Tyrosinase Inhibitors Naturally Present in Plants and Synthetic Modifications of These Natural Products as Anti-Melanogenic Agents: A Review

Tyrosinase is a key enzyme target to design new chemical ligands against melanogenesis. In the current review, different chemical derivatives are explored which have been used as anti-melanogenic compounds. These are different chemical compounds naturally present in plants and semi-synthetic and synthetic compounds inspired by these natural products, such as kojic acid produced by several species of fungi; arbutin—a glycosylated hydroquinone extracted from the bearberry plant; vanillin—a phenolic aldehyde extracted from the vanilla bean, etc. After enzyme inhibition screening, various chemical compounds showed different therapeutic effects as tyrosinase inhibitors with different values of the inhibition constant and IC50. We show how appropriately designed scaffolds inspired by the structures of natural compounds are used to develop novel synthetic inhibitors. We review the results of numerous studies, which could lead to the development of effective anti-tyrosinase agents with increased efficiency and safety in the near future, with many applications in the food, pharmaceutical and cosmetics industries.


Introduction
Melanin is a darkish macromolecular pigment present in bacteria, fungi, insects, plants, invertebrates, and vertebrates [1][2][3]. In mammals, there are sorts of melanin, namely, eumelanin and pheomelanin, that are accountable for brown-to-black and yellow-to-pink colorations, respectively [4][5][6]. The basic characteristic of melanin in animals is the representation of apparent colors in the skin, hair, feathers, and pupils [7]. Melanin is secreted with the aid of using melanocytes positioned within side the basal epidermal layer [8]. Melanosomes are organelles discovered in melanocytes that biosynthesize melanin by using of a method called melanogenesis, which entails a series of complicated enzymatic and chemical processes [9]. The capabilities of melanocytes are managed by using intrinsic factors, including α-melanocyte stimulating hormone, and extrinsic factors, including chemical compounds and UV rays. Although melanin protects pores and skin from UV radiation and from different chemical compounds, its overaccumulation can cause hyperpigmentation-related diseases, esthetic problems and even skin cancer [10].
Tyrosinase is a copper-containing enzyme successfully used as an inhibitor for the treatment of melanogenesis [11]. Mushroom tyrosinase is a soluble tetrameric enzyme discovered within the cytoplasm, while mammalian tyrosinases, including human tyrosinase, are glycosylated monomeric enzymes anchored to the melanosome membrane [12]. Comparative sequence analysis showed that mushroom tyrosinase has 22-24% sequence identity with mammalian tyrosinases. Therefore, most tyrosinase inhibitors of melanogenesis have been designed based mushroom tyrosinase due to its low price and business  83 The biological effects of hormones on human melanocytes are still being investigated 84 [30]. There are multiple hormones that have mitogenic and/or melanogenic effects on hu-85 man melanocytes [31]. For example, melanotropic hormones, such as α-melanocyte-stim- 86 ulating hormone (α-MSH) and adrenocorticotrophic hormone (ACTH), are responsible 87 for skin darkening. The mechanism of up-regulation of tyrosinase activity in human nor- 88 mal melanocytes by melanotropins through the cyclic AMP (cAMP) signaling pathway is 89 The biological effects of hormones on human melanocytes are still being investigated [30]. There are multiple hormones that have mitogenic and/or melanogenic effects Molecules 2023, 28, 378 3 of 13 on human melanocytes [31]. For example, melanotropic hormones, such as α-melanocytestimulating hormone (α-MSH) and adrenocorticotrophic hormone (ACTH), are responsible for skin darkening. The mechanism of up-regulation of tyrosinase activity in human normal melanocytes by melanotropins through the cyclic AMP (cAMP) signaling pathway is now recognized to play a key role in the regulation of skin pigmentation. α-MSH is known to elevate intracellular cAMP levels through the melanocortin 1 receptor (MC1R) and this plays a critical role in the regulation of melanogenesis [30]. Two important female hormones estrogen and progesterone are also responsible for skin pigmentation during pregnancy due to their increased levels in female body [32]. Melasma commonly known as mask of pregnancy develops on the skin due to imbalance of these hormonal changes which ultimately results in dark patches on the body particularly on both sides of the face and pelvic regions [33]. Moreover, local changes in skin pigmentation such as age spots (solar lentigines) can occur with ageing [34].
The other skin disorders including acne, atopic dermatitis or lichen planus occur by localized post-inflammatory hyperpigmentation (PIH) resulting from overproduction of melanin or abnormal melanin deposition in the epidermis or dermis regions [35]. Furthermore, the patients suffering from Addison's disease which is primary known as adrenal insufficiency also suffer from hyperpigmented skin due to elevated levels of Adrenocorticotropic hormone (ACTH), a polypeptide tropic hormone produced and secreted by the anterior pituitary gland that stimulates adrenal glands to release cortisol [36]. Vitiligo is an acquired pigmentary disorder of unknown etiology characterized by the localized loss of skin color [37]. Moreover, several genetic diseases, called genodermatoses, affect skin pigmentation. One example is albinism, which includes a group of inherited disorders that are characterized by little or no production of the pigment melanin, impacting skin, hair and eye color [38]. Melatonin is another hormone produced in the glandula pinealis that follows a circadian light-dependent rhythm of secretion. Melatonin is implicated in skin functions such as hair cycling and fur pigmentation, and melatonin receptors are expressed in many skin cell types including normal and malignant keratinocytes, melanocytes and fibroblasts [39,40].
The basic aim of this article is to review various novel tyrosinase inhibitors which have been extracted from plants or synthesized in chemical laboratory to address melanogenesis. Additionally, another significant aspect is the development of skin whitening agents which is a basic pillar of cosmetic industry. The present article reviews chemical compounds along with their structures and inhibitory potentials against tyrosinase and is intended for the medicinal chemists to screen out and to synthesize novel chemical inhibitors.

Simple Phenolic Derivatives
Phenolic compounds having at least one fragrant ring and one (or more) hydroxyl organization may be categorized primarily based on the quantity of their carbon atoms and association among them [29]. There is a large variety of phenolic compounds from small to large and complicated tannins and derived polyphenols differing by their molecular weight and quantity of fragrant-rings and mostly all of these products are promising inhibitors of melanogenesis [41]. There are multiple phenolic derivatives which have been reported in different studies in simple or conjugated forms [42-46]. Chen et al. observed that alkylhydroquinone 10'(Z)-heptadecenylhydroquinone, obtained from the sap of the lacquer tree Rhus succedanea, can inhibit the activity of tyrosinase and suppress melanin production in animal cells. The half-maximal inhibitory concentration IC 50 of this compound (37 µM) is much less than the IC 50 of hydroquinone (70 µM), which is an acknowledged inhibitor of tyrosinase ( Figure 2). They have concluded that the potent inhibitory impact of this compound on tyrosinase activity is possibly due to its heptadecenyl chain, which allows the oxidation of the hydroquinone ring [43,44]. lacquer tree Rhus succedanea, can inhibit the activity of tyrosinase and suppress melani production in animal cells. The half-maximal inhibitory concentration IC50 of this com pound (37 µM) is much less than the IC50 of hydroquinone (70 µM), which is an acknow edged inhibitor of tyrosinase ( Figure 2). They have concluded that the potent inhibitor impact of this compound on tyrosinase activity is possibly due to its heptadecenyl chain which allows the oxidation of the hydroquinone ring [43,44]. Isotachioside, a chemical compound used to treat melanogenesis found in plants Iso tachis japonica and Protea neriifolia, and its glycoside derivatives are categorized as analog of arbutin, a standard molecule used for tyrosinase inhibition screening. The isotachiosid derivatives such as glucoside, xyloside, cellobioside and maltoside are acting as tyrosinas inhibitors [47,48]. Among those novel inhibitors, glucoside derivative (IC50 = 417 µM) i the most effective, indicating that the structural mixture of resorcinol and glucose induce enormous inhibitory effect [29,49]. Hydroquinone derivatives, inclusive of α and β-ar butin, are recognized as tyrosinase inhibitors [50,51]. Earlier studies showed that deoxyar butin, along with its derivatives, can be used to ameliorate hyperpigmented lesions o lighten pores and skin due to much less toxicity and powerful inhibitory effects [52,53 Plants produce a large diverse class of polyphenols including phenolic acids, flavonoids stilbenes and lignans which can be used as weak or potent inhibitors of tyrosinase [54]. Isotachioside, a chemical compound used to treat melanogenesis found in plants Isotachis japonica and Protea neriifolia, and its glycoside derivatives are categorized as analogs of arbutin, a standard molecule used for tyrosinase inhibition screening. The isotachioside derivatives such as glucoside, xyloside, cellobioside and maltoside are acting as tyrosinase inhibitors [47,48]. Among those novel inhibitors, glucoside derivative (IC 50 = 417 µM) is the most effective, indicating that the structural mixture of resorcinol and glucose induces enormous inhibitory effect [29,49]. Hydroquinone derivatives, inclusive of α and β-arbutin, are recognized as tyrosinase inhibitors [50,51]. Earlier studies showed that deoxyarbutin, along with its derivatives, can be used to ameliorate hyperpigmented lesions or lighten pores and skin due to much less toxicity and powerful inhibitory effects [52,53]. Plants produce a large diverse class of polyphenols including phenolic acids, flavonoids, stilbenes and lignans which can be used as weak or potent inhibitors of tyrosinase [54].

Flavonoids
The flavonoid derivatives that are mainly present in natural plants are used as the best tyrosinase inhibitors [55][56][57]. There is a substantial correlation between the inhibitory efficiency of flavonoids on mushroom tyrosinase and melanin synthesis in melanocytes [58]. In searching for powerful tyrosinase inhibitors from herbal products, many flavonoid compounds have been extracted and evaluated for their inhibitory effects on mushroom tyrosinase [59]. Flavonoids are generally categorized into flavones, flavonols, isoflavones, flavanones, flavanoles and anthocyanidins, dihydroflavones, flavan-3,4-diols, coumarins, chalcones, dihydrochalcones and aurones [60]. Additionally, prenylated and vinylated flavonoids, such as flavonoid glycosides, represent different subclasses of flavonoids. Some flavonoid glycosides including myricetin 3-galactoside and quercetin 3-O-β-galactopyronaside, are used as supplements possessing antioxidant activity. Interestingly, it has been proven that deglycosylation of a few flavonoid glycosides by using far-infrared irradiation can increase their tyrosinase inhibitory activity [61].

Flavones
The most common flavones are luteolin, apigenin, baicalein, chrysin and their glycosides [62]. It has been shown that apigenin and nobiletin from the methanolic extract of the heartwood of Artocapus altilis with 11 other phenolic compounds demonstrate inhibitory activities on tyrosinase [63]. In another work, it has been observed that derivative of flavone, called 7,8,4'-trihydroxyflavone inhibits diphenolase activity of tyrosinase with an IC 50 value 10.31 ± 0.41 µM in a noncompetitive manner with the inhibitor constant K i of 9.50 ± 0.40 µM [19]. Computational analysis reported that the binding process involves hydrogen bonds and hydrophobic interactions between the ligands and the residues His244 and Met280 of active site [64]. Moreover, there are some different studies which also explore the similar binding results of different compounds at same binding position in the tyrosinase [53,65].

Flavonoles
The major derivatives of flavonoles, such as myricetin, kaempferol, quercetin, morin, isorhamnetin, galangin and their glycosides, have been identified as tyrosinase inhibitors [66,67]. Kinetics studies show that morin reversibly inhibits tyrosinase through a multi-phase kinetic process and binds to tyrosinase at a single binding site mainly by hydrogen bonds and van der Waals interactions [68,69]. Furthermore, galangin, kaempferol and quercetin inhibit the oxidation of L-DOPA catalyzed by mushroom tyrosinase and presumably this inhibitory activity originates from their copper chelating ability [69]. Interestingly, quercetin behaves as a cofactor and does not inhibit monophenolase activity, while galangin inhibits monophenolase activity and does not act as a cofactor, whereas kaempferol neither acts as a cofactor nor inhibits monophenolase activity [70].

Isoflavones
Isoflavones including daidzein and genistein, both occurring in soybeans, glycitein found in soy food products, formononetin occurring in green beans and clover sprouts, and their glycosides, are mostly present in medicinal herbs [71]. It has been shown that the natural derivatives of o-dihydroxyisoflavone are potent antioxidatives [72]. Additionally, glabridin found in the root extract of licorice (Glycyrrhiza glabra) with an IC 50 of 0.43 µM exhibits excellent inhibitory effects on tyrosinase [73].

Flavanones
Flavanone derivatives such as naringenin, hesperetin, eriodictyol and their glycosides and flavanonols (taxifolin) are mainly found in citrus fruits and the medicinal herbs [74]. A copper chelator flavanone named hesperetin, the main flavonoid in lemons and sweet oranges, inhibits tyrosinase activity reversibly and competitively [75]. The 8-anilino-1naphthalenesulfonic acid (ANS)-binding fluorescence analysis shows that hesperetin disrupts tyrosinase structure by hydrophobic interactions. In addition, hesperetin chelates a copper ion coordinated with three histidine residues (His-61, His-85, and His-259) within the active site pocket of the enzyme, as shown by docking simulation studies [76][77][78].

Anthocyanidins and Curcuminoids
The most common anthocyanidins are cyanidin, delphinidin, malvidin, peonidin, petunidin, and pelargonidin, occurring in the medicinal herbs [79]. It has been observed that there is a significant correlation between anthocyanin content and inhibitory activity for both human and mushroom tyrosinases [80]. Furthermore, a couple of phenolic ligands such as curcumin and desmethoxycurcumin significantly inhibit activity of tyrosinase in comparison to standard kojic acid [81,82].

Chalcones and Dihydrochalcones
Chalcones (butein, phloretin, sappan-chalcone, carthamin, etc.), or 1,3-diphenyl-2propen-1-ones, are some of the most important classes of flavonoids occurring in foods, vegetables, tea, and spices [54]. It has been shown that some natural and synthetic chalcones and their derivatives are identified as new potent depigmentation agents and tyrosinase inhibitors. Synthetic chalcones and their derivatives evaluated by various research studies include oxindole-based chalcones [87], 1-(2-cyclohexylmethoxy-6-hydroxy-phenyl)-3-(4hydroxymethyl-phenyl) propenone derivatives [88] and isoxazole chalcone derivatives [89]. It has been observed that the efficacy of a chalcone depends upon the location of the hydroxyl groups on both aromatic rings as well as the presence of a catechol moiety, and it does not correlate with increasing tyrosinase inhibition potency [90].

Stilbenes
The best-known stilbene that inhibits tyrosinase activity is resveratrol occurring in grapes and red wine [54]. There are multiple stilbene derivatives from natural and synthetic sources that have been investigated for their tyrosinase inhibition activity [91,92]. However, enzymatic assay studies have shown that resveratrol does not inhibit the diphenolase activity of tyrosinase, but L-tyrosine oxidation by tyrosinase was suppressed in presence of 100 µM resveratrol [93,94]. Oxyresveratrol a stilbenoid found in the heartwood of Artocarpus lakoocha is also known as not an inhibitor but an alternative tyrosinase substrate [95].

Conclusions and Future Prospects
Tyrosinase is a vital enzyme involved in the browning of food and depigmentation disorders in humans [65]. Despite the diversity of naturally occurring and semi-synthetic and synthetic tyrosinase inhibitors that have recently been studied, there is still an urgent need to find compounds that are both highly effective and safe, without any noticeable side effects that could be widely applied in medicine, food industry and cosmetology. To achieve this goal, researchers have used appropriately designed scaffolds inspired by the structures of natural compounds and are developing novel synthetic inhibitors. In this review, different tyrosinase inhibitors have been discussed that exhibit promising therapeutic potential for treatment of melanogenesis. However, despite the existence of a wide range of tyrosinase inhibitors from both natural and synthetic sources, only a few of them, in addition to being effective, are known as safe, non-toxic compounds and probably with reduced side effects [54]. Therefore, screening such products in in vitro, in vivo and computational studies are the most important tasks to select potent inhibitors for treating melanogenesis. Taken together, the information provided in this review is the result of numerous efforts of many research groups studying this problem, which could lead in near future to the development of effective anti-tyrosinase agents with increased efficiency and safety in the food, pharmaceutical and cosmetics industries.