50% reduction in activity (IC50) was estimated to be 0.239 mM. Additionally, the results also showed that α-Na8SiW11CoO40 treatment could significantly decrease the browning process of apple slices and inhibit the polyphenol oxidase (PPO) activity. Moreover, application of α-Na8SiW11CoO40 resulted in higher peroxidase activity and promoted high amounts of phenolic compounds and ascorbic acid. This study may provide a promising method for the use of polyoxometalates to inhibit tyrosinase activity and control the browning of fresh-cut apples.

Functionality study of Na6PMo11FeO40 as a mushroom tyrosinase inhibitor.

The inhibitory effects of iron-substituted phosphomolybdic acid (Na6PMo11FeO40, abbreviated as PMo11Fe) on mushroom tyrosinase were investigated. The Native-PAGE results show that PMo11Fe has an inhibitory effect on tyrosinase. A spectrophotometric analysis shows that PMo11Fe is a reversible and noncompetitive inhibitor with KI=KIS=0.47 mmol L(-1). The effects of PMo11Fe on the preservation of lotus root slices were studied. The results show that PMo11Fe can significantly slow the browning rate of lotus root slices, inhibit the activity of polyphenol oxidase (PPO), enhance the activity of peroxidase (POD) and superoxide dismutase (SOD), maintain high levels of glutathione (GSH), ascorbic acid (ASA) and sucrose and control the peroxidation degree of fresh-cut fruits and vegetables. This study may help to elucidate the design of mushroom tyrosinase inhibitors of polyoxometalates (abbreviated as POMs) and a theoretical basis for the use of POMs to inhibit fruit spoilage.