Two potent suicide substrates of mushroom tyrosinase: 7,8,4'-trihydroxyisoflavone and 5,7,8,4'-tetrahydroxyisoflavone

Natural products as tyrosinase inhibitors

Tyrosinase is a crucial copper-containing enzyme involved in the production of melanin. Melasma, age spots, and freckles are examples of hyperpigmentation diseases caused by excess production of melanin. Inhibiting tyrosinase activity is a crucial method for treating these disorders along with various applications such as cosmetics, food technology, and medicine. Natural products have proven a rich source of tyrosinase inhibitors, with several molecules from plant, marine, and microbial sources showing potential inhibitory action. This chapter provides a complete overview of natural compounds that have been found as tyrosinase inhibitors, with emphasis on their structures, modes of action, and prospective applications.

Naturally-Occurring Tyrosinase Inhibitors Classified by Enzyme Kinetics and Copper Chelation

Currently, there are three major assaying methods used to validate in vitro whitening activity from natural products: methods using mushroom tyrosinase, human tyrosinase, and dopachrome tautomerase (or tyrosinase-related protein-2, TRP-2). Whitening agent development consists of two ways, melanin synthesis inhibition in melanocytes and downregulation of melanocyte stimulation. For melanin levels, the melanocyte cell line has been used to examine melanin synthesis with the expression levels of TRP-1 and TRP-2. The proliferation of epidermal surfaced cells and melanocytes is stimulated by cellular signaling receptors, factors, or mediators including endothelin-1, α-melanocyte-stimulating hormone, nitric oxide, histamine, paired box 3, microphthalmia-associated transcription factor, pyrimidine dimer, ceramide, stem cell factors, melanocortin-1 receptor, and cAMP. In addition, the promoter region of melanin synthetic genes including tyrosinase is upregulated by melanocyte-specific transcription factors. Thus, the inhibition of growth and melanin synthesis in gene expression levels represents a whitening research method that serves as an alternative to tyrosinase inhibition. Many researchers have recently presented the bioactivity-guided fractionation, discovery, purification, and identification of whitening agents. Melanogenesis inhibition can be obtained using three different methods: tyrosinase inhibition, copper chelation, and melanin-related protein downregulation. There are currently four different types of inhibitors characterized based on their enzyme inhibition mechanisms: competitive, uncompetitive, competitive/uncompetitive mixed-type, and noncompetitive inhibitors. Reversible inhibitor types act as suicide substrates, where traditional inhibitors are classified as inactivators and reversible inhibitors based on the molecule-recognizing properties of the enzyme. In a minor role, transcription factors can also be downregulated by inhibitors. Currently, the active site copper iron-binding inhibitors such as kojic acid and chalcone exhibit tyrosinase inhibitory activity. Because the tyrosinase catalysis site structure is important for the mechanism determination of tyrosinase inhibitors, understanding the enzyme recognition and inhibitory mechanism of inhibitors is essential for the new development of tyrosinase inhibitors. The present review intends to classify current natural products identified by means of enzyme kinetics and copper chelation to exhibit tyrosinase enzyme inhibition.

Paving the way towards effective plant-based inhibitors of hyaluronidase and tyrosinase: a critical review on a structure-activity relationship

Human has used plants to treat many civilisation diseases for thousands of years. Examples include reserpine (hypertension therapy), digoxin (myocardial diseases), vinblastine and vincristine (cancers), and opioids (palliative treatment). Plants are a rich source of natural metabolites with multiple biological activities, and the use of modern approaches and tools allowed finally for more effective bioprospecting. The new phytochemicals are hyaluronidase (Hyal) inhibitors, which could serve as anti-cancer drugs, male contraceptives, and an antidote against venoms. In turn, tyrosinase inhibitors can be used in cosmetics/pharmaceuticals as whitening agents and to treat skin pigmentation disorders. However, the activity of these inhibitors is stricte dependent on their structure and the presence of the chemical groups, e.g. carbonyl or hydroxyl. This review aims to provide comprehensive and in-depth evidence related to the anti-tyrosinase and anti-Hyal activity of phytochemicals as well as confirming their efficiency and future perspectives.

Advances in heterocycles as DNA intercalating cancer drugs

The insertion of a molecule between the bases of DNA is known as intercalation. A molecule is able to interact with DNA in different ways. DNA intercalators are generally aromatic, planar, and polycyclic. In chemotherapeutic treatment, to suppress DNA replication in cancer cells, intercalators are used. In this article, we discuss the anticancer activity of 10 intensively studied DNA intercalators as drugs. The list includes proflavine, ethidium bromide, doxorubicin, dactinomycin, bleomycin, epirubicin, mitoxantrone, ellipticine, elinafide, and echinomycin. Considerable structural diversities are seen in these molecules. Besides, some examples of the metallo-intercalators are presented at the end of the chapter. These molecules have other crucial properties that are also useful in the treatment of cancers. The successes and limitations of these molecules are also presented.

An outlook on suicide enzyme inhibition and drug design

There have been recent renewed interests in the importance of suicide enzyme inhibition. The principal objective of this review is to investigate all types of suicide inhibitions for natural enzymes, artificial biocatalysts as well as therapeutic potential of enzyme suicide inhibition. It is discussed the suicide inhibition beneficial in drug design and treatments and non-beneficial achievements for some industrial enzymes such as HRP peroxidase enzyme. The design of biomimetic artificial enzymes explained to prevent inhibition by protecting the active site via environmental conditions. Suicide enzyme inhibition development can be the key mechanism against sever diseases such as SARS. In this report, suicide enzyme inactivation classes are classified based on target enzyme groups via their substrates.

Synthesis, Characterization, and Antioxidant Activity of 8-Hydroxygenistein

It was reported that 8-hydroxygenistein (8-OHG) was synthesized by methylation, bromination, methoxylation, and demethylation using cheap and readily available biochanin A as raw material. All synthesized products were structurally confirmed by ultra-high-performance liquid chromatography (UHPLC), infrared spectroscopy, mass spectrometry, 1H-nuclear magnetic resonance (NMR), and 13C-NMR. In addition, we examined the antioxidant capacity of 8-OHG using 6 different methods such as 1,1-diphenyl-2-picrylhydrazyl radical scavenging, 2,2′-azinobis(3-ethylbenzothiazoline-6-sulfonate) radical (ABTS) scavenging, nitric oxide radical (NO) scavenging, superoxide radical (O2 −•) scavenging, reducing power assay, and total antioxidant activity using ascorbic acid (VC) as a positive control. Compared with VC, 8-OHG exhibited higher total antioxidant activity and stronger scavenging activity on ABTS, NO, and O2 −•. These results indicate that 8-OHG is an excellent antioxidant agent and may be effective in preventing damage induced by free radical.

A comprehensive review on tyrosinase inhibitors

Tyrosinase is a multi-copper enzyme which is widely distributed in different organisms and plays an important role in the melanogenesis and enzymatic browning. Therefore, its inhibitors can be attractive in cosmetics and medicinal industries as depigmentation agents and also in food and agriculture industries as antibrowning compounds. For this purpose, many natural, semi-synthetic and synthetic inhibitors have been developed by different screening methods to date. This review has focused on the tyrosinase inhibitors discovered from all sources and biochemically characterised in the last four decades.

Potential Industrial Production of a Well-Soluble, Alkaline-Stable, and Anti-Inflammatory Isoflavone Glucoside from 8-Hydroxydaidzein Glucosylated by Recombinant Amylosucrase of Deinococcus geothermalis

8-Hydroxydaidzein (8-OHDe), an ortho-hydroxylation derivative of soy isoflavone daidzein isolated from some fermented soybean foods, has been demonstrated to possess potent anti-inflammatory activity. However, the isoflavone aglycone is poorly soluble and unstable in alkaline solutions. To improve the aqueous solubility and stability of the functional isoflavone, 8-OHDe was glucosylated with recombinant amylosucrase of Deinococcus geothermalis (DgAS) with industrial sucrose, instead of expensive uridine diphosphate-glucose (UDP-glucose). One major product was produced from the biotransformation, and identified as 8-OHDe-7-α-glucoside, based on mass and nuclear magnetic resonance spectral analyses. The aqueous solubility and stability of the isoflavone glucoside were determined, and the results showed that the isoflavone glucoside was almost 4-fold more soluble and more than six-fold higher alkaline-stable than 8-OHDe. In addition, the anti-inflammatory activity of 8-OHDe-7-α-glucoside was also determined by the inhibition of lipopolysaccharide-induced nitric oxide production in RAW 264.7 cells. The results showed that 8-OHDe-7-α-glucoside exhibited significant and dose-dependent inhibition on the production of nitric oxide, with an IC₅₀ value of 173.2 µM, which remained 20% of the anti-inflammatory activity of 8-OHDe. In conclusion, the well-soluble and alkaline-stable 8-OHDe-7-α-glucoside produced by recombinant DgAS with a cheap substrate, sucrose, as a sugar donor retains moderate anti-inflammatory activity, and could be used in industrial applications in the future.

Bioconversion of fructus sophorae into 5,7,8,4'-tetrahydroxyis oflavone with Aspergillus aculeatus

A fungus identified as Aspergillus aculeatus was used to biotransform genistein and glycosides to polyhydroxylated isoflavones. The strain was identified on the basis of colony morphology features and ITS rDNA sequence analysis. Phylogenetic tree was constructed to determine its taxonomic status. Genistein and glycosides were transformed by Aspergillus aculeatus to 5,7,8,4’- tetrahydroxyisoflavone. The chemical structure of the product was identified by high performance liquid chromatography(HPLC), liquid chromatography-mass spectrometry(LC/MS), Infrared spectroscopy (IR) and NMR spectrometer methods. The ITS rDNA sequence of the strain had 100% similarity with Aspergillus. Furthermore, it was ultimately identified as Aspergillus aculeatus. The metabolite of genistein and glycosides was identified as 5,7,8,4’-tetrahydroxyisoflavone. 120 mg 5,7,8,4’-tetrahydroxyisoflavone was made from 20 g fructus sophorae, which was bioconverted unconditionally by Aspergillus aculeatus for 96 h, and the purity was 96%. On the basis of the findings, Aspergillus aculeatus was a novel strain with specific ability to convert genistein and glycosides into 5,7,8,4’-tetrahydroxyisoflavone which had potential applications.

Recent advances in the microbial hydroxylation and reduction of soy isoflavones

Soy isoflavones are naturally occurring phytochemicals, which are biotransformed into functional derivatives through oxidative and reductive metabolic pathways of diverse microorganisms. Such representative derivatives, ortho-dihydroxyisoflavones (ODIs) and equols, have attracted great attention for their versatile health benefits since they were found from soybean fermented foods and human intestinal fluids. Recently, scientists in food technology, nutrition and microbiology began to understand their correct biosynthetic pathways and nutraceutical values, and have attempted to produce the valuable bioactive compounds using microbial fermentation and whole-cell/enzyme-based biotransformation. Furthermore, artificial design of microbial catalysts and/or protein engineering of oxidoreductases were also conducted to enhance production efficiency and regioselectivity of products. This minireview summarizes and introduces the past year's studies and recent advances in notable production of ODIs and equols, and provides information on available microbial species and their catalytic performance with perspectives on industrial application.

Attenuation of melanogenesis by Nymphaea nouchali (Burm. f) flower extract through the regulation of cAMP/CREB/MAPKs/MITF and proteasomal degradation of tyrosinase

Medicinal plants have been used to treat diseases from time immemorial. We aimed to examine the efficacy of the ethyl acetate fraction of Nymphaea nouchali flower extract (NNFE) against melanogenesis process, and the underlying mechanisms in vitro and in vivo. Paper spray ionisation mass spectroscopy and (+) mode electrospray ionisation revealed the presence of seven flavonoids, two spermidine alkaloids, 3,4,8,9,10-pentahydroxy-dibenzo[b,d]pyran-6-one, and shoyuflavone C in NNFE. NNFE (100 µg/mL) significantly inhibited the monophenolase and diphenolase activities of mushroom tyrosinase at 94.90 ± 0.003% and 93.034 ± 0.003%, respectively. NNFE significantly suppressed cellular tyrosinase activity and melanin synthesis in vitro in melan-a cells and in vivo in HRM2 hairless mice. Furthermore, NNFE inhibited tyrosinase (TYR), tyrosinase-related protein (TYRP)-1, TYRP-2, and microphthalmia-associated transcription factor (MITF) expression, thereby blocking melanin synthesis. In particular, NNFE suppressed cAMP production with subsequent downregulation of CREB phosphorylation. Additionally, it stimulated MAP kinase phosphorylation (p38, JNK, and ERK1/2) and the proteasomal debasement pathway, leading to degradation of tyrosinase and MITF and the suppression of melanin production. Moreover, selective inhibitors of ERK1/2, JNK, and p38 attenuated NNFE inhibitory effects on melanogenesis, and MG-132 (a proteasome inhibitor) prevented the NNFE-induced decline in tyrosinase protein levels. In conclusion, these findings indicate that NNFE is a potential therapy for hyperpigmentation.

Production of New Isoflavone Glucosides from Glycosylation of 8-Hydroxydaidzein by Glycosyltransferase from Bacillus subtilis ATCC 6633

8-Hydroxydaidzein (8-OHDe) has been proven to possess some important bioactivities; however, the low aqueous solubility and stability of 8-OHDe limit its pharmaceutical and cosmeceutical applications. The present study focuses on glycosylation of 8-OHDe to improve its drawbacks in solubility and stability. According to the results of phylogenetic analysis with several identified flavonoid-catalyzing glycosyltransferases (GTs), three glycosyltransferase genes (BsGT110, BsGT292 and BsGT296) from the genome of the Bacillus subtilis ATCC 6633 strain were cloned and expressed in Escherichia coli. The three BsGTs were then purified and the glycosylation activity determined toward 8-OHDe. The results showed that only BsGT110 possesses glycosylation activity. The glycosylated metabolites were then isolated with preparative high-performance liquid chromatography and identified as two new isoflavone glucosides, 8-OHDe-7-O-β-glucoside and8-OHDe-8-O-β-glucoside, whose identity was confirmed by mass spectrometry and nuclear magnetic resonance spectroscopy. The aqueous solubility of 8-OHDe-7-O-β-glucoside and 8-OHDe-8-O-β-glucoside is 9.0- and 4.9-fold, respectively, higher than that of 8-OHDe. Moreover, more than 90% of the initial concentration of the two 8-OHDe glucoside derivatives remained after 96 h of incubation in 50 mM of Tris buffer at pH 8.0. In contrast, the concentration of 8-OHDe decreased to 0.8% of the initial concentration after 96 h of incubation. The two new isoflavone glucosides might have potential in pharmaceutical and cosmeceutical applications.

Correlation between the Potency of Flavonoids on Mushroom Tyrosinase Inhibitory Activity and Melanin Synthesis in Melanocytes

Twenty-seven flavonoids isolated from Dalbergia parviflora with vast structural diversity were screened for inhibitory activity against mushroom and murine tyrosinases using l-DOPA as the substrate. Among the flavonoids tested, only four—khrinone (5), cajanin (9), (3RS)-3′-hydroxy-8-methoxy vestitol (21), and (6aR,11aR)-3,8-dihydroxy-9-methoxy pterocarpan (27)—reacted with mushroom tyrosinase, with IC₅₀ values of 54.0, 67.9, 67.8, and 16.7 μM, respectively, and only compound 27 showed inhibitory activity against murine tyrosinase. With cell-based assays, only compounds 9 and 27 effectively inhibited melanogenesis in B16-F10 melanoma cells (by 34% and 59%, respectively), at a concentration of 15 μM, without being significantly toxic to the cells. However, the crude extract of D. parviflora and some of the flavonoid constituents appeared to increase melanin production in B16-F10 cells, suggesting that there are flavonoids with both inhibitory and stimulatory melanogenesis in the crude extract. Studies on the correlation between the enzyme-based and cell-based assays showed that only the flavonoids with IC₅₀ values below 50 μM against mushroom tyrosinase could inhibit the mammalian tyrosinase, and thus, reduce melanogenesis in B16-F10. Flavonoids with the IC₅₀ values greater than 50 μM, on the other hand, could not inhibit the mammalian tyrosinase, and had either no effect or enhancement of melanogenesis. In conclusion, the tyrosinase enzyme from mushroom is not as selective as the one from mammalian source for the enzyme-based melanogenesis inhibitory screening, and the mammalian cell-based assay appears to be a more reliable model for screening than the enzyme-based one.

Understanding the inhibitory mechanism of tea polyphenols against tyrosinase using fluorescence spectroscopy, cyclic voltammetry, oximetry, and molecular simulations

Inhibiting the activity of tyrosinase is a very effective and safe way to prevent enzymatic browning in food and to resist pests in agriculture. Tea polyphenols (TPs), regarded as safe and non-toxic food additives, have been reported due to their potential inhibitory capability against tyrosinase, but their ambiguous inhibitory mechanisms have severely limited their application. In the present work, fluorescence spectroscopy, cyclic voltammetry (CV), oximetry and molecular simulation approaches were employed to shed light on the underlying inhibitory mechanisms of TPs with different structures including (+)-catechin, (-)-epicatechin gallate (ECG) and (-)-epigallocatechin gallate (EGCG) against tyrosinase. Fluorescence spectra show that the three TPs are capable of binding tyrosinase with a molar proportion of 1 : 1. The analysis of CV curves and oxygen utilization suggests that these three TPs can be oxidized by tyrosinase, revealing that these three TPs are suicide inhibitors of tyrosinase. Furthermore, ECG and catechin make tyrosinase irreversibly inactivated due to their catechol group (ring B) being catalyzed by tyrosinase through a cresolase-like pathway, while EGCG inhibits the activity of tyrosinase by competing with or delaying the oxidation of substrate. Molecular simulations further confirm that ring B of ECG and catechin makes a significant contribution to tyrosinase inhibitory activities, and has a direct interaction with the coupled binuclear copper ions in the optimal orientation required by the cresolase-like pathway.

Substituent effect of benzaldehydes on tyrosinase inhibition

Benzaldehyde inhibited the oxidation of 4-t-butylcatechol catalyzed by mushroom tyrosinase with an IC₅₀ of 31.0 μM. The inhibition kinetics analyzed by Dixon plot indicated that it acts as a partial noncompetitive inhibitor. Further studies of several benzaldehydes, particularly those having a substitution at C-4, suggested that the partial inhibitory property diminished when using a bulk substituent. For example, 4-penthylbenzaldehyde showed a full and mixed type inhibition on diphenolase activity. Therefore, 4-substituted benzaldehyde on the aromatic ring primarily reflected the rate of product formation as it may act as a tight hydrophobic cover on the catalytic center of tyrosinase.

Regioselective ortho-Hydroxylations of Flavonoids by Yeast

Natural flavonoids, such as naringenin, hesperetin, chrysin, apigenin, luteolin, quercetin, epicatechin, and biochanin A, were subjected to microbiological transformations by Rhodotorula glutinis. Yeast was able to regioselectively C-8 hydroxylate hesperetin, luteolin, and chrysin. Naringenin was transformed to 8- and 6-hydroxyderivatives. Quercetin, epicatechin, and biochanin A did not undergo biotransformation. A metabolic pathway for the degradation of chrysin has been elucidated. The metabolism of chrysin proceeds via an initial C-8 hydroxylation to norwogonin, followed by A-ring cleavage to 4-hydroxy-6-phenyl-2H-pyran-2-one.

Free-Radical-Scavenging, Antityrosinase, and Cellular Melanogenesis Inhibitory Activities of Synthetic Isoflavones

In this study, we examined the potential of synthetic isoflavones for application in cosmeceuticals. Twenty-five isoflavones were synthesized and their capacities of free-radical-scavenging and mushroom tyrosinase inhibition, as well as their impact on cell viability of B16F10 murine melanoma cells and HaCaT human keratinocytes were evaluated. Isoflavones that showed significant mushroom tyrosinase inhibitory activities were further studied on reduction of cellular melanin formation and antityrosinase activities in B16F10 melanocytes in vitro. Among the isoflavones tested, 6-hydroxydaidzein (2) was the strongest scavenger of both ABTS(.+) and DPPH(.) radicals with SC50 values of 11.3 ± 0.3 and 9.4 ± 0.1 μM, respectively. Texasin (20) exhibited the most potent inhibition of mushroom tyrosinase (IC50 14.9 ± 4.5 μM), whereas retusin (17) showed the most efficient inhibition both of cellular melanin formation and antityrosinase activity in B16F10 melanocytes, respectively. In summary, both retusin (17) and texasin (20) exhibited potent free-radical-scavenging capacities as well as efficient inhibition of cellular melanogenesis, suggesting that they are valuable hit compounds with potential for advanced cosmeceutical development.

Isolation, bioactivity, and production of ortho-hydroxydaidzein and ortho-hydroxygenistein

Daidzein and genistein are two major components of soy isoflavones. They exist abundantly in plants and possess multiple bioactivities. In contrast, ortho-hydroxydaidzein (OHD) and ortho-hydroxygenistein (OHG), including 6-hydroxydaidzein (6-OHD), 8-hydroxydaidzein (8-OHD), 3'-hydroxydaidzein (3'-OHD), 6-hydroxygenistein (6-OHG), 8-hydroxygenistein (8-OHG), and 3'-hydroxygenistein (3'-OHG), are rarely found in plants. Instead, they are usually isolated from fermented soybean foods or microbial fermentation broth feeding with soybean meal. Accordingly, the bioactivity of OHD and OHG has been investigated less compared to that of soy isoflavones. Recently, OHD and OHG were produced by genetically engineering microorganisms through gene cloning of cytochrome P450 (CYP) enzyme systems. This success opens up bioactivity investigation and industrial applications of OHD and OHG in the future. This article reviews isolation of OHD and OHG from non-synthetic sources and production of the compounds by genetically modified microorganisms. Several bioactivities, such as anticancer and antimelanogenesis-related activities, of OHD and OHG, are also discussed.

Purification of a fucoidan from kelp polysaccharide and its inhibitory kinetics for tyrosinase

High-speed countercurrent chromatography (HSCCC) was used to separate kelp polysaccharide. HSCCC was performed using an aqueous two-phase solvent system composed of PEG1000-K₂HPO₄-KH₂PO₄-H₂O (0.5:1.25:1.25:7.0, w/w) by eluting a lower aqueous phase at 2.0 mL/min at 600 rpm, yielding two separate fractions, KPS-1 and KPS-2. The KPS-2 fraction was further purified by DEAE-Sepharose fast flow anion-exchange column chromatography to provide 3 fractions, KPS-2-1, KPS-2-2 and KPS-2-3. GPC-HPLC analysis indicated that KPS-2-1 fraction was a purified fucoidan. FT-IR analysis showed that KPS-2-1 was a sulphated polysaccharide. An analysis of enzymatic kinetics showed that the purified fucoidan was a competitive inhibitor of tyrosinase toward l-tyrosine, and the inhibitory constant Ki obtained from double-reciprocal plots was 0.9907 mg/mL.

A potential daidzein derivative enhances cytotoxicity of epirubicin on human colon adenocarcinoma Caco-2 cells

In this study, we evaluated the effects of 8-hydroxydaidzein (8HD), an isoflavone isolated from fermented soy germ koji, and epirubicin (Epi), an antineoplastic agent, on the production of reactive oxygen species (ROS). We subsequently correlated the ROS levels to the anticancer mechanisms of Epi and 8HD in human colon adenocarcinoma Caco-2 cells. 8HD enhanced cytotoxicity of Epi and generated a synergistic effect. Epi and/or 8HD treatments increased the hydrogen peroxide and superoxide levels. Combined treatment markedly decreased mRNA expression levels of multidrug resistance protein 1 (MDR1), MDR-associated protein (MRP) 1, and MRP2. 8HD significantly intensified Epi intracellular accumulation in Caco-2 cells. 8HD and/or Epi-induced apoptosis, as indicated by the reduced mitochondrial membrane potential and increased sub-G1 phase in cell cycle. Moreover, 8HD and Epi significantly enhanced the mRNA expressions of Bax, p53, caspases-3, -8, and -9. To our best knowledge, this study verifies for the first time that 8HD effectively circumvents MDR in Caco-2 cells through the ROS-dependent inhibition of efflux transporters and p53-mediated activation of both death receptor and mitochondrial pathways of apoptosis. Our findings of 8HD shed light on the future search for potential biotransformed isoflavones to intensify the cytotoxicity of anticancer drugs through simultaneous reversal of pump and nonpump resistance.

Effects of ortho-dihydroxyisoflavone derivatives from Korean fermented soybean paste on melanogenesis in B16 melanoma cells and human skin equivalents

In this study we investigated the inhibitory effects and possible mechanisms of action of 8'-hydroxydaidzein and 3'-hydroxydaidzein, two ortho-dihydroxyisoflavone derivatives from Korean fermented soybean paste, on melanogenesis in B16 murine melanoma cells. The two hydroxydaidzeins reduced melanin synthesis comparably to treatment with kojic acid, a proven whitening agent, in B16 melanoma cells. Furthermore, when in vitro human skin equivalents were treated with the hydroxydaidzeins, the levels of melanogenesis were significantly reduced relative to a kojic acid control. The RT-PCR results demonstrated that depigmentation was due to transcriptional repression of several melanogenesis genes, including microphthalmia-associated transcription factor (MITF), by the hydroxydaidzeins. The immunoblotting results confirmed that diminution of MITF expression subsequently decreased expression of tyrosinase, and tyrosinase-related proteins 1 and 2. Cumulatively, these results suggest that hydroxydaidzeins would be potent attenuators of melanin synthesis as well as effective inhibitors of hyperpigmentation in human skin.

Kinetic characterisation of o-aminophenols and aromatic o-diamines as suicide substrates of tyrosinase

We study the suicide inactivation of tyrosinase acting on o-aminophenols and aromatic o-diamines and compare the results with those obtained for the corresponding o-diphenols. The catalytic constants follow the order aromatic o-diamines<o-aminophenols<o-diphenols, which agrees with the view that the transfer of the proton to the peroxide group of the oxy-tyrosinase form is the slowest step in the catalytic cycle. As regards the apparent inactivation constant, it remains within the same order of magnitude, although slightly lower in the case of the aromatic o-diamines and o-aminophenols than o-diphenols: o-diamines<o-aminophenols<o-diphenols. The efficiency of the second nucleophilic attack of substrate on CuA seems to be the determining factor in the bifurcation of the inactivation and catalytic pathways. This attack is more efficient in o-diamines (where it attacks a nitrogen atom) than in o-aminophenols and o-diphenols (where it attacks an oxygen atom), favouring the catalytic pathway and slowing down the inactivation pathway. The inactivation step is the slowest of the whole process. The values of r, the number of turnovers that 1mol of enzyme carries out before being inactivated, follows the order aromatic o-diamines<o-aminophenols<o-diphenols. As regards the Michaelis constants, that of the o-diamines is slightly lower than that of the o-diphenols, while that of the o-aminophenols is slightly greater than that observed for the o-diphenols. As a consequence of the above, the inactivation efficiency, λ(max)/K(m)(S), follows this order: o-diphenols>o-aminophenols>aromatic o-diamines.

Inhibitory effect of homochlorcyclizine on melanogenesis in α-melanocyte stimulating hormone-stimulated mouse B16 melanoma cells

The histamine receptor H1 antagonist homochlorcyclizine (HC) has been widely used as an antihistamine agent for the treatment of allergies. However, the effect of HC on skin pigmentation is not known. In the present study, we investigated the inhibitory effect of HC on melanogenesis in mouse B16 melanoma cells. Our results showed that HC inhibited melanogenesis in either α-melanocyte stimulating hormone (α-MSH)- or 3-isobutyl-1-methylxanthin (IBMX)-stimulated B16 cells in a dose-dependent manner. Despite the strong inhibition of melanogenesis by HC, it was surprisingly found that HC did not reduce either cellular or melanosomal tyrosinase activity in α-MSH-stimulated B16 cells. In addition, HC also did not directly inhibit either murine or mushroom tyrosinase activity in the cell-free system. Moreover, western blotting and reverse-transcription polymerase chain reaction (RT-PCR) analyses respectively confirmed that HC did not downregulate levels of tyrosinase protein and its mRNA in α-MSH-stimulated B16 cells. These results clearly demonstrated that HC inhibits melanogenesis of B16 cells by a mechanism other than reduction of the cellular tyrosinase activity. From the present study, HC was proven to be a good candidate as a skin-whitening agent for treatment of skin hyperpigmentation, and this generic drug might be suitable for use in combination with other depigmenting agents due to its unique inhibition mechanism.

Resveratrol as a kcat type inhibitor for tyrosinase: potentiated melanogenesis inhibitor

Resveratrol exhibited the inhibitory activity against mushroom tyrosinase (EC1.14.18.1) through a k(cat) inhibition. Resveratrol itself did not inhibit tyrosinase but rather was oxidized by tyrosinase. In the enzymatic assays, resveratrol did not inhibit the diphenolase activity of tyrosinase when l-3,4-dihydroxyphenylalanin (L-DOPA) was used as a substrate; however, L-tyrosine oxidation by tyrosinase was suppressed in presence of 100 μM resveratrol. Oxidation of resveratrol and inhibition of L-tyrosine oxidation suggested the inhibitory effects of metabolites of resveratrol on tyrosinase. After the 30 min of preincubation of tyrosinase and resveratrol, both monophenolase and diphenolase activities of tyrosinase were significantly suppressed. This preincubational effect was reduced with the addition of L-cysteine, which indicated k(cat) inhibition or suicide inhibition of resveratrol. Furthermore, investigation was extended to the cellular experiments by using B16-F10 murine melanoma cells. Cellular melanin production was significantly suppressed by resveratrol without any cytotoxicity up to 200 μM. trans-Pinosylvin, cis-pinosylvin, dihydropinosylvin were also tested for a comparison. These results suggest that possible usage of resveratrol as a tyrosinase inhibitor and a melanogenesis inhibitor.

Tetrahydrofolic Acid is a potent suicide substrate of mushroom tyrosinase

The coenzyme tetrahydrofolic acid is the most rapid suicide substrate of tyrosinase that has been characterized to date. A kinetic study of the suicide inactivation process provides the kinetic constants that characterize it: λ(max), the maximum apparent inactivation constant; r, the partition ratio or the number of turnovers made by one enzyme molecule before inactivation; and k(cat) and K(m), the catalytic and Michaelis constants, respectively. From these values, it is possible to establish the ratio λ(max)/K(m), which represents the potency of the inactivation process. Besides acting as a suicide substrate of tyrosinase, tetrahydrofolic acid reduces o-quinones generated by the enzyme in its action on substrates, such as l-tyrosine and l-DOPA (o-dopaquinone), thus inhibiting enzymatic browning.

Suicide inactivation of the diphenolase and monophenolase activities of tyrosinase

The suicide inactivation mechanism of tyrosinase acting on its phenolic substrates has been studied. Kinetic analysis of the proposed mechanism during the transition phase provides explicit analytical expressions for the concentrations of o-quinone versus time. The electronic, steric, and hydrophobic effects of the phenolic substrates influence the enzymatic reaction, increasing the catalytic speed by three orders of magnitude and the inactivation by one order of magnitude. To explain this suicide inactivation, we propose a mechanism in which the enzymatic form oxy-tyrosinase is responsible for the inactivation. In this mechanism, the rate constant of the reaction would be directly related with the strength of the nucleophilic attack of the C-1 hydroxyl group, which depends on the chemical shift of the carbon C-1 (delta(1)) obtained by (13)C-NMR. The suicide inactivation would occur if the C-2 hydroxyl group transferred the proton to the protonated peroxide, which would again act as a general base. In this case, the coplanarity between the copper atom, the oxygen of the C-1 and the ring would only permit the oxidation/reduction of one copper atom, giving rise to copper (0), hydrogen peroxide, and an o-quinone, which would be released, thus inactivating the enzyme. One possible application of this property could be the use of these suicide substrates as skin depigmenting agents.

Hypopigmentary action of dihydropyranocoumarin D2, a decursin derivative, as a MITF-degrading agent

In this study, the decursin derivative dihydropyranocoumarin D2 (1) was selected for its effects on melanogenesis using a spontaneously immortalized mouse melanocyte cell line (Mel-Ab). The results showed that 1 effectively inhibited melanin synthesis in a concentration-dependent manner, but that it did not inhibit tyrosinase in a cell-free system. In addition, the changes in ERK, Akt, and microphthalmia-associated transcription factor (MITF) in response to treatment with 1 were assessed. The results revealed that ERK was dramatically up-regulated and MITF was down-regulated in response to treatment with 1, but that Akt was unchanged. Therefore, the effects of 1 on melanogenesis were examined in the absence or presence of PD98059 (a specific inhibitor of the ERK pathway). PD98059 restored hypopigmentation and the down-regulation of MITF induced by 1. Finally, MITF down-regulation by 1 was clearly restored by both chloroquine, a lysosomal proteolysis inhibitor, and MG132, a proteasome inhibitor.

The hypopigmentary action of KI-063 (a new tyrosinase inhibitor) combined with terrein

Resorcinol derivatives are known to inhibit melanin synthesis. In this study, resorcinol derivatives were synthesized and screened for their activity on melanogenesis. KI-063 (a tyrosinase inhibitor) was examined for its effects on melanogenesis using a spontaneously immortalized mouse melanocyte cell line (Mel-Ab). In a cell-free system, KI-063 directly inhibited tyrosinase, the rate-limiting melanogenic enzyme. Moreover, in a cell system, it inhibited melanin synthesis in a concentration-dependent manner. In addition, KI-063 inhibited the activity of cellular tyrosinase. Thus, this study examined the effects of a combination of KI-063 with terrein, an agent that down-regulates microphthalmia-associated transcription factor. The data suggest that KI-063 has an additive effect in combination with terrein. Thus, the suppression of tyrosinase activity by KI-063 and the inhibition of tyrosinase production by terrein appear to be an optimal combination for skin whitening.

Evaluation of depigmenting activity by 8-hydroxydaidzein in mouse B16 melanoma cells and human volunteers

In our previous study, 8-hydroxydaidzein (8-OHDe) was demonstrated to be a potent and unique suicide substrate of mushroom tyrosinase. In this study, the compound was evaluated for in vitro cellular tyrosinase and melanogenesis inhibitory activities in mouse B16 melanoma cells and for in vivo skin-whitening activity in human volunteers. Tyrosinase activity and melanogenesis in the cell culture incubated with 10 μM of 8-OHDe were decreased to 20.1% and 51.8% of control, respectively, while no obvious cytotoxicity was observed in this concentration. In contrast, a standard tyrosinase inhibitor, kojic acid, showed 69.9% and 71.3% of control in cellular tyrosinase and melanogenesis activity, respectively, at a concentration as high as 100 μM. Hence, 8-OHDe exhibited more than an inhibitory effects on melanin production in B16 cells 10-fold stronger than kojic acid. In addition, when a cream containing 4% 8-OHDe was applied to human skin in an in vivo study, significant increases in the dL*-values were observed after three weeks. Moreover, the increase in the dL*-values after 8-week treatment with 4% 8-OHDe (from −0.57 to 1.94) is stronger than those of 2% 8-OHDe treatment (from 0.26 to 0.94) and 2% ascorbic acid-2-glucoside treatment (from 0.07 to 1.54). From the results of the study, it was concluded that 8-OHDe, the potent suicide substrate of mushroom tyrosinase, has depigmenting activities in both mouse melanoma cells and in human volunteers. Thus, the compound has significant potential for use in cosmetics as a skin-whitening ingredient.

Antiplasmodial isoflavanones from the roots of Sophora mollis

Six new prenylated isoflavanones named sophoronols A-F (1-6), together with eight phenolic constituents, were isolated from the roots of Sophora mollis. Their structures and stereochemistry were established by 1D and 2D NMR techniques, especially HMBC and NOESY as well as CD results. Componds 3 and 5 exhibited moderate anitplasmodial activity against the CQS D10 strain of Plasmodium falciparum, with IC(50) values of 12.9 and 12.8 microM, respectively.

An updated review of tyrosinase inhibitors

Tyrosinase is a multifunctional, glycosylated, and copper-containing oxidase, which catalyzes the first two steps in mammalian melanogenesis and is responsible for enzymatic browning reactions in damaged fruits during post-harvest handling and processing. Neither hyperpigmentation in human skin nor enzymatic browning in fruits are desirable. These phenomena have encouraged researchers to seek new potent tyrosinase inhibitors for use in foods and cosmetics. This article surveys tyrosinase inhibitors newly discovered from natural and synthetic sources. The inhibitory strength is compared with that of a standard inhibitor, kojic acid, and their inhibitory mechanisms are discussed.

Initial studies on the quantitation of bruise damage and freshness in mushrooms using visible-near-infrared spectroscopy

Identification of mushrooms that have been physically damaged and the measurement of time elapsed from harvest are very important quality issues in industry. The purpose of this study was to assess whether the chemical changes induced by physical damage and the aging of mushrooms can: (a) be detected in the visible and near infrared absorption spectrum and (b) be modeled using multivariate data analysis. The effect of pre-treatment and the use of different spectral ranges to build PLS models were studied. A model that can identify damaged mushrooms with high sensitivity (0.98) and specificity (1.00), and models that allow estimation of the age (1.0-1.4 days root mean square error of cross-validation) were developed. Changes in water matrix and alterations caused by enzymatic browning were the factors that most influenced the models. The results reveal the possibility of developing an automated system for grading mushrooms based on reflectance in the visible and near infrared wavelength ranges.