A comprehensive review on tyrosinase inhibitors

Abstract 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.


Introduction
Browning of fruits, fungi and vegetables and hyperpigmentation in human skin are two common undesirable phenomena. Tyrosinase is the main enzyme recognised as responsible for this enzymatic browning and melanogenesis in mammals 1,2 . This encouraged researchers and scientists to focus on the identification, isolation, synthesis and characterisation of new potent tyrosinase inhibitors for various application in the food 3 , cosmetics 4 and medicinal industries. However, very few inhibitors are qualified for clinical use and skin-whitening agents. Moreover, as the clinical and industrial demands for tyrosinase inhibitors increase, in vitro assays and improved screening techniques are also undergoing rapid development for in vitro high-throughput screening tyrosinase inhibitors and putative skin-whitening agents 5 . In other words, sensitive and correct assay methods for screening and development of effective tyrosinase inhibitors are of great importance. For this purpose, several spectrophotometric 6-10 , chromatographic [11][12][13][14][15][16][17] , electrophoretic [18][19][20][21][22] , radiometric 23,24 and electrochemical [25][26][27] assays have been applied and developed by researchers so far. Recently, a novel fluorescent biosensor 28 and tyrosinase-based thin-layer chromatography-autography have been suggested for tyrosinase inhibitor screening 29 .
The present review also focuses on the tyrosinase inhibitors discovered from all sources, including synthetic compounds, extracts and active ingredients of natural products, virtual screening and structure-based molecular docking studies published in the last four decades. We hope that the knowledge offered in this review serves as an updated comprehensive database contributing to the development of new safe and efficient anti-tyrosinase agents for the prevention of browning in plant-derived foods, seafood and hyperpigmentation treatments.
(TYRP1 and TYRP2) have a critical role in melanin synthesis. Tyrosinase is a multifunctional copper-containing metalloenzyme with dinuclear copper ions, which plays as a rate-limiting enzyme in the synthesis of melanin (Figure 1) 52,67 . Also, tyrosinase constitutes the primary cause for undesired browning of fruits and vegetables as well as diseases resulting from overproduction of melanin. Therefore, controlling the activity of enzyme by tyrosinase inhibitors is an essential endeavor for treating hypopigmentary disorders of mammals and enzymatic browning of fruits and fungi. To date, numerous effective inhibitors are identified and developed for using in the medical and cosmetic products, as well as food bioprocessing and agricultural industries and environmental industries. However, in medicine, tyrosinase inhibitors are a class of important clinical antimelanoma drugs but only a few compounds are known to serve as effective and safe tyrosinase inhibitors.

Mushroom tyrosinase properties
Tyrosinases have been isolated and purified from different sources such as some plants, animals and microorganisms. Although many of them (such as human) have been sequenced, only few of them have been characterised. Recently, a novel tyrosinase produced by Sahara soil actinobacteria have been isolated and biochemically charactrised with the aim to identify novel enzymes with exclusive features for biotechnological applications [68][69][70][71][72][73][74][75][76][77][78][79][80] . However, among different sources of tyrosinase, mushroom tyrosinase from Agaricus bisporus is a major and cheap source of tyrosinase with high similarity and homology compared to human tyrosinase 78 . Because of these good properties, the structural, functional and biochemical characteristics of mushroom tyrosinase have been studied extensively as a model system for screening of tyrosinase inhibitors and melanogenic studies, enzyme-catalysed reactions and enzyme-inhibitor structural studies so far 78,[81][82][83][84][85][86][87][88][89][90] . Tyrosinase from Agaricus bisporus is a 120 kDa tetramer with two different subunits, heavy and light 91 , which was the first isolated by Bourquelot and Bertrand 92 in 1895. It has three domains and two copper binding sites which bind to six histidine residues and interact with molecular oxygen in the tyrosinase active site. Also, a disulfide linkage stabilise its structure 93 . Recently, a 50 kDa tyrosinase isoform from Agaricus bisporus (H-subunit) have been purified with a high specific tyrosinase activity of more than 38,000 U/mg 94 .

Reaction mechanism
Tyrosinase (EC 1.14.18.1) has two activities in its catalytic cycle, see Figure 2 95,96 , a monophenolase activity where it hydroxylates monophenols (e.g L-tyrosine) to o-diphenols (e.g. L-dopa) and a diphenolase activity where tyrosinase oxidises o-diphenols to oquinones (o-dopaquinone). At the same time of these enzymatic reactions, there are different chemical reactions coupled where two molecules of o-dopaquinone react their-selves generating an o-diphenol molecule (L-dopa) and a dopachrome molecule.
Diphenolase activity can be independently studied, when tyrosinase reacts with an o-diphenol (see Figure 2). The form mettyrosinase (E m ) binds the o-diphenol (D) originating the complex E m D. This complex oxidises the o-diphenols transforming it to o-quinone and the enzyme is converted into the form deoxytyrosinase (E d ). E d has a very big affinity for the molecular oxygen originating the form oxy-tyrosinase (E ox ), which binds another o-diphenol molecule and originating the complex E ox D. After that, the o-diphenol is oxidised again to o-quinone and the form E m is formed again completing the catalytic cycle. However, after these enzymatic reactions, two o-quinone molecules (e.g. o-dopaquinone) react generating dopachrome and regenerating a molecule of o-diphenol.
As mentioned before, we can independently study the diphenolase activity. However, it is not applicable for the monophenolase activity, see Figure 2, because the chemical reactions of diphenolase activity have to occur at the same time of monophenolase activity. Tyrosinase shows the monophenolase activity with a lag period. This period is the time that the enzyme requires to accumulate a quantity of o-diphenol in reaction medium and is proportional to the quantity of monophenol used. Figure 2 shows the new complexes appeared in the monophenolase activity: E ox M (oxy-tyrosinase bound to monophenol) and E m M (met-tyrosinase bound to monophenols). E ox M is active and is transformed into E m D, which is an intermediate of the catalytic cycle 95 . o-Quinones formed by these two oxidation cycle spontaneously react with each other to form oligomers 97 .

Tyrosinase inhibition
Due to the critical role of tyrosinase in the melanogenesis and browning process, several investigations have been reported for the identification of tyrosinase inhibitor from both natural (fungi, bacteria, plants) and synthetic sources so far. General speaking, tyrosinase inhibitors are examined in the presence of a monophenolic substrate such as tyrosine or a diphenolic substrate such as L-dopa, and activity is assessed based on dopachrome formation.

Inhibition mechanism
Among different types of compounds such as specific tyrosinase inactivators and inhibitors, o-dopaquinone scavengers, alternative enzyme substrates, nonspecific enzyme inactivators and denaturants, only specific tyrosinase inactivators and reversible inhibitors actually bind to the enzyme as true inhibitors and really inhibit its activity: a. Specific tyrosinase inactivators. They are called suicide inactivators or mechanism-based inhibitors. This group of compounds can be considered very interested from a pharmacological point of view, in hyperpigmentation processes ( Figure 3) 98 .
To explain the suicide inactivation of tyrosinase, mainly two mechanisms have been proposed 98,99 . Accordingly, Haghbeen et al. have suggested that the conformational changes, triggered by the substrate then mediated by the solvent molecules, in the tertiary and quaternary structures of tyrosinase, might be the real reason for the suicide inactivation 100 . On the other hand, however, based on reports, it was found that acetylation of tyrosine residues with N-acetylimidazole protects mushroom tyrosinase from the suicide inactivation in the presence of its catecholic substrate, 4-[(4-methylbenzo) azo]-1,2-benzenediol without any major impact on the secondary structure of enzyme 101 .
The studies about the kinetics of suicide inactivation of tyrosinase have been carried out with several o-diphenolic substrates 102 , ascorbic acid 103 , L-and D-dopa 104 and with different aminophenols and o-diamines 105 . The authors have established that the suicide inactivation could occur after the transference of a proton to the peroxide group on the active site of oxy-tyrosinase 98,106 , also it has been proposed that the monophenols do not inactivate the enzyme 107,108 .
The chemical structure of the different substrates is diverse, but the process always requires a step of oxidation/reduction: o-diphenols 102 b. Generally, the mode of inhibition by "true inhibitors" is one of these four types: competitive, uncompetitive, mixed type (competitive/uncompetitive), and noncompetitive. A competitive inhibitor can bind to a free enzyme and prevents substrate binding to the enzyme active site. Regarding the property that tyrosinase is a metalloenzyme, copper chelators such as many aromatic acids, phenolic and poly-phenolic compounds, a few non-aromatic compounds, can inhibit tyrosinase competitively by mimicking the substrate of tyrosinase 52,60 . Recently, it was found that D-tyrosine negatively regulates melanin synthesis by inhibiting tyrosinase activity, competitively 113 . In addition, L-tyrosine has been shown as an inhibitor 114 .
In contrast, an uncompetitive inhibitor can bind only to the enzyme-substrate complex and a mixed (competitive and uncompetitive mixed) inhibitor can bind to both forms of free enzyme and enzyme-substrate complex. Finally, noncompetitive inhibitors bind to a free enzyme and an enzyme-substrate complex with the same equilibrium constant 115 . Non-competitive and mixedinhibition are frequent modes observed in the kinetics studies on mushroom tyrosinase activities. Phthalic acid and cinnamic acid hydroxypyridinone derivatives 116 are two examples of mixed type inhibitors of mono-phenolase activity 117 . Also, some compounds such as phthalic acid 46 and terephthalic acid 118 , D-(À)arabinose 119 , brazilein 120   and p-alkylbenzaldehydes 127 inhibited catecholase activity of mushroom tyrosinase uncompetitively. Some derivatives of thiazoles are examples for noncompetitive tyrosinase inhibition 128 .
In addition to determining the inhibition mechanism, inhibitory strength which is expressed as the IC 50 value (the concentration of inhibitor at which 50% of your target is inhibited) should be calculated in the enzyme kinetics studies and inhibitor screening to compare the inhibitory strength of an inhibitor with others. However, the IC 50 values may be incomparable due to the varied assay conditions (different substrate concentrations, incubation time, and different sources of tyrosinase) but a positive control can be used for this purpose 52 . Although, some researchers have not calculated IC 50 and have not applied a positive control in their studies but, fortunately, in most studies conducted for screening new tyrosinase inhibitors, the popular whitening agents, such as kojic acid, arbutin or hydroquinone, were used as a positive control 129 at the same time. However, among different types of mushroom tyrosinase inhibitors, some inhibitors such as hydroquinone 49 arbutin, kojic acid 15,49 , azelaic acid, L-ascorbic acid, ellagic acid and tranexamic acid have been reported as skin-whitening agents in the cosmetic industry but there are a few reports failed to confirm their effect as an agent to lighten skin in clinical trials despite the safety of this compound 5 .
Recently, Mann et al., have compared the inhibitory effects of hydroquinone, arbutin and kojic acid by human tyrosinase and mushroom tyrosinase. They have found hydroquinone and arbutin and kojic acid (IC 50 > 500 mmol/L) weekly inhibits human tyrosinase. In contrast, a resorcinyl-thiazole derivative, thiamidol, is a most potent inhibitor of human tyrosinase (IC 50 of 1.1 mmol/L) but inhibits mushroom tyrosinase weakly (IC 50 ¼ 108 mmol/L) 130 . Also, deoxyarbutin, a novel reversible tyrosinase inhibitor with effective in vivo skin lightening potency, have been reported due to its increased skin penetration and binding affinity to human tyrosinase 131 . In another research, Sugimoto et al. have investigated a comparison of inhibitory effects of alpha-arbutin and arbutin with human tyrosinase and they have found a-arbutin is stronger than arbutin 132 .

Natural tyrosinase inhibitor sources
Natural sources including plants, bacteria and fungi have recently become of increasing interest for their antityrosinase activity by producing bioactive compounds. A number of researchers prefer to identify inhibitors from natural sources due to their less toxicity and better bioavalibility, especially for food, cosmetic and medicinal applications.

Plants
It is well known that phenolic compounds are the largest group of phytochemicals found in plants, which are mainly the factors responsible for the activities in plant extracts 52 . Tyrosinase inhibitory activity of many plant extracts was carried out to find new sources of anti-tyrosinase compounds. For example, antityrosinase activities of the following plants have been reported by various researchers: Asphodelus microcarpus 133 147 and Inula britannica L. 146 . Also, tyrosinase inhibitory activity of 91 native plants from central Argentina was carried out by Chiari et al. 138,147 . Their results approved the inhibitory activity of these extracts against tyrosinase: Achyrocline satureioides, Artemisia verlotiorum, Cotoneaster glaucophylla, Dalea elegans, Flourensia campestris, Jodina rhombifolia, Kageneckia lanceolata, Lepechinia floribunda, Lepe-chinia meyenii, Lithrea molleoides, Porlieria microphylla, Pterocaulon alopecuroides, Ruprechtia apetala, Senna aphylla, Sida rhombifolia, Solanum argentinum, Tagetes minuta, and Thalictrum decipiens.

Fungi and bacteria
Fungi from different genera such as Aspergillus sp. 164 , Trichoderma sp. 165 , Paecilomyces sp. 166 , Phellinus linteus 167 , Daedalea dickinsii 168 , Dictyophora indusiata 169 along with a liquid culture of Neolentinus lepideus 170 have been reported as a source of novel tyrosinase inhibitor by producing bioactive compounds. Also, there have been several reports on tyrosinase inhibitors from some marine fungi species such as Myrothecium sp. isolated from algae 171 and Pestalotiopsis sp. Z233 172 . Also, there are several reports on tyrosinase inhibition by bacterial species and their metabolites.
Among them, Streptomyces sp., such as S. hiroshimensis TI-C3 isolated from soil 173 , an actinobacterium named Streptomyces swartbergensis sp. Nov. 174 and Streptomyces roseolilacinus NBRC 12815 175 are potential bacterial sources of tyrosine inhibitors. Moreover, some tyrosinase inhibitors have been reported from a gram-negative marine bacterium Thalassotalea sp. Pp2-459 176 and a toxic strain of the cyanobacterium, Oscillatoria agardhii 177 . Interestingly, some probiotics such as Lactobacillus sp. 178 which are used in the fermentation process have been investigated as natural tyrosinase inhibitor sources. Based on the studies, it has been confirmed that the physiological activities of fermented extracts are considerably higher than those of unfermented extracts and their cytotoxic activity is lower as compared to unfermented extracts 179 . Recently, tyrosinase inhibitory four different lactic acid bacteria (LAB) strains isolated from dairy cow feces have been proved by Ji et al. 180 .
Finally, in an updated review by Fernandes from reported findings, tyrosinase inhibitors produced by microorganisms have been summarised 61 . This review shows that diverse tyrosinase inhibitors isolated from plant sources and fungi are mostly phenolic compounds, steroids, and alkaloids structurally comparable with each other. In contrast, tyrosinase inhibitors from bacteria comprise a smaller group of alkaloids, macrolides, and polyphenols, which competitively inhibit the enzyme 61 .

Simple phenols
Phenolic compounds which are characterised by having at least one aromatic ring and one (or more) hydroxyl group can be classified based on the number and arrangement of their carbon atoms. These compounds are commonly found to be conjugated to sugars and organic acids. Phenolics range from simple to large and complex tannins and derived polyphenols due to their molecular-weight and number of aromatic-rings 180 .
The simple phenols such as hydroquinone 181,182 and its derivatives 183,184 , deoxyarbutin 185,186 and its derivatives 187 , 4-(6-Hydroxy-2-naphthyl)-1,3-bezendiol, resorcinol (or resorcin) 188 and 4-n-butylresorcinol 189 , vanillin 190 and its derivatives 191,192   activity of tyrosinase and suppress melanin production in animal cells. The IC 50 of this compound (37 mM) is less than hydroquinone (70 mM) as a known inhibitor of tyrosinase. They have suggested that the potent inhibitory effect of this derivative on tyrosinase activity is likely due to its heptadecenyl chain, which facilitates the oxidation of the hydroquinone ring 183,184 .
Isotachioside, a methoxy-hydroquinone-1-O-beta-D-glucopyranoside isolated from Isotachis japonica and Protea neriifolia and its glycoside derivatives (glucoside, xyloside, cellobioside, and maltoside) are categorised as analogs of arbutin. However, isotachioside and arbutin could not be determined as potent inhibitor. But, glucoside, xyloside, cellobioside and maltoside derivatives, missing methyl and benzoyl groups, acted as tyrosinase inhibitors with IC 50 s of 417, 852, 623 and 657 mM, respectively. Among these novel inhibitors, glucoside derivative (IC 50 ¼ 417 mM) was the most potent, indicating that the structural combination of resorcinol and glucose was significant for inducing the inhibitory effect 193 .
Hydroquinone and some of its known derivatives, including a and b-arbutin, are described as both a tyrosinase inhibitor and a substrate 194,195 . Deoxyarbutin and its second-generation derivatives have been proposed as promising agents to ameliorate hyperpigmented lesions or lighten skin due to less toxicity at their effective inhibitory dose 185,186 .
Monophenolic compounds such as L-tyrosine, L-a-methyltyrosine and tyramine are substrates of tyrosinase. o-Quinone evolves in the medium of reaction accumulating o-diphenol and this accumulation provokes that met-tyrosinase (E m ) is transformed into oxy-tyrosinase (E ox ), which is the active form of the tyrosinase for monophenols and o-diphenols. Therefore, tyrosinase is active with monophenols such as: umbelliferone 196 , hydroquinone 197,198 p-hydroxybenzyl alcohol 199 , 4-hexylresorcinol 200 , oxyresveratrol 201 , 4-n-butylresorcinol 202 , resorcinols 203 , a and b-arbutin 195 and p-coumaric acid 204,205 when we add the following reagents to medium of reaction: hydrogen peroxide (transforms E m to E ox ), an o-diphenol or a reducing agent such as ascorbic acid transforming E m to E d which, with molecular oxygen, is transformed into E ox . A particular case is deoxyarbutin, which acts as a substrate of tyrosinase even if any reagent is not added to the medium of reaction 206 . Taking into consideration all the previous comments, several methods have been developed to discriminate between true inhibitors and alternative substrates of the enzyme 98,207 .

Polyphenols
Plants produce a large diverse class of polyphenols including phenolic acids, flavonoids, stillbenes and lignans 208,209 . A large number of these compounds have been reported as a weak or potent inhibitor of tyrosinase from natural [210][211][212][213][214][215] and synthetic 216-219 sources.

Flavonoids
Among polyphenolic compounds, some of the flavonoid derivatives mostly found in herbal plants, fruits and synthetic sources have been raveled to be the potent inhibitors of tyrosinase 133,211,[220][221][222][223][224][225] . There is a significant correlation between the inhibitory potency of flavonoids on mushroom tyrosinase and melanin synthesis in melanocytes 226 . In searching effective tyrosinase inhibitors from natural products, many flavonoid compounds have been isolated and evaluated for their inhibitory activity on mushroom tyrosinase from different natural sources such as Trifolium nigrescens Subsp. Petrisavi 227 , mung bean (Vigna radiatae L.) 228 239 and other various medicinal plants 240 .
Interestingly, it has even demonstrated that deglycosylation of some flavonoid glycosides by far-infrared irradiation can be improved tyrosinase inhibitory activity 243   Based on the thermodynamics parameters, the binding process involved hydrogen bonds and van der Waals forces. Also, docking simulation illustrated hydrogen bonds between this compound and the residues His244 and Met280 of active site 245 .
In addition, several hydroxyflavones including baicalein, 6hydroxyapigenin, 6-hydroxygalangin and 6-hydroxy-kaempferol 246 and tricin (5,7,4 0 -trihydroxy-3 0 ,5 0 -dimethoxyflavone) 247 have been demonstrated as inhibitors of diphenolase activity of tyrosinase. The mechanism of inhibition by baicalein (IC 50 ¼ 0.11 mM) indicated a mix-type (K i of 0.17 mM, a ¼ 0.56). A single binding site with a binding constant of 2.78 Â 10 5 M À 1 was obtained from the quenching fluorescence analysis for this compound. Thermodynamic parameters suggested spontaneous binding through hydrogen bonding and van der Waals forces. Furthermore, circular dichroism spectra indicated a reduction in the content of a-helix from 32.67% to 29.00% due to this binding. Docking simulations also indicated that baicalein mainly bound tyrosinase via its Met280 residue 248 . While, tricin was found as a noncompetitive inhibitor of tyrosinase with good efficacy compared to its control. Based on circular dichroism spectra, the interactions between tricin and tyrosinase did not change the secondary structure. Fluorescence quenching revealed that the interaction of tricin with residues in the hydrophobic pocket of tyrosinase is stabilised by hydrophobic interactions and hydrogen bonding. Also, docking results implied that the stereospecific effects of tricin on substrates or products and flexible conformation alterations of tyrosinase produced by weak interactions between tricin and this enzyme are the possible inhibitory mechanisms of this compound 247  Based on kinetics studies, morin reversibly inhibited tyrosinase through a multi-phase kinetic process and bind to tyrosinase at a single binding site mainly by hydrogen bonds and van der Waals forces. It inhibited tyrosinase reversibly in a competitive manner with K i ¼ 4.03 ± 0.26 mM and the binding of morin to tyrosinaseinduced rearrangement and conformational changes of the enzyme 254 . Furthermore, it was reported that three flavonols including galangin 235 , kaempferol 251 and quercetin inhibit the oxidation of L-DOPA catalysed by mushroom tyrosinase and presumably this inhibitory activity comes from their copper chelating ability. While their corresponding flavones, chrysin, apigenin and luteolin, are not identified as copper chelator, Kubo et al. believed that the chelation mechanism by flavonols may be attributed to the free 3-hydroxyl group 251 . Interestingly, quercetin behaves as a cofactor and does not inhibit monophenolase activity. In contrast, galangin inhibits monophenolase activity and does not act as a cofactor, and kaempferol neither acts as a cofactor nor inhibits monophenolase activity. However, inhibiting of diphenolase activity by chelating copper in the enzyme is the common feature of these three flavonols 160 .
Recently, 8-prenylkaempferol as a competitive tyrosinase inhibitor along with Kushenol A (noncompetitive) isolated from Sophora flavescens 256 , have been investigated with IC 50 values less than 10 mM. Finally, based on the literature review, many flavonol inhibitors are usually competitive inhibitors due to the 3-hydroxy-4keto moiety of the flavonol structure, which chelates the copper in the active site 251 . Also, among all these compounds, quercetin-4'-O-beta-D-glucoside with a IC 50 value of 1.9 mM is revealed stronger tyrosinase inhibition than their positive control, kojic acid 236 . While the other flavonol inhibitors listed above are very weak inhibitors and have little potential as skin whitening or food antibrowning.
Isoflavones. Isoflavones such as daidzein, genistein, glycitein, formononetin, and their glycosides (e.g. genistin, daidzin) mostly are detected in the medicinal herbs 209   6,7,4'-trihydroxyisoflavone, daidzein, glycitein and genistein 257 . Also, 6,7,4'-trihydroxyisoflavone was identified as a potent competitive inhibitor of monophenolase activity of tyrosinase by Chang et al., with an IC 50 value of 9.2 mM, which is six times potent than kojic acid 258 . But, its analogs, glycitein, daidzein, and genistein showed little anti-tyrosinase activity. Therefore, they have suggested that C-6 and C-7 hydroxyl groups of the isoflavone skeleton might play an important role in the tyrosinase inhibitory activity. Furthermore, two other isoflavone metabolites, 7,8,4'-trihydroxyisoflavone and 5,7,8,4'-tetrahydroxyisoflavone isolated from soygerm koji, were investigated by Chang et al. 259 . These compounds inhibited both monophenolase and diphenolase activities with an irreversible inhibition manner. Interestingly, by using HPLC analysis and kinetic studies, they have found that 7,8,4'-trihydroxyisoflavone and 5,7,8,4'-tetrahydroxyisoflavone are potent suicide substrates of mushroom tyrosinase. It may be concluded that the hydroxyl groups at both the C7 and C8 positions could completely change the inhibitory mechanism of the isoflavones from the reversible competitive to the irreversible suicide form 52 .
Recently, a noncompetitive inhibitor, glabridin (IC 50 ¼ 0.43 mM), isolated from the root of Glycyrrhiza glabra Linn, has exhibited excellent inhibitory effects on tyrosinase. The quenching analysis of tyrosinase by glabridin showed a static mechanism 260 . Notably, a drug delivery system by using glabridin microsponge-loaded gel as a new approach for hyperpigmentation disorders have been proposed by Deshmukh et al. 261 . In another research, Jirawattanapong et al. have identified a synthetic glabridin, 3'',4''-dihydroglabridin, with higher activity than glabridin (IC 50 ¼ 11.40 mM) against tyrosinase. They have suggested the more effective interaction with the enzyme may be due to more conformational flexibly of this compound that has occurred by the 4-substituted resorcinol skeleton and the lacking of double bond between carbon atom 3'' and 4'' in its structure 262 . Also, Nerya et al. have reported that another isoflavone, glabrene, in the licorice extract can inhibit both monophenolase and diphenolase tyrosinase activities 263 . In the study reported by Heo et al., two new isoflavones desmodianone H and uncinanone B have been identified as novel tyrosinase inhibitors. However, uncinanone B has higher anti-tyrosinase rate than desmodianone H 264 . Glyasperin C from Glycyrrhiza glabra is another kind of isoflavone identified as tyrosinase inhibitor 265 . Furthermore, some other isoflavones, formononetin, genistein, daidzein, texasin, tectorigenin, odoratin and mirkoin isolated from the stems of Maackia fauriei, have been investigated by Kim et al. for their tyrosinase inhibition activity. Based on their results, among these falvonoids, mirkoin (IC 50 ¼ 5 mM) revealed stronger tyrosinase inhibition than the positive control, kojic acid and inhibited tyrosinase reversibly in a competitive mode 232 . Recently, two isoflavonoids lupinalbin (IC 50 ¼ 39.7 ± 1.5 mM), and 2 0 -hydroxygenistein-7-O-gentibioside (IC 50 ¼ 50.0 ± 3.7 mM) from Apios americana were identified as competitive inhibitors, with K i values of 10.3 ± 0.8 mM and 44.2 ± 1.7 mM, respectively 266 .
Flavanones. Flavanones such as naringenin, hesperetin, eriodictyol and their glycosides (e.g. naringin, hesperidin, and liquiritin) and flavanonols (taxifolin) are mainly found in citrus fruits and the medicinal herbs 209 . A copper chelator flavanone named hesperetin inhibits tyrosinase reversibly and competitively. Based on the ANSbinding fluorescence analysis, hesperetin disrupted of tyrosinase structure by hydrophobic interactions. In addition, hesperetin chelates a copper ion coordinating with 3 histidine residues (HIS61, HIS85, and HIS259) within the active site pocket of the enzyme due to docking simulation results 267 . In another study, Chiari et al. have illustrated tyrosinase inhibitory activity of a 6-isoprenoid-substituted flavanone isolated from Dalea elegans 268 . Also, Steppogenin is a natural flavanone with a strong tyrosinase inhibitory activity (IC 50 ¼ 0.98 ± 0.01 mM), from Morus alba L 249 . Recently, a new isoprenylated sanggenon-type flavanone, nigrasin K, along with some other analogs including sanggenon M, C and O, chalcomoracin, sorocein H and kuwanon J isolated from the twigs of Morus nigra have been identified as potent tyrosinase inhibitors by Hu et al. 269 . Among these natural inhibitors, sanggenon D revealed stronger tyrosinase inhibition than the positive control, kojic acid or arbutin.
Flavanoles and flavan-3,4-diols. Flavan-3-ols are the most complex subclass of flavonoids ranging from the simple monomers (þ)catechin and its isomer (À)-epicatechin to the oligomeric and polymeric proanthocyanidins, which are also known as condensed tannins. Flavanols, such as catechin, epicatechin, epi-gallocatechin, epicatechin gallate (ECG), epigallocatechin gallate (EGCG) and proanthocyanidins are widespread in the medicinal herbs and higher plants 231,270 . Alphitonia neocaledonica (Rhamnaceae) is an endemic tree of New Caledonia, which has been identified as an anti-tyrosinase source due to the presence of tannins and gallocatechin 228 . Moreover, a catechin compound isolated from the ethanol extract of Distylium racemosum branches, with IC 50 value of 30.2 mg/mL, showed higher tyrosinase inhibition activity than arbutin as a positive control 271 . Also, a proanthocyanidins from Clausena lansium demonstrated potent mushroom tyrosinase inhibition in a mixed competitive manner and illustrated strong inhibition of the melanogenic activity of B16 cells. The IC 50 values for the monophenolase and diphenolase activities were 23.6 ± 1.2 and 7.0 ± 0.2 mg/mL, respectively. Furthermore, from the inhibition mechanism of this compound, it can be concluded that a chelation between the hydroxyl group on the B ring of the proanthocyanidins and dicopper ions of the enzyme has been occurred 39 .
Another investigation revealed that procyanidin-type proanthocyanidins, purified from cherimoya (Annona squamosa) pericarp could powerfully inhibit the activities of monophenolase and diphenolase of tyrosinase, competitively 272 . In addition, Kim et al. have demonstrated that (þ)-catechin-aldehyde polycondensates inhibit the L-tyrosine hydroxylation and L-DOPA oxidation by chelation to the active site of tyrosinase 273 . Recently, another tyrosinase inhibitor from this class, condensed tannins (mixtures of procyanidins, propelargonidins, prodelphinidins) and their acyl derivatives (galloyl and p-hydroxybenzoate) from Longan Bark indicated the reversible and mixed (competitive is dominant) inhibition of tyrosinase 274 .
Curcuminoids. Two phenolic compounds, namely curcumin and desmethoxycurcumin have been isolated from the methanolic extract of the heartwood of Artocapus altilis and showed more potent tyrosinase inhibitory activities than the positive control kojic acid 190 . Also, a curcumin included in Chouji and Yakuchi extracts inhibited the enzyme competitively 192 . In addition, some synthetic curcumin derivative compounds 217,276 and its analogs possessing m-diphenols and o-diphenols have been investigated as potent inhibitors of mushroom tyrosinase 216 . Based on the results, 4-hydroxyl groups in curcumin analogs containing 4-hydroxyl-substituted phenolic rings with C-2/C-4or C-3/C-4-dihydroxyl-substituted diphenolic rings make them more active than kojic acid 217 .
In addition to these compounds, 3-phenylbenzoic acid (3-PBA) was revealed to be the most potent inhibitor against monophenolase (noncompetitive, IC 50 ¼ 6.97 mM) and diphenolase (mixed type inhibition, IC 50 ¼ 36.3 mM) activity of mushroom tyrosinase. Also, Oyama et al. have found that some modification such as esterification can abrogate this inhibitory activity of tyrosinase 318 .

Lignans
Lignans are complex and diverse structures, which are formed from three primary precursors. So far, lignans and lignan glycosides isolated from exocarp of Castanea henryi 342 , Marrubium velutinum and Marrubium cylleneum 343 , Pinellia ternate 344 and Crataegus pinnatifida 345 have been evaluated for their tyrosinase inhibitory potentials. However, these compounds mostly displayed a moderate mushroom tyrosinase inhibitory activity.

Quinone derivatives
The quinones are a class of small molecules that are mostly derived from aromatic compounds such as benzene or naphthalene. Among these compounds, Aloin, an anthraquinone-C-glycoside from Aloe vera 349 , anthraquinones from Polygonum cuspidatum 350 and tanshinone IIA (IC 50 ¼ 1214 mM) have been verified as tyrosinase inhibitors 239 .

Kojic acid analogs
Kojic acid is a well-known tyrosinase inhibitor. When DL-DOPA, norepinephrine and dopamine are oxidised by tyrosinase, Kojic acid inhibits effectively the rate of formation of pigmented product(s) and of oxygen uptake 411 . Furthermore, several of its derivatives have demonstrated a potent tyrosinase inhibitory activity 361,412-418 . Noh et al. have modified kojic acid with amino acids and screened their tyrosinase inhibitory activity. Among them, kojic acid-phenylalanine amide showed a strong noncompetitive inhibition 417 . Interestingly, some kojic acid derivatives despite their depigmenting activities did not display tyrosinase inhibitory activitiy 419 .

Benzaldehyde derivatives
Benzaldehyde 420 and its derivatives 421 , hydroxy-or methoxysubstituted benzaldoximes and benzaldehyde-O-alkyloximes 422 , piperonal or 4-(methylenedioxy) benzaldehyde mesoionic derivatives 423 , 4-hydroxybenzaldehyde derivatives 424 , anisaldehyde 425 have been investigated for their inhibitory activities against tyrosinase ( Figure 15). Among these derivatives, 3,4-dihydroxybenzaldehyde-Oethyloxime (IC 50 ¼ 0.3 ± 0.1 mM) is of the same magnitude as one of the best tyrosinase known inhibitors tropolone (IC 50 ¼ 0.13 ± 0.08 mM) 422 . However, in benzaldehyde derivatives, the presence of the aldehyde group and the terminal methoxy group in C4 was found to play an important role in its inhibitory effect. But, due to their lower activity levels or serious side effects, unfortunately, most 4-substituted benzaldehyde derivatives cannot be considered for practical use 421 .
Based on the findings investigated by Gheibi et al., aliphatic carboxylic acids have dual effects on the monophenolase and diphenolase activities of mushroom tyrosinase. They have found that optimal diphenolase activity of tyrosinase takes place in the presence of n-alkyl acids (pyruvic acid, acrylic acid, propanoic acid, 2-oxo-butanoic acid, and 2-oxo-octanoic acid). While, the monophenolase activity is inhibited by all types of n-alkyl acids. They have believed that there is a physical difference in the docking of mono-and o-diphenols to the tyrosinase active site. On the other hand, the binding of acids occurs through their carboxylate group with one copper ion of the binuclear site. So these carboxylic acid compounds completely block the monophenolase reaction, by preventing monophenol binding to the oxyform of the enzyme 124 .

Xanthate derivative
The inhibitory effect of some synthesised xanthates including C 3 H 7 OCS 2 Na, C 4 H 9 OCS 2 Na, C 5 H 11 OCS 2 Na, C 2 H 5 OCS 2 Na, and C 6 H 13 OCS 2 Na have been examined for inhibition of both monophenolase and diphenolase activities of mushroom tyrosinase. Based on the reports, C 3 H 7 OCS 2 Na and C 4 H 9 OCS 2 Na showed a mixed inhibition pattern on monophenolase activity but C 5 H 11 OCS 2 Na and C 6 H 13 OCS 2 Na showed a competitive and C 2 H 5 OCS 2 Na showed uncompetitive inhibition pattern. For diphenolase activity, C 3 H 7 OCS 2 Na and C 2 H 5 OCS 2 Na showed mixed inhibition but C 4 H 9 OCS 2 Na and C 5 H 11 OCS 2 Na and C 6 H 13 OCS 2 Na showed competitive inhibition 427 . According to their results, it seems that the lengthening of the hydrophobic tail of the xanthates leads to a decrease of the K i values for monophenolase inhibition and an increase of the K i values for diphenolase inhibition 428 .

Conclusion
Due to the vital role of tyrosinase in the enzymatic browning of food and depigmentation disorders in humans, its inhibitors have been considered by researchers, extensively. As mentioned above, natural sources such as plants and microorganisms and their effective compounds have wonderful potential as organic antityrosinase sources.
However, the majority of the compounds identified from natural sources were isolated from plants but, recently, microorganisms are considered as potential sources of tyrosinase inhibitors. It is interesting that despite the diversity of natural inhibitors, a large number of tyrosinase inhibitors are phenolic-based structures. Many researchers have designed appropriate scaffold inspired by the structure of natural compounds and developed novel synthetic inhibitors. In this paper, many natural, semisynthetic and synthetic inhibitors have been summarised and the inhibitory effects of these compounds on the tyrosinase activity are discussed.
Based on the results, phenolic compounds (simple phenols and polyphenols) and their derivatives and several compounds including terpenoid, phenyl, pyridine, piperidine, pyridinone, hydroxypyridinone, thiosemicarbazone, thiosemicarbazide, azole, thiazolidine, kojic acid, benzaldehyde and xanthate derivatives were characterised as potent tyrosinase inhibitors. The appropriate functionalisation of these inhibitors such as C-6 and C-7 hydroxyl groups of the isoflavone skeleton, 4-functionalisation thiophene-2carbaldehyde thiosemicarbazone with a methoxyacetyl group and the aldehyde group and methoxy group in C4 of benzaldehyde derivatives may be improved the inhibitory activity of these inhibitors. Furthermore, in cholcone derivatives, the location of the hydroxyl groups on both aromatic rings and the number of hydroxyls is an important factor in the efficacy of a chalcone. In contrast, some modifications such as the prenylation or the vinylation of some flavonoid molecules do not enhance their tyrosinase inhibitory activity while deglycosylation of some flavonoid glycosides by far-infrared irradiation can be improved tyrosinase inhibitory activity. Interestingly, among different inhibitors, some compounds, especially hydroquinone and its known derivatives (a and b-arbutin), are described as both a tyrosinase inhibitor and a substrate.
Actually, the main objective of this review is to provide a useful source of effective tyrosinase inhibitors. However, despite the existence of a wide range of tyrosinase inhibitors from natural and synthetic sources, only a few of them, in addition to being effective, are known as safe compounds. Therefore, it is recommended to examine the efficacy and safety of inhibitors by in vivo models, along with in vitro and docking experiments, especially for the application of such materials in food and medicinal products. Finally, we hope that the information provided in this study, which is the result of numerous researchers' efforts, could serve as leads in the search for effective anti-tyrosinase agents from natural and synthetic sources with increased efficiency and safety in the food and cosmetics industries.

Disclosure statement
The authors report that they have no conflicts of interest.

Funding
This work was financially supported by Research Council of both University of Tehran and IAU Jahrom Branch.