ABSTRACT

Tyrosinase is an important enzyme in controlling the formation of melanin in melanosomes and plays a key role in the pigmentation of hair and skin. The abnormal expression or activation of tyrosinase is associated with several diseases such as albinism, vitiligo, melanoma, and Parkinson's disease. Excessive deposition of melanin could cause diseases such as freckles and brown spots in the human body, and it is also closely related to the browning of fruits and vegetables and insect molting. Detecting and inhibiting the activity of tyrosinase is of extraordinary value in the progress of diagnosis and treatment of these diseases. Therefore, many selective optical detection probes and small molecular inhibitors have been developed, and have made significant contributions to the basic and clinical research on these diseases. In this paper, the detection and inhibition of tyrosinase and its application in whitening products are reviewed, with special emphasis on the development of fluorescent probes and inhibitors. Hopefully, this review will help design more efficient and sensitive tyrosinase probes and inhibitors, as well as shed light on novel treatments for diseases such as melanoma.

According to relevant studies,cistanche is a common herb that is known as "the miracle herb that prolongs life". Its main component is cistanoside, which has various effects such as antioxidant, anti-inflammatory, and immune function promotion. The mechanism between cistanche and skin whitening lies in the antioxidant effect of cistanche glycosides. Melanin in human skin is produced by the oxidation of tyrosine catalyzed by tyrosinase, and the oxidation reaction requires the participation of oxygen, so the oxygen-free radicals in the body become an important factor affecting melanin production. Cistanche contains cistanoside, which is an antioxidant and can reduce the generation of free radicals in the body, thus inhibiting melanin production.

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1. Introduction 

Tyrosinase (EC 1.14.18.1; catechol oxidase; polyphenol oxidase [1] or diphenolase) is considered the most important copper-containing enzyme in melanin formation. Tyrosinase can catalyze the hydroxylation and subsequent oxidation of monophenol unit to orthoquinone under the action of molecular oxygen. It is found in melanosomes, the site for the synthesis, storage, and transport of melanin. The pH values of mildly and severely melanized melanosomes are about 4.5 and 3, respectively. The optimum pH for tyrosinase activity is 6.8 [2]. Raper [3] and Mason [4] were the first to clarify the biosynthetic pathways of melanin formation in various organisms, which were recently embellished by Schallreuter et al. [5] and Cooksey et al. (Fig. 1) [6].

Tyrosinase is widely found in plants, animals, and microorganisms. Given the involvement of tyrosinase in the pathogenesis of melanoma, monitoring and pharmacologically regulating its activity will help the diagnosis and treatment of the disease [7]. Probes were developed to specifically detect tyrosinase activity in physiological environments. Inhibitors that could make tyrosinase inactive were also developed, and some of them were used clinically to treat diseases. For example, kojic acid and arbutin [8], as specific inhibitors of tyrosinase, were clinically utilized as a whitening products. With advances in drug discovery, an increasing number of effective compounds that can detect/inhibit tyrosinase activity have been designed and synthesized recently. This would greatly facilitate the progress of diagnosing and treating those diseases resulting from aberrant melanin production. In this review, we will discuss the progress in the development of small-molecule-based probes and inhibitors for endogenous tyrosinase activity. Hopefully, it will help us learn more about tyrosinase and find more functional compounds targeting/regulating tyrosinase.

2. Structure of tyrosinase 

The tyrosinase active site presents a dual-core copper center structure composed of two copper ions (Fig. 2), which bind to histidine residues in the protein. The two copper ion centers are connected by an endogenous coordination bridge. Tyrosine and other substances are complexed with the enzyme, through the bond between the active center of the enzyme and the hydroxyl group. In the process of the catalytic reaction of melanin, the catalytic site is categorized into three forms: oxidation state (Eoxy), reduction state (Emet), and deoxygenation state (Edeoxy), the difference lies in the structure of binuclear copper ion active center (Fig. 2).

Epoxy is composed of two square copper (II) atoms; each atom is made up of two strong equators and the ligand is one weaker axial NH [9]. The exogenous oxygen molecule binds and bridges the two copper centers in the form of peroxides. The CueCu bond length is about 0.35 nm. The combination of oxygen molecules leads to the formation of (m-h2:h2 -peroxo) structure [10], so the Eoxy active center can be written as [Cu(II)eO2eCu(II)]. The electronic structure of peroxides plays a crucial role in the biological functions of Eoxy. Due to the strong s* acceptor action, the peroxide has a less negative charge, while the p electron acceptor interacts with the electrons in the s* orbital of the peroxide, which greatly weakens the strength of the oxygen-oxygen bond, rendering the nucleus of tyrosinase active center easily breakable [9]. Mettyrosinase (Emet) is similar to Eoxy and also contains two tetragonal copper (II) atoms coupled through an endogenous bridge. The difference is that the bridging ligand between copper ions is hydroxide instead of peroxide [2]. In terms of oxidative properties, Emet and Eoxy are also slightly different. Emet is not able to oxidize mono phenolic compounds. In the absence of substrates, Emet exists as the main form in organisms. Deoxytyrosinase (Edeoxy), similar to deoxy hemocyanin, has a symmetrical structure [(Cu(I)eCu(I)]. There is no bridging ligand such as peroxide or hydroxide between binuclear copper; thus hydroxide in water is an essential bridging ligand.

3. Mechanism of the action of tyrosinase

Although researchers have conducted in-depth studies on tyrosinase and its related proteins, the mechanism of its catalytic reaction is still controversial. For example, the catalytic activity at the same active site of tyrosinase is different. The catalytic center of tyrosinase contains a binuclear copper center, named Cu(A) and Cu(B), respectively. Each copper ion in the active center is coordinated with three different histidine residues. There is a large difference between mycophenolate activity and diphenolase activity, and reduced tyrosinase has a lag phase when reacting with monophenol. These are important topics that need to be continuously explored and studied [11].

The reaction between tyrosinase and related substrates is mainly through the formation of an effective coordination bond between the hydroxyl group on the substrate and the tyrosinase active center. Olivares et al. [12] proposed that Eoxy and suitable substrates in mammals trigger mycophenolate activity and diphenolase activity. During mycophenolate activity, monophenols (L-Tyrosine) are oxidized to form o-quinones (o-dopaquinone), an important precursor of melanin, and Edeoxy. During diphenolase activity, Eoxy and Emet can also oxidize o-diphenols (L-DOPA) to produce o-dopaquinone [13]. This mechanism is generally accepted by researchers as it can most accurately reflect the kinetic characteristics of tyrosinase, in which the rate-limiting step in melanin production is the monophenol cycle [14].

3.1. Mechanism of mycophenolate activity

During melanin synthesis, the main function of the enzyme is to oxidize monophenol substrates to o-quinone by Eoxy. This process is an important feature that distinguishes tyrosinase from other oxidoreductases such as catechol oxidase. During the monophenol cycle (Fig. 2), the oxygen atom on the deprotonated phenol is coordinated with the copper ion of the oxidized tyrosinase active center to form mycophenolate Eoxy complex (EoxyM), and then the phenol is ortho-electrophilically substituted to diphenolase Emet complex (EmetD). EmetD undergoes a cleavage process to directly generate o-quinone and Edeoxy. Edeoxy directly combines with oxygen molecules to re-form Eoxy. This is the cyclic process of mycophenolate activity. This process does not end until the substrate reaction is complete. In the reaction process of the monophenol cycle, if the Emet in the natural state meets a monophenol substrate, it will undergo an extremely slow oxidation reaction and hinder the normal progress of the monophenol reaction. Therefore, this period is called the ‘lag period’ because Emet itself cannot bind oxygen molecules [12].

3.2. Mechanism of diphenolase activity

4. Function of tyrosinase

Tyrosinase is part of the type 3 copper family and exists in the early process of melanin formation. It mainly participates in the following two reaction processes [17]: (1) hydroxylation of L-tyrosine to L-DOPA; (2) oxidation of L-DOPA to form dopaquinone. Dopaquinone will eventually form melanin through a series of reactions. The other two family members of the type 3 copper family are catechol oxidase and hemocyanin. Catechol oxidase only exhibits diphenolase activity and hemocyanin is on the lymph of many mollusks and arthropods oxygen carrier. Although the active centers of the type 3 copper family proteins are conserved in terms of total structure and ability to connect oxygen molecules, their potential activities of enzymatic are slightly distinct due to the variability of the substrate’s attachment to the enzyme center or the uncontrollability that the substrate can reach.

In addition to participating in the process of melanin production, tyrosinase also has other important physiological functions. In sponges, plants, and some invertebrates, tyrosinase is mainly involved in the process of wound healing and primary immune response [18]. In arthropods, tyrosinase can promote the hardening process of the stratum corneum of animals after molting. Bacterial tyrosinase can be secreted in soil and participate in the process of random coupling of different aromatic compounds to form humus. And, it was discovered that tyrosinase can also be a potential antidote for benzene toxic substances [13,19]. In addition, it plays an indispensable role in killing parasitic plants against phenolic symbiotic bacteria, the production of natural pigments, and the synthesis of amino acid antibiotics such as lincomycin. The common point of these effects is inseparable from the tyrosinase using redox reactions with oxygen molecules [11].

5. Tyrosinase-related diseases

The distribution of tyrosinase is closely connected with the physiological functions of plants and animals. It is generally believed that the colors of feathers, hair, eyes, insect epidermis, seeds, and other pigments are the results of tyrosinase [10]. Tyrosinase has varying but important functions in different organisms. In most insects under normal physiological conditions, tyrosinase exists in the form of the zymogen, and different types of tyrosinase exist in specific parts of insects to complete specific physiological functions [20]. In addition to participating in the production of melanin, insect tyrosinase is the only enzyme involved in keratosis. Insect-hardened keratin can block the invasion of microorganisms and foreign bodies and protect the soft invertebrate body. In arthropods, tyrosinase also participates in two other important physiological processes, namely, defense response and wound healing. The melanin produced by tyrosinase in mammals is secreted into the keratinocytes of the epidermis and hair, discoloring the body surface, thereby protecting the skin and eyes, resisting ultraviolet radiation, and preventing internal tissues from overheating [10]. Tyrosinase found in mammals is commonly discovered in melanocytes, which are highly specific cells that exist in the skin, hair follicles, and eyes to produce pigments [4,21]. When tyrosinase function is reduced or missing, it will affect melanin metabolism and cause diseases such as epilepsy and albinism. Autosomal recessive diseases in animals and humans are also related to the loss of tyrosinase or decreased activity [22].

6. Probes of tyrosinase

Probes are substances that specifically recognize the target and release detectable signals that reflect the presence and activity of the target. The traditional colorimetric method for tyrosinase analysis has been limited due to its low sensitivity [23]. At first, several other detection methods based on electrochemistry and gold nanoparticles were reported by Willner’s group [24e27], which not only increases the versatility of detection but also greatly updates the colorimetry in terms of sensitivity. A fluorescent strategy has also been introduced to design highly sensitive tyrosinase probes as shown in Fig. 3. Initially developed quantum dots and conjugated polymer fluorescent probes may be applied to monitor the activity of tyrosinase [28]. Nevertheless, small-molecule fluorescent probes are particularly attractive because of their special advantages such as sensitivity, specificity, and compatibility. In 2008, Zhu’s team synthesized a new water-soluble oligo (phenylenevinylene) (Pr1) as an FL fluorophore containing tyrosine warhead (WH) as a fluorescent probe for tyrosinase. It was demonstrated that Pr1 is suitable for detecting the activity of tyrosinase so far in an aqueous buffer solution even in agarose gel [29]. First, a cyanine-based near-infrared (NIR) fluorescent probe (Pr2) was used to monitor the activity of tyrosinase in 2010 by Ma et al. [30]. The significant color change before and after the reaction could be detected by the naked eye. However, these probes also show a turn-off mode caused by the quinone moiety generated by tyrosinase-catalyzed oxidation. For functional use, however, the best approach is to implement a bioassay in the turn-on mode because of its sensitivity and more suitability for bioimaging of tyrosinase in living systems. In 2010, Kim et al. [31] proposed a BODIPY-based turn-on fluorogenic probe to detect endogenous tyrosinase activity in live melanoma cells (Pr3, Fig. 4). Based on previous research of tyrosinase applied to remove the amines protecting groups [32], Yan et al. [33] prepared tyrosinase probes, Pr4 and Pr5, in which a phenol group and a naphthylamine group were connected through a urea linkage in 2012. Importantly, Pr4 was the first two-photon turn-on fluorogenic probe designed to detect the activity of tyrosinase in aqueous buffer and living cells. In 2013, Wang et al. [34] showed that the NBD-NH2-based fluorescent probe (Pr6 and Pr7) containing phenol moieties (WH) can be used to detect the activity of tyrosinase and screen for potential tyrosinase inhibitors with a “turn-on” strategy. However, there is no biological experiment related to cell imaging in this work. In 2016, Li and colleagues designed a group of new probes based on 7- amino-4-(trifluoromethyl)-coumarin as an FL fluorophore and synthesized them, Pr8-11, with varying distances between FL fluorophore and phenols. Pr9 was found to be a highly sensitive and selective “turn-on” fluorogenic probe for imaging live melanoma cells [35]. In 2018, Wu’s team was the first to make use of an FL fluorescent probe (Pr12) to diagnose early melanoma in rodent mouse models (Figs. 4 & 5A). The probe can be activated by tyrosinase-mediated oxidation and then hydrolyze urea bonds to produce a fluorescence signal. At the same time, it can also monitor the level of endogenous tyrosinase in living cells and zebrafish sensitively and selectively (Fig. 5 B/C) [36].

In 2016, Ma’s group [37] developed a novel fluorogenic probe named Mela-TYR (Pr13, Fig. 4) to target melanosomes for detecting the activity of tyrosinase. Pr13 was designed by incorporating phthalimide with morpholine and 4-amino-phenol-derived urea. The probe shows a highly sensitive and selective turn-on response to tyrosinase through an oxidization-cleavage reaction. The fluorescence probes described above mainly contained a 4-hydroxyphenyl group as recognition moiety (WH) and showed parallel fluorescence response to several reactive oxygen species (ROS) and tyrosinase, thus interfered with by ROS. Ma’s group discovered a new tyrosinase-recognition moiety, 3-hydroxy benzyloxy (WH), which showed diverse reaction mechanisms for tyrosinase and ROS [38]. A NIR fluorescence probe (Pr14, Fig. 4) was developed by installing 3- hydroxy benzyloxy into a NIR fluorophore (HXPI), and showed a highly specific off-on response to tyrosinase instead of ROS, thus overcoming the interference. The presence of the 3- hydroxy group facilitates the hydroxylation by tyrosinase at the 4-position vacancy but not by ROS, and the intermediate would undergo spontaneous 1,6-rearrangement elimination, releasing free fluorophore. The high specificity of the developed probe was proved by imaging and detection of endogenous tyrosinase activity in living cells and zebrafishes, and the high specificity of the probe was further verified by an enzyme-linked immunosorbent assay (Figs. 4 and 6). Ma’s group subsequently developed another turn-on fluorogenic probe (Pr15, Fig. 4) based on resorufin incorporated with a 3-hydroxyphenyl group [39]. It was used to detect and image the activity of endogenous tyrosinase in a variety of living cells. Inspired by the above design, Zhang and co-workers [40] proposed a fluorogenic tyrosinase probe (Pr16, Fig. 4) with resorufin as a fluorophore, and the m-tolyl boronic acid pinacol ester (WH) as a new tyrosinase-recognition moiety. The probe showed high selectivity for tyrosinase over other biological substances, including ROS. However, it was severely interfered with by H2O2. In 2019, Hu et al. [41] reported a new fluorogenic probe with high chemical selectivity based on fluorescein (Pr17, Fig. 4), which can track tyrosinase in vitro and in vivo, and realize the high chemoselective detection of tyrosinase. In addition, the probe reacted in an aqueous solution and exhibited a fluorescence enhancement of more than 24 times in the presence of tyrosinase. In addition, Pr17 showed great cell membrane and tissue permeability characteristics, which helped its success in following endogenous tyrosinase activity in distinct helped living cells and zebrafish models. Ding’s group constructed a novel water-soluble NIR fluorescence probe (Pr18, Fig. 4) that can specifically recognize tyrosinase, which is highly stable within the physiological temperature and pH and can accurately detect tyrosinase in biological systems without being disturbed by ubiquitous entities. It can be used for imaging tyrosinase in living cells, zebrafish, and xenogeneic mouse models [42].

Sidhu et al. devised and synthesized a ratiometric fluorescent probe (Pr19, Fig. 4) based on naphthalimide. Pr19 has high selectivity and sensitivity for tyrosinase, and the limit of detection (LOD) is quite low [43]. The excitation or emission spectrum shift occurs after the probe is combined with reactants. It can be recorded using the ratio of the fluorescence intensity measured at two different wavelengths, which is called ratio measurement. Ratiometric fluorescent probes based on this principle show their sensitivity and selectivity and can be used for the study of enzyme function in live systems [44]. Guo’s team proposed a new ratiometric and turn-on NIR fluorescent probe (Pr20, Fig. 4) for real-time detection of endogenous tyrosinase activity. These special characteristics of Pr20, combined with rare cytotoxicity, superior photophysical characters, and cell membrane permeability, make it ideal for the quantitative detection of endogenous tyrosinase activity [45]. Cyanine derivatives, as typical NIR fluorescent dyes, can be controllably aggregated in aqueous solutions and thus exhibit many significantly different spectral properties. The chlorine atom in the center of the cyanine skeleton is easily substituted by other functional groups. Based on this function, Zhang et al. [46] developed a new cyanine-based fluorescent probe (Pr21, Fig. 4) for the ratiometric fluorescence detection of tyrosinase activity (Fig. 7). The ratio determination obtained a good signal-to-noise ratio, and the LOD value of tyrosinase activity was 0.02 U/mL. In addition, Pr21 was successfully employed in imaging the endogenous tyrosinase activity in B16 cells and qualitatively distinguishing it from other cancer/normal cells in the absence of tyrosinase (Fig. 7).

7. Inhibitors of tyrosinase

Inhibitors are generally divided into reversible inhibitors and irreversible inhibitors based on whether the inhibitors interacting with enzymes cause permanent inactivation of the enzyme. The inhibition characteristic of tyrosinase is reversible inhibition. For the inhibition characterized by reversible inhibition, the combination of inhibitor and enzyme is a reversible dynamic equilibrium process [47e49]. Increasing the concentration of the inhibitor will cause the enzyme activity to decrease but the inhibitor only inhibits the enzyme activity rather than permanently inactivates the enzyme. When the inhibitor concentration decreases, the tyrosinase activity will increase. Meanwhile, the irreversible inhibition will be the permanent inactivation of tyrosinase. According to the different sites and methods of tyrosinase inhibitors interacting with the enzyme, they can be divided into four forms: competitive, noncompetitive, mixed, and slow binding.

Flavonoids are a set of compounds composed of two benzene rings connected by a three-carbon chain. Because the hydroxyl, methoxy, and glycoside side-chain groups are in the benzene rings, the arrangement can be subdivided into flavonols, chalcones, dihydroflavones, and orange ones (Fig. 8) [56]. Flavonoids are extensively distributed in the leaves, seeds, skins, and followers of plants, and researchers have verified more than 4000 flavonoids. For some plants, flavonoids and their derivatives have protection against UV rays, pathogens, and herbivores [57]. Analysis of the structure of licorice root extract shows that the tyrosinase inhibitory ability of neo glycyrrhizin, glycyrrhizin, isoliquiritigenin, and glycyrrhizin is related to their lipophilicity. Among them, the inhibition of monophenol is more effective than that of diphenol, indicating that it is a rate-limiting reaction in the first step of the oxidation reaction [58].

Some flavones containing a 3-hydroxy-4-ketone structure can competitively inhibit enzyme activity by chelating copper at the active site of tyrosinase, resulting in the irreversible inactivation of tyrosinase. After chelating tyrosinase, the molecule theoretically loses its planar structure and becomes distorted. Competitive inhibitors are usually parallel in structure to the substrate, so the molecule easily enters the tyrosinase active site and prevents the entry of L-DOPA [59,60]. Jeong et al. extracted two flavonols from the leaves of Zanthoxylum piperitum. The flavonols can inhibit the tyrosinase activity of mushrooms, which is a competitive inhibition, but it cannot inhibit the melanin production of Streptomyces bikiniensis. Later, it was found that the flavonoid compound isolated from the Philippine Formosa can also inhibit tyrosinase, and the inhibitory effect is better than that of kojic acid [62]. Liang et al. found that safflower yellow pigment can also inhibit mushroom tyrosinase activity, with a half-maximal inhibitory concentration (IC50) value of 1.01 mg/mL. This inhibition relationship appears to be dose-dependent [63]. (2R, 3R)-(þ)-purpurin extracted from Shuiliao inhibits 70% of tyrosinase activity and the concentration is 0.50 mM. The inhibitory ability is better than that of kojic acid and arbutin [64].

7,8,40 -trihydroxyflavone is a flavonoid derivative that inhibits tyrosinase diphenolase activity in a non-competitive way with an IC50 value of 10.31 ± 0.41 mM and a Ki of 9.50 ± 0.40 mM. The mechanism of action of this compound with tyrosinase is a static mechanism and shows a single binding site with a binding constant of (7.05 ± 1.20) × 104 M-1 at 298 K. Thermodynamic parameters show that the bonding process is related to hydrogen bonding and van der Waals forces [51].

3,8-hydroxyquinoline (In1, Fig. 9 [65]) separated from Scolopendra subsidies mutilans can inhibit the production and oxidation of melanin in Melan-a cells. In1 shows a concentration-developed antioxidant effect and significantly inhibits mushroom tyrosinase activity through noncompetitive inhibition. At the same time, In1 is found to be non-cytotoxic in the study as shown in Table 1 [65]. The ethyl acetate fraction of Nymphaea nuchal flower extract (NNFE) (100 mg/mL) can effectively reduce melanin production and inhibit mushroom tyrosinase activity. The underlying mechanism involves interfering with transcription factors and universal signaling pathways in melanin synthesis [66]. Capsaicin (In2, Fig. 9 [67]) and dihydrocapsaicin (In3, Fig. 9) extracted from pepper can inhibit tyrosinase activity. The result shows that the IC50 value of In2 is 1.73 times smaller than that of In3. The inhibition constant (Ki) also supports the inhibitory activity of In3 (0.39 mM, Table 1) on tyrosinaseislower than that of In2 (0.30mM, Table 1) [67]. It was found that caffeine (In4, Fig. 9 [68]) extracted from camellia pollen shows strong inhibitory activity towards mushroom tyrosinase in a non-competitive model as shown in Table 1. In4 changes the binding site of L-tyrosine and the ring conformation adjacent to the active center by binding to tyrosinase. The experimental results show that In4 has an obvious inhibitory effect on the tyrosinase activity in the cells and melanin generation of B16F10 melanoma cells is related to the concentration [68]. The caftaric acid (In5, Fig. 9) extracted from grapes can competitively inhibit tyrosinase, and the IC50 value (Table 1) is lower than that of the relational compounds, caffeic and chlorogenic acids [47]. Phloretin (In6, Fig. 9) can bind to tyrosinase through a static process, which causes the conformation of tyrosinase to change, thereby inhibiting its activity. At the same time, In6 has a strong antioxidant capacity and the ability to reduce o-dopaquinone to LDOPA [48].

Researchers have evaluated two spiro acridines (AMTAC-01, In7, Fig. 9) and (AMTAC-02, In8, Fig. 9) as inhibitors of tyrosinase. The results show that acridine derivatives interact strongly with mushroom tyrosinase. In8 is more effective in inhibiting enzyme activity than In7 as shown in Table 1, which indicates that the methoxy group of In8 is highly correlated with the inhibitory activity [49]. Several 21 halogenated thiosemicarbazones (TSCs) were synthesized and investigated. It was found that TSCs 6, 12, and 21 (In9/10/11, Fig. 9 [69]) exhibit potent inhibitory properties with different IC50, respectively (Table 1). They demonstrate a mutually reversible and competitive mechanism to inhibit tyrosinase. Among the compounds studied, para-substituted acetophenone derivatives of thiosemicarbazones have the highest affinity to the enzyme [69]. Penicillin V (In12, Fig. 9) is a bacteriolytic b-lactam antibiotic drug. Studies have found that In12 can inhibit mycophenolate and diphenolase activity. Fluorescence quenching and molecular docking studies have shown that In12 can form a static interaction near the catalytic pocket of the enzyme, thereby hindering substrate transport to the active site and reducing the plasticity of copper for catalysis [70]. Recently, Raza et al. studied the inhibitory potential of N-(substituted-phenyl)-4-{(4-[(E)-3- phenyl-2-propenyl]-1-piperazinyl) butanamides (5a-e) on tyrosinase. All compounds were found to be biologically active, with 5b (In13, Fig. 9) showing the highest inhibitory potential [71]. Mahajan et al. designed and synthesized quinazolinone ben amides 4a-h (In14e21, Fig. 9). Through the study of the inhibitory effect of compounds on the activity of tyrosinase, it was found that all compounds show lower IC50 values than standard kojic acid as shown in Table 1 [72]. Shen et al. found a new tyrosinase inhibitor, the peptide ECGYF (EF-5, In22) as shown in Fig. 9. The binding between In22 and tyrosinase mainly depends on hydrogen bonding and hydrophobic interactions, and the effect of inhibiting tyrosinase is stronger than that of arbutin and glutathione [73]. He et al. isolated three tamariscinols 1/2/3 (In23/24/25) and two phenolics 4 & 5 (In26 & 27) as shown in Fig. 9. Experiments have shown that all isolates have inhibitory effects on mushroom tyrosinase, with In23 being the most effective one (Table 1) [74].

For more info: david.deng@wecistanche.com  WhatApp:86 13632399501