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Flavonoids are an important type of natural tyrosinase inhibitor, but their inhibitory activity and mechanism against tyrosinase are very different because of their different structures. In this study, the inhibitory activity and mechanism differences between norartocarpetin and luteolin for tyrosinase were investigated by a combination of kinetic studies and computational simulations. The kinetic analysis showed that norartocarpetin reversibly inhibited tyrosinase in a competitive manner, whereas luteolin caused reversible noncompetitive inhibition. Both norartocarpetin and luteolin showed a single type of quenching and a static-type quenching mechanism. A computational simulation indicated that the hydroxyl groups of the B ring of norartocarpetin interacted with tyrosinase residues Asn81 and His85 in the active pocket, while the hydroxyl groups of the B ring of luteolin bound residues Asn81 and Cys83. HPLC and UPLC-MS/MS further confirmed that luteolin acted as a substrate or a suicide inhibitor, yet norartocarpetin acted as an inhibitor., (Copyright © 2016 Elsevier Ltd. All rights reserved.)

The twigs of Morus alba L. were found to show strong tyrosinase inhibition activity, and the responsible active components in the extract were further investigated in this study. A flavone, named morusone (1), and sixteen known compounds 2-17 were isolated from M. alba twigs and their structures were identified by interpretation of the corresponding ESI-MS and NMR spectral data. In the tyrosinase inhibitory test, the compounds steppogenin (IC50 0.98 ± 0.01 µM), 2,4,2',4'-tetrahydroxychalcone (IC50 0.07 ± 0.02 µM), morachalcone A (IC50 0.08 ± 0.02 µM), oxyresveratrol (IC50 0.10 ± 0.01 µM), and moracin M (8.00 ± 0.22 µM) exhibited significant tyrosinase inhibition activities, much stronger than that of the positive control kojic acid. These results suggest that M. alba twig extract should served as a good source of natural tyrosinase inhibitors for use in foods as antibrowning agents or in cosmetics as skin-whitening agents.

Chalcones and their derivatives have attracted great interests in recent years for their comprehensive biological activities. In this study, 2,4,2',4'-tetrahydroxychalcone and its two derivatives, 1,3,5-tris-(2,4-dihydroxy-phenyl)pentane-1,5-dione (new compound) and 7,2',4'-trihydroxyflavanone, were synthesized through one-pot green procedure catalyzed by boric acid in polyethylene glycol 400. Their structures were identified by ESI-MS and NMR spectral. Tyrosinase inhibitory activity and antibrowning test results showed that compounds 1-3 exhibited strong tyrosinase inhibitory activities and significant antibrowning effects on the fresh-cut lotus root slices at room temperature in 48 h. Among them, 0.01% 1,3,5-tris-(2,4-dihydroxy-phenyl)pentane-1,5-dione combined with 0.5% VC showed the best antibrowning ability. In brief, this study offers a protocol for one-pot green synthesis of high efficiency tyrosinase inhibitors which may be suitable as antibrowning agents for fresh-cut vegetables. More important, this study developed a new type of 1,5-dione derivative which may serve as new lead structures for novel tyrosinase inhibitors discovery., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

A new method was developed for one-pot green synthesis 1,3,5-triarylpentane-1,5-dione, triarylmethane, and flavonoid derivatives from the reaction between 2,4-dihydroxybenzaldehyde and hydroxyacetophenones via Aldol, Michael, and Friedel-Crafts additions using boric acid as catalyst in polyethylene glycol 400. The synthetic compounds demonstrated significant tyrosinase inhibitory activities much stronger than that of kojic acid. More important, 1,3,5-triarylpentane-1,5-dione and triarylmethane derivatives were found to be a new class of tyrosinase inhibitors., (Copyright © 2015 Elsevier Ltd. All rights reserved.)

Norartocarpetin is a flavone widely distributed in Moraceae plants with strong tyrosinase inhibitory activity. However, its poor solubility in aqueous systems and in food grade solvents (oils) limits its extensive applications. The aim of this study was to improve the solubility of norartocarpetin by developing an oil-in-water (o/w) microemulsion with food grade components. A microemulsion was developed and characterized, and its chemical and physical stabilities were also evaluated within 8 weeks. Using the construction of pseudoternary phase diagrams, the optimized formulation of 2% w/w of ethyl oleate, 12% w/w of Tween 80, 6% w/w of polyethylene glycol 400, and 80% w/w of water was obtained, with a maximum solubility of norartocarpetin up to 1.73 ± 0.21 mg/mL, at least a 1000-fold increase in solubility. The microemulsion base and norartocarpetin-loaded microemulsion were demonstrated to be stable after accelerated and long-term conditions (8 weeks). The norartocarpetin microemulsion still showed strong tyrosinase inhibitory activity and antibrowning effect on fresh-cut apple slices. These combined results indicated that norartocarpetin microemulsion may be suitable as an antibrowning agent for fresh-cut fruits.

The twigs of Cudrania tricuspidata were found to show strong tyrosinase inhibitory activity, and further detailed component analysis resulted in the isolation of a new flavanol glucoside, (2S,3S)-2,3-trans-dihydromorin-7-O-β-d-glucoside (1), plus twenty-seven known compounds (2-28). Their structures were elucidated on the basis of ESI-MS and NMR spectral data. Among the isolated compounds, trans-dihydromorin (8), oxyresveratrol (9), and steppogenin (12) were found to exhibit significant tyrosinase inhibition activities. Moreover, the structure-activity relationship of these isolated compounds was also discussed., (Copyright © 2012 Elsevier B.V. All rights reserved.)

The phytochemical profiles of Morus australis roots, stems and twigs were firstly compared by HPLC analysis. It was found that Morus australis stem extract mainly contained one known tyrosinase inhibitor, oxyresveratrol, while its root and twig extract might contain some unknown potential tyrosinase inhibitors. The root extract of Morus australis was further investigated in this study. One new compound, austraone A, together with 21 known compounds, was isolated and their structures were identified by interpretation of MS and NMR data. In the tyrosinase inhibitory testing, some of them, such as oxyresveratrol, moracenin D, sanggenon T, and kuwanon O, exhibited stronger tyrosinase inhibitory activities than that of kojic acid. These results suggested the Morus australis root extract as a good source of natural tyrosinase inhibitors with a great potential to be used in foods as anti-browning agents and in cosmetics as skin-whitening agents., (Copyright © 2012 Elsevier B.V. All rights reserved.)

The phytochemcal profiles of Cudrania cochinchinensis leaf, twig, stem and root were compared by HPLC analysis. It was found that C. cochinchinensis stem extract contained some unknown natural products with potential tyrosinase inhibitory activities. Therefore, the chemical constitutes in extract (95% ethanol) of C. cochinchinensis stem were further investigated in this study. A new racemic mixture, (±)2,3-cis-dihydromorin, and fifteen known phenolic compounds, dihydrokaempferol 7-O-β-d-qlucopyranoside, skimmin, quercetin-7-O-β-d-glucoside, 2,3-dihydroquercetin 7-O-β-d-glucoside, kaempferol-7-O-β-glucopyranoside, quercetin-3,7-di-O-β-d-glucoside, morin-7-O-β-d-glucoside, 1,3,5,8-tetrahydroxyxanthen-9-one, 2,3-trans-dihydromorin, aromadendrin, oxyresveratrol, genistin, protocatechuic acid, kaempferol 3,7-di-O-β-glucopyranoside, and naringenin were isolated. Spectral techniques (MS, (1)H NMR and (13)C NMR) were utilized for their structural identification and their inhibitory activities on mushroom tyrosinase were also evaluated. The results showed that tyrosinase inhibitory activities of (±)2,3-cis-dihydromorin (IC(50) = 31.1 μM), 2,3-trans-dihydromorin (IC(50) = 21.1 μM), and oxyresveratrol (IC(50) = 2.33 μM), were more potent than that of kojic acid (IC(50) = 50.8 μM), a well-known tyrosinase inhibitor, indicating that Cudrania cochinchinensis stem will be a great potential agent for the development of effective natural tyrosinase inhibitors.

The phytochemical profiles of Morus nigra roots and twigs were compared by HPLC with those of the old and young twigs of Morus alba which are known to contain oxyresveratrol and mulberroside A as major components. It was found that M. nigra root extract contains some unknown natural products with potential tyrosinase inhibitory activity. The extract (95% ethanol) of the roots of M. nigra was further investigated in this study. One new compound, 5'-geranyl-5,7,2',4'-tetrahydroxyflavone, and twenty-eight known phenolic compounds were isolated. Their structures were identified by mass spectrometry and NMR spectroscopy. Nine compounds, 5'-geranyl-5,7,2',4'-tetrahydroxyflavone, steppogenin-7-O-beta-D-glucoside, 2,4,2',4'-tetrahydroxychalcone, moracin N, kuwanon H, mulberrofuran G, morachalcone A, oxyresveratrol-3'-O-beta-D-glucopyranoside and oxyresveratrol-2-O-beta-D-glucopyranoside, showed better tyrosinase inhibitory activities than kojic acid. It was noteworthy that the IC(50) values of 2,4,2',4'-tetrahydroxychalcone and morachalcone A were 757-fold and 328-fold lower than that of kojic acid, respectively, suggesting a great potential for their development as effective natural tyrosinase inhibitors.