Microorganisms as a source of tyrosinase inhibitors: a review

Abstract

Tyrosinase is the main enzyme responsible for enzymatic browning of fruits post-harvest and melanogenesis in mammals, an undesirable phenomenon. This encouraged researchers to seek potent tyrosinase inhibitors for application in the food and cosmetics industries. Despite an increased knowledge of tyrosinase inhibitors from plants and synthetic sources in the past few years, inhibitors of microbial origin are under-explored. Thus, this article surveys tyrosinase inhibitors produced by microorganisms and hence, serves as an updated database of tyrosinase inhibitors from microbial sources.

Introduction

Over the past several years, tyrosinase (EC 1.14.18.1) has been studied extensively in a wide area of research. Tyrosinase enzyme is ubiquitous in nature, found in both prokaryotes as well as eukaryotes. There are several examples of well-characterized tyrosinases from prokaryotes. The first well-described tyrosinase was reported in Streptomyces sp. (Lerch and Ettinger 1972; Katz et al. 1983); however, this enzyme has also been reported from other genera, such as Bacillus megaterium, Rhizobium sp., Symbiobacterium thermophilum, Pseudomonas maltophilia, Sinorhizobium meliloti, Marinomonas mediterranea, Thermomicrobium roseum, Bacillus thuringiensis, Pseudomonas putida F6 and Ralstonia solanacearum (Liu et al. 2004; Ruan et al. 2005; Claus and Decker 2006; Dalfard et al. 2006; Hernández-Romero et al. 2006; McMahon et al. 2007; Shuster and Fishman 2009). In eukaryotes, they serve several other functions apart from melanin production. They are important for wound healing and serve as primary immune response in plants, sponges, and many invertebrates (Van Gelder et al. 1997; Cerenius and Söderhäll 2004; Müller et al. 2004), and are also involved in sclerotization in arthropods (García‐Borrón and Solano 2002).

Recently, enzyme inhibitors have been gaining attention as indispensable tools, not only for the study of the respective enzyme structure but also for their potential in pharmaceuticals and agriculture (Imada 2004). Tyrosinase plays a key role in melanogenesis in mammals and enzymatic browning in fruits and fungi, through a series of reactions leading to the formation of a dark pigment, melanin (Chang 2009). Although melanin plays an important role in the phytoprotection of human skin from UV rays, depigmentation is an esthetic problem in a wide range of human populations (Solano et al. 2006; Brenner and Hearing 2008). In addition, browning of fruits and mushrooms post-harvest is undesirable, as it reduces the commercial value of the product. The development of tyrosinase inhibitors has also become a better alternative in controlling insect pests, as the enzyme also plays an important role in developmental and defensive functions in insects (Sugumaran 2002). Due to these varied applications, tyrosinase inhibitors have been gaining importance as the best alternative for these approaches.

Tyrosinase inhibitors have been discovered and reviewed from various natural and synthetic sources (Kim and Uyama 2005; Khan 2007; Parvez et al. 2007; Schurink et al. 2007; Likhitwitayawuid 2008; Lin et al. 2008; Chang 2009, 2012a; Loizzo et al. 2012; Chan et al. 2014; Chen et al. 2015; Kilimnik and Dembitsky 2016). However, limited literature has been reviewed about tyrosinase inhibitors produced by microorganisms. Microorganisms produce several bioactive compounds and have potential as important new sources of tyrosinase inhibitors. Hence, this article reviews several tyrosinase inhibitors produced by microorganisms in the literature for use in the depigmentation of hyperpigmented skin and other applications.

Biochemical characteristics of tyrosinase

In this section, we give a brief overview of tyrosinase from bacteria, plants and fungi, with more emphasis on mushroom tyrosinase. Because of difficulties in producing tyrosinase from humans in large quantities, its three-dimensional structure is still unknown. Tyrosinase is a polyphenol oxidase enzyme which uses molecular oxygen to catalyze sequential reactions, such as (i) hydroxylation of monophenols to o-diphenols, followed by (ii) oxidation of o-diphenols to o-quinones. The quinones self-polymerize or react with other substances to form melanin. They belong to a large group of proteins, namely type 3 copper proteins, responsible mainly for the first step in melanin synthesis. Both copper atoms are coordinated by conserved three histidine residues. In melanin synthesis, three types of tyrosinase, namely oxy, met, and deoxy, with different binuclear copper structures are involved. The resting form of tyrosinase consists of a mixture of met and oxy forms, with 85% of the met form (Sánchez-Ferrer et al. 1995; Kim and Uyama 2005; Claus and Decker 2006).

The first crystal structure of tyrosinase was determined from Streptomyces castaneoglobisporus (Matoba et al. 2006). The low sequence homology between tyrosinase of different sources can be related to the differences in their structure and function. In fungal tyrosinases, one histidine residue is linked by thioether bond to the side chain of a cysteine residue. This feature is not found in bacterial tyrosinase. Haudecoeur et al. (2014) reported that there was some similarity and difference between the binding sites of tyrosinase from different origins using the same set of molecules. Selinheimo et al. (2007) also compared the characteristics of fungal and plant tyrosinases and suggested that the enzymes showed different features in terms of substrate specificity, stereo-specificity, inhibition, and ability to crosslink the model protein; however, they had similar reaction mechanisms to produce identical quinone radicals. In a recent study, it was found that, although monophenols and diphenols bind and orient identically at the active site, only monophenols rotate during the reaction, thus enabling enzymes with only diphenolase activity to have two constraints to prevent monophenolase activity. They also proposed a conserved water molecule at the active site that mediates deprotonation of monophenol at the active site (Goldfeder et al. 2014). Kanteev et al. (2015) also suggested that the active site flexibility and substrate deprotonation is crucial for the monophenolase activity of type 3 copper proteins. Asn and Glu residues are highly conserved in type 3 copper proteins and are assumed to play a role in the activation of the conserved water molecule. We have listed in Table 1 tyrosinases from different microbial sources.

Table 1 Tyrosinase of different origins

Melanogenesis in mammals

Melanin is an important pigment in mammals, synthesized and distributed in the skin and hair bulbs, that absorb free radicals generated within the cytoplasm and also protect the host from various types of ionizing radiation (Seiberg et al. 2000; Schaffer and Bolognia 2001). In mammals, a mixture of two types of melanin, eumelanin (brown or black pigment) and pheomelanin (red or yellow pigment), are found. The formation of melanin occurs through a series of oxidative reaction, where tyrosine is converted to dihydroxyphenylalanine (DOPA) and, further, to dopaquinone by tyrosinase. Dopaquinone is further auto-oxidized to dihydroxyindole or to dihydroxyindole-2-carboxylic acid (DHICA) by dopachrome tautomerase and DHICA oxidase to form eumelanin. Subsequently, pheomelanin is formed (Raper 1928; Kobayashi et al. 1995; Borges et al. 2001).

Melanogenesis is regulated by three different signaling pathways: protein kinase C-mediated pathway, cAMP-mediated pathway, and mitogen-activated protein kinase (MAPK) pathway. Although there are three enzymes active in the process of melanogenesis, tyrosinase plays the key role in the formation of melanin, whereas the rest adjust the type of pigment formed (Kobayashi et al. 1995). Microphthalmia-associated transcription factor (MITF) is phosphorylated by MAPK, which is essential for its activation as well as degradation. cAMP serves as a starting point of several interacting signaling cascades in melanin synthesis as well as regulating melanin production and PI3K. Stimulation with cAMP inhibits PI3K signaling, thereby increasing the synthesis of melanin via increased transcription of tyrosinase and TRP-1 (tyrosinase-related protein 1). Therefore, the activation of PI3K or protein kinase B (AKT) signaling reduces melanogenesis via the downregulation of MITF expression, as AKT is an effector of PI3K (Bertolotto et al. 1998; Hemesath et al. 1998; Meinkoth et al. 1991; Xu et al. 2000; Hennessy et al. 2005).

Due to the increased treatments for skin fairness, there has been a demand for the prevention of skin pigmentation in the cosmetics industry. This has lead to an increased interest on potent tyrosinase inhibitors, to prevent melanogenesis. Although several tyrosinase inhibitors have been reported from natural and synthetic sources, only a few of them are used as skin-whitening agents. Solano et al. (2006) suggests that, although tyrosinase inhibition is the most common approach, a new innovative combined approach improved the transdermal delivery system and enabled efficient screening tests for validating their efficacy and safety. Currently, arbutin, gentisic acid, hydroquinone, and aloesin isolated from plants as well as 4-n-butylresorcinol, deoxyarbutin, kojic acid, ascorbic acid, and azelaic acid are used in the cosmetics industry, with strong inhibition against tyrosinase (Solano et al. 2006; Parvez et al. 2007; Lin et al. 2008; Gillbro and Olsson 2011).

Enzymatic browning of plant-derived foods

The browning of fruit and vegetables is of great concern in the food industry, as it reduces its economic value. Browning occurs due to various reasons, such as microbial spoilage, mechanical damage and enzymatic reactions. Due to their thin and epidermal layer, the respiration rate of vegetables and fruits is high; hence, they tend to lose their quality post-harvest. Enzymatic browning is a major concern in damaged fruits during post-harvest handling and processing, where tyrosinase enzyme plays a key role (Mayer 1987). Tyrosinase causes oxidation of the phenolic compounds in fruits, causing undesirable changes in color, flavor and texture, thereby reducing its marketability. The extent of browning depends on various factors, such as concentration of the enzyme and substrate, oxygen availability, pH and temperature (Zheng et al. 2008). Tyrosinase catalyzes the hydroxylation of phenolic substrate tyrosine to DOPA via its monophenolase activity, which is further oxidized to dopaquinone by its diphenolase activity. Further, these quinones are powerful electrophiles, which can be attacked by water, other polyphenols, amino acids, peptides and proteins, leading to Michael-type additions. This is further converted to melanin through a series of reactions (Busch 1999).

The appearance of a product has been an essential attribute in the food industry and, therefore, several methods have been incorporated to reduce or stop enzymatic browning, such as blanching, microwave, autoclaving, application of chemicals, modified atmospheric packing, controlled atmospheric control, etc. (Singh et al. 2010; Ioannou and Ghoul 2013). However, these processes alter the quality, texture, and nutrient content of the product. Several enzyme inhibitors, namely citric acid, ascorbic acid and kojic acid, have been used for the prevention of browning (Loizzo et al. 2012; Ioannou and Ghoul 2013). However, since safety is the main concern in the food industry, the search for a considerably safe tyrosinase inhibitor from a natural source is an eminent topic of research.

Tyrosinase inhibitors

Tyrosinase inhibitors are widely used in cosmetology and agriculture. There are several tyrosinase inhibitors derived from natural and synthetic sources (Parvez et al. 2007; Lin et al. 2008). Some authors use “melanogenesis inhibitors” as the terminology for tyrosinase inhibitors; however, this is attributed to the inhibition of melanin synthesis, regardless of its mode of action. Thus, tyrosinase inhibition could be due to one of the following reasons, which could mislead the definition of an enzyme inhibitor:

• 1. Reducing agents causing chemical reduction of dopaquinone, e.g., ascorbic acid

• 2. o-Dopaquinone scavengers which react with dopaquinone to form a colorless product, e.g., thio-containing compounds

• 3. Alternative substrate with good affinity for the enzyme forming a different product, e.g., phenolic compounds

• 4. Non-specific enzyme inactivators such as acids and bases which inactivate the enzyme

• 5. Specific enzyme inactivators or suicide substrates

• 6. True inhibitors which bind to the enzyme and inhibit its activity

The true inhibitors can be subdivided further into three categories based on their mode of inhibition, such as competitive inhibitors, mixed type inhibitors, and non-competitive inhibitors (Chang 2009, 2012b). The inhibitors mainly comprise copper-binding agents and compounds binding on active sites (Mayer and Harel 1979; Robb 1984). Substrate analogues include numerous aromatic acids, phenols and their derivatives, and a few non-aromatic compounds, which mainly behave as competitive inhibitors (Walker and McCallion 1980; Menon et al. 1990; Nicolas et al. 1994). As the enzyme is a metalloenzyme, metal chelaters such as carbon monoxide, cyanide, azide ions, thiourea derivatives, kojic acid, tropolone, etc. could inhibit its activity. Inhibitors from natural sources have been preferred over synthetic sources, with microbial sources being an important area for exploration of some novel and safe inhibitors for application in various sectors.

Tyrosinase inhibitors from fungi

Fungi produce diverse bioactive compounds, including antibiotics, enzymes, enzyme inhibitors, growth promoters, etc., exploited in the agriculture, food, and pharmaceutical industries. Fungi from different genera have been found to demonstrate anti-tyrosinase activity. One of the genera, Aspergillus, was found to produce several compounds having tyrosinase inhibitory activity (Fig. 1). Kojic acid (5-hydroxy-2-(hydroxymethyl)-gamma-pyrone), a well-studied tyrosinase inhibitor, was reported from A. albus (Saruno et al. 1979), A. candidus (Wei et al. 1991), A. niger (Vasantha et al. 2014), and Penicillium sp., a good chelator and also a scavenger of free radicals. Saruno et al. (1979) reported kojic acid with 80% inhibition by A. albus, whereas Vasantha et al. (2014) reported A. niger S16 producing kojic acid that showed 84% competitive inhibition of mushroom tyrosinase with an IC₅₀ value of 61.9 μM. Based on several studies, kojic acid at a minimum level of exposure or consumption was found to have negligible toxicity to humans (Burdock et al. 2001; Nohynek et al. 2004). Apart from kojic acid, the Aspergillus genus produces diverse compounds with anti-tyrosinase activity. Aspergillus niger produces metallothioneins, which are strong tyrosinase inhibitors having strong avidity to chelate copper at its active site (Goetghebeur and Kermasha 1996). An inhibitor of melanin formation, decumbenone A, was isolated from P. decumbens and A. sulphureus; in addition, the Aspergillus genus also produced a new potent decaline derivative, decumbenone C, showing cytotoxic activity against human melanoma cells with an IC₅₀ value of 0.9 μM (Fujii et al. 2002; Zhurayleva et al. 2012). Terrein was isolated for the first time from A. terreus, which inhibited melanin synthesis by the downregulation of MITF via the induction of ERK activity and inhibition of MITF promoter activity (Raistrick and Smith 1935; Kim et al. 2007, 2008). A melanogenesis inhibitor isolated from Penicillium sp. 20135 was also identified as terrein, which inhibited melanin formation in B16 melanoma cells; however, neither inhibited mushroom tyrosinase nor demonstrated cytotoxic activity in a cell-based assay (Park et al. 2004; Kim et al. 2005). In addition, Chang et al. (2007) reported seven isoflavones from soygerm koji fermented with A. oryzae BCRC 32288 having anti-tyrosinase activity. Five compounds, 6,7,4′-trihyroxyisoflavone (IC₅₀ = 9 μM), daidzein (IC₅₀ = 203 μM), glycitein (IC₅₀ = 218 μM), daidzin (IC₅₀ = 267 μM), and genistin (IC₅₀ = 343 μM), showed inhibitory activity against the monophenolase activity of tyrosinase by competitive inhibition. The other two compounds, 7,8,4′-trihyroxyisoflavone and 5,7,8,4′-tetrahydroxyisoflavone, irreversibly inhibited both monophenolase with IC₅₀ values of 191 μM and 184 μM, respectively, as well as diphenolase activity with IC₅₀ values of 181 μM and 212 μM, respectively, of tyrosinase. Additionally, dietary daidzein, a phytoestrogen component of soy, did not show toxicity to the female reproductive tract in rats (Lamartiniere et al. 2002). Tyrosinase inhibition activity (56.18%) was also found in rice bran fermented with A. oryzae (Razak et al. 2015).

Fig. 1

Structures of tyrosinase inhibitors from Aspergillus sp. (n.d not defined)

Another genus found to produce diverse compounds having anti-tyrosinase activity is Trichoderma (Fig. 2). Lee et al. (1995) reported a particular strain of T. harzianum MR304 to produce a melanin synthesis inhibitor, MR304-1, identified as an isocyanide compound, which inhibited melanogenesis inhibition in S. bikinienesis, B16 melanoma cells [minimum inhibitory concentration (MIC) = 0.05 μg/mL], and mushroom tyrosinase (IC₅₀ = 0.25 μg/mL). Trichoderma harzianum isolated from soil was also reported to produce several melanin synthesis inhibitors. Two new tyrosinase inhibitors, MR566A (IC₅₀ = 1.72 μM) and MR566B (IC₅₀ = 47 μM), along with a new oxazole compound MR93B (IC₅₀ > 6000 μM), six known isocyanide compounds, and MR93A (IC₅₀ > 6000 μM), were isolated showing inhibition against mushroom tyrosinase, melanogenesis inhibition in S. bikinienesis, and B16 melanoma cells. The isocyanide compounds were identified as 1-(1,4,5-trihydroxy-3-isocyanocyclopenten-2-enyl)ethanol, 2-hydroxy-4-isocyano-α-methyl-6-oxabiocyclo[3.1.0]hex-3-ene-3methanol, 4-hydroxy-8-isocyano-1-oxaspiro[4.4]cyclonon-8-en-2-one, MR304A, methyl-3-(1,5-dihydroxy-3-isocyanocyclopent-3-enyl)prop-2-enoate, and an unidentified compound with IC₅₀ values of 3.6, 4.9, 0.089, 47, 1.72, and 0.0014 μM, respectively (Lee et al. 1997a, b). Lee et al. (1997a, b) proposed that the isocyano group in the compounds plays a vital role in inhibiting the activity of mushroom tyrosinase enzyme. Imada et al. (2001) reported mushroom tyrosinase inhibitor produced by Trichoderma sp. H1-7 isolated from a marine environment as having 1000–2500 U/mL inhibitory activity. A competitive inhibitor of tyrosinase (5.4 × 10⁵ U/mL) similar to the structure of homothallin II was isolated from T. viridae strain H1-7 from marine sediments which inhibited the enzyme by binding to the copper active site. In addition, this strain produced seven different melanogenesis inhibitors, with not all of them showing inhibition of tyrosinase (Tsuchiya et al. 2008).

Fig. 2

Structures of tyrosinase inhibitors from Trichoderma sp.

Marine fungi live in a unique environment with stressful conditions of pH, temperature, salinity, oxygen nutrients, and light, and, therefore, serve as promising candidates for novel bioactive compounds. On investigation, few known and novel compounds with tyrosinase inhibition activity have been reported from marine-derived fungi (Fig. 3). Two derivatives of kojic acid, kojic acid dimethyl ether and kojic acid monomethyl ether, as well as phomaligol A, were identified from broth of marine-derived fungi Alternaria sp. isolated from marine green algae having tyrosinase inhibitory activity (Li et al. 2003). Similarly, two compounds, 6-n-pentyl-α-pyrone and myrothenone A, identified from marine-derived fungi Myrothecium sp. MFA 58 isolated from algae were stronger than kojic acid (IC₅₀ = 7.7 μM), with IC₅₀ values of 0.8 and 6.6 μM, respectively (Li et al. 2005). Zhang et al. 2007 reported a pyrone derivative, 6-[(E)-hept-1-enyl]-α-pyrone, exhibiting anti-tyrosinase activity (IC₅₀ = 4.5 μM) isolated from Botrytis sp. Two sesquiterpene compounds were isolated from a marine-derived fungi Pestalotiopsis sp. Z233, isolated from algae, 1β,5α,6α,14-tetraacetoxy-9α-benzoyloxy-7βH-eudesman-2β,11-diol and 4α,5α-diacetoxy-9α-benzoyloxy-7βH-eudesman-1β,2β,11-tetraol, having tyrosinase inhibitory activity. These compounds were induced by abiotic stress elicitation by CuCl₂ with IC₅₀ values of 14.8 μM and 22.3 μM, respectively (Wu et al. 2013).

Fig. 3

Structures of tyrosinase inhibitors from marine-derived fungi (n.d not defined)

Apart from marine fungi, several other fungal groups are reported for anti-tyrosinase activity (Fig. 4). Azelaic acid (1,7-heptanedicarboxylic acid) produced by yeast, Pityrosporum ovale, has a cytotoxic effect on the melanocytes of primary cutaneous melanoma. It is a straight chain, saturated dicarboxylic acid which inhibits tyrosinase by competing for the α-carboxylate binding site of the L-tyrosine substrate of the enzyme (Schallreuter and Wood 1990). Nevertheless, azelaic acid is a known compound that has been previously reported as non-toxic (Töpert et al. 1989). In addition, yeasts also produce cytosolic proteins, metallothioneins characterized by the selective binding of a large amount of heavy metal ions and high cysteine content. Neurospora crassa is also reported to produce a copper metallothionein, which serves as a metal donor for apotyrosinase (Lerch 1981). Tanaka et al. (1996) reported an anti-melanoma compound from Talaromyces sp. FO-3182, which reduced the melanin content of B16 melanoma cells. Melanocin A was isolated from the fermentation broth and mycelia extract of Eupenicillium shearii F80695, showing inhibition against mushroom tyrosinase (IC₅₀ = 0.009 μM) and B16 melanoma cells (MIC = 0.9 μM) due to the presence of isocyanide group in the compound (Kim et al. 2003). Two steroids were isolated from the fungus Cunninghamella elegans, 17α-ethynyl-11α,17β-dihyroxyandrost-4-en-3-one (IC₅₀ = 5950 μM) and 17α-ethyl-11α,17β-dihyroxyandrost-4-en-3-one (IC₅₀ = 1720 μM), having tyrosinase inhibition activity (Choudhary et al. 2005). Entomopathogenic fungi are a source of several potential bioactive compounds. Three new polyphenolic tyrosinase inhibitors were isolated from an entomopathogenic fungi Paecilomyces gunnii, paecilomycones A, B, and C, having IC₅₀ values of 110, 170, and 140 μM, respectively, which compete for the active binding site of the enzyme and, in addition, the number of hydroxyl groups present in these compounds also plays a vital role in its inhibitory activity (Lu et al. 2014).

Fig. 4

Structures of tyrosinase inhibitors from other fungi (n.d not defined)

There have been several studies of secondary metabolites from Basidiomycetes with different biological activities, with few studies on tyrosinase inhibition and depigmentation of skin. We have reviewed compounds serving as tyrosinase or melanogenesis inhibitors isolated from mycelia or fruiting bodies of mushrooms (Fig. 4). Two tyrosinase inhibitors have been isolated, purified, and characterized from the mushroom Agaricus hortensis with competitive and non-competitive inhibition, respectively (Madhosingh and Sundberg 1974). Similarly, two isomeric compounds having tyrosinase inhibitory activity were isolated from the lipophilic fractions Albatrellus confluens and identified as neogrifolin (IC₅₀ = 25 μM) and grifolin (IC₅₀ = 760 μM), the activities of which are affected by the position of the farnesyl group on the aromatic ring (Misasa et al. 1992). Neogrifolin was also isolated from mushroom Polyporus confluens, which showed 100% tyrosinase inhibition at 50 ppm (Minosasa et al. 1991). Melanogenesis inhibitor, 2-amino-3H-phenoxazin-3-one was identified from the mushroom A. bisporus (Lu et al. 2002). Sharma et al. (2004) reported the methanolic extract of an edible mushroom Dictyophora indusiata non-competitively inhibiting mushroom tyrosinase activity and was identified as 5-hydroxymethyl-2-furfural (HMF). However, the carcinogenic potential of HMF in food was found to be contradictory due to limited data from toxicity studies and, therefore, there is a need for improvement in the risk assessment for HMF (Abraham et al. 2011; Capuano and Fogliano 2011). Two tyrosinase inhibitors, 5-hydroxymethyl-2-furaldehyde (IC₅₀ = 720 μM) and protocatechualdehyde (IC₅₀ = 2.896 μM), were isolated from the fruiting body of a medicinal mushroom Phellinus linteus. Protocatechualdehyde competitively binds to the copper active site with its hydroxyl group and possibly chelating the copper in tyrosinase, whereas 5-hydroxymethyl-2-furaldehyde is a non-competitive inhibitor which may form a Schiff base with primary amino groups in the enzyme, rather than binding to the active site (Kang et al. 2004). A chromene type compound, daedalin A (IC₅₀ = 194 μM), was reported from the mycelia culture broth of the mushroom Daedalea dickinsii, which competitively inhibited tyrosinase, for its substrate L-tyrosine. Further studies on the application of this compound in an in vitro human skin model substantiated its activity on suppressing melanogenesis without affecting cell viability by directly inhibiting tyrosinase activity in melanocytes (Morimura et al. 2007, 2009).

Tyrosinase inhibitors from bacteria

Bacterial metabolites represent a diverse array of chemical compounds with different biological activities. Several reports of tyrosinase inhibition by bacteria have been discussed in this article (Fig. 5). Among them, Streptomyces sp. serves as a potential source of several bioactive compounds, including enzyme inhibitors (Umezawa 1972). There have been several reports on tyrosinase inhibition from the genus Streptomyces. Melanostatin isolated from the fermentation broth of S. claviver N924-2 inhibited melanin formation in B16 melanoma cells (IC₅₀ > 703.34 μM) (Ishihara et al. 1991). Three compounds, OH-3984 K1, OH-3984 K3, and albocycline K3, a macrocyclic compound isolated from Streptomyces sp. OH-3984, inhibited melanogenesis of B16 melanoma cells at concentrations of 7.5, 3.8, and 15 μg/mL respectively; however the mechanism of action is unknown (Takamatsu et al. 1993, 1996). Arai et al. (1997) reported melanogenesis inhibitor produced by Streptomyces sp. KP-3052, which was identified as amphistin with IC₅₀ = 6.8 μM against the growth of B16 melanoma cells. Amphistin is a pseudotripeptide with activity similar to melanostatin and feldamycin, which inhibits tyrosinase through post-translational modification of the enzyme or other modulatory proteins. Imada et al. (2001) screened and reported two bacterial isolates, one being actinobacteria producing tyrosinase inhibitor, having 19 and 6 U/mL inhibitory activity, respectively. Chang and Tseng (2006) isolated and screened actinobacteria from forest soil for anti-tyrosinase activity; one bacterial strain, Streptomyces sp. TI-B10, showed the highest tyrosinase activity (46 U/mL), which was further improved to 73 U/mL when cultured in YMG medium at pH 8.0 and 30 °C. Chang et al. (2008) reported S. hiroshimensis TI-C3 isolated from soil, showing anti-tyrosinase activity (498 U/mL) with enhanced activity (905 U/mL) using glucose and malt extract as the sole carbon and nitrogen sources, respectively. Streptomyces roseolilacinus NBRC 12815 produced two compounds, 12815 A (IC₅₀ = 9 μM) and B (IC₅₀ = 1086 μM), showing anti-tyrosinase activity against mushroom and mammalian tyrosinases. However, 12815 A was further identified as streptochlorin, which was found to be a competitive inhibitor of tyrosinase with anti-nematode activity and cytotoxicity (Nakashima et al. 2009). This study also suggested that compound 12815 A produced by S. roseolilacinus and its companions could be a common feature in related species.

Fig. 5

Structures of tyrosinase inhibitors from bacterial source (n.d not defined)

Several studies on melanogenesis inhibitors have been reported from Gram-negative bacteria. Takahashi et al. (2007) reported an Enterobacter sp. B20 isolated from soil produced a novel potent melanogenesis inhibitor, byelyankacin, which inhibited tyrosinase (IC₅₀ = 0.0021 μM) by binding its isocyanide group to the copper active site of the enzyme, and also inhibited melanogenesis of B16-2D2 melanoma cells (IC₅₀ = 0.03 μM). Burkholderia cepacia TKU025, a Gram-negative bacteria isolated from soil, also produced tyrosinase inhibitor (2890 U/mL) in nutrient broth, which was maximized after cultivation in 1% squid pen as a sole C/N source to 5000 U/mL. The inhibitor was stable at varying pH conditions (pH 2–12) and thermostable at 100 °C for 60 min. The partially purified methanol extract of the metabolite exhibited an IC₅₀ value of 2 μg/mL (Hsu et al. 2014; Liang et al. 2015). In addition, tyrosinase inhibitors are reported from a marine Gram-negative bacterium, Thalassotalea sp. PP2-459 isolated from a marine bivalve and identified as thalassotalic acid A, B, and C, with IC₅₀ values of 130, 470, and 280 μM, respectively. Thalassotalic acids are N-acyl dehydrotyrosine derivatives produced by this bacterium, thalassotalic acid A being comparable to the inhibitory activity of arbutin and could be used as a whitening agent or in preventing browning of foods. They suggest that the presence of a carboxylic acid and a straight aliphatic chain increased enzyme inhibition within this structural class of inhibitors (Deering et al. 2016).

Probiotics such as Lactobacillus sp. and Bifidobacterium sp. have been used in several fermented food products. In addition, the fermented by-products of such probiotic bacteria have been recently explored for bioactive compounds with applications in cosmetics. Several investigators have reported fermented substrates that inhibit tyrosinase activity and melanogenesis. Lactobacilli and bifidobacteria are the two major bacteria involved in fermentation, resulting in producing metabolites suppressing melanogenesis. Lactobacillus helveticus produced a novel tyrosinase inhibitor, identified as a cyclic tetra peptide, cyclo(-L-Pro-L-Tyr-L-Pro-L-Val-), by Kawagishi et al. (1993). Lactobacillus plantarum M23 isolated from raw milk showed better tyrosinase inhibitory activity as compared to commercial lactic acid bacteria, showing 52.1% tyrosinase inhibition and 32% inhibition of melanoma B16 cells. Tyrosinase inhibition activity was enhanced to 84.05% in fermented milk by the addition of yeast extract and grape, incubated at 37.1 °C for 14.8 h (Heo et al. 2007; Lim and Kim 2012). In addition, Kuwaki et al. (2012) reported a plant-based paste fermented by a lactic acid bacteria and yeast, and extracted with PBS, which demonstrated anti-tyrosinase activity with an IC₅₀ value 58.5 mg/mL. Bifidobacterium adolescentis culture filtrate was found to decrease melanogenesis of melanoma cell by inhibiting tyrosinase activity mediated by its antioxidant property (Huang and Chang 2012). Tsai et al. (2013) reported L. rhamnosus spent culture supernatant showing 71.3% tyrosinase inhibitory activity, where the supernatant showed no difference in activity on heating at 100 °C for 30 min. Chen et al. (2013) reported extracts from L. plantarum TWK10 fermented soy milk to inhibit tyrosinase activity (38.33%) and melanin production in B16F0 melanocytes (27.56%) compared to non-fermented soy milk, structurally elucidated as an aglycone isoflavone similar to daidzein, equol, or genistein. These isoflavones have been known to be non-toxic to the reproductive tract of female rats (Fritz et al. 1998; Lamartiniere et al. 2002). Chen et al. (2013) further report the inhibition of melanogenesis by suppressing tyrosinase activity and expression through a positive regulator, microphthalmia-associated transcription factor (MITF) and p38 MAPK inactivation. Daidzein and equol reduced the melanin content by suppressing gene encoding melanocortin receptor-1, interfering with phosphorylation of p38 MAPK, phosphorylation of extracellular signal regulated kinase and glycogen synthase kinase, and decreasing the expression of tyrosinase, TRP-1, and TRP-2 (Chang and Tsai 2016). Kim et al. (2015) further report a cell wall component of L. plantarum, lipoteichoic acid, to inhibit melanogenesis in B16F10 mouse melanoma cells by reducing the activity and expression of tyrosinase and, also, likely by degrading MITF via the regulation of signaling and RNA stability of proteins involved in melanogenesis. Interestingly, the metabolite had no effect on mushroom tyrosinase. Lactobacillus plantarum TWK10, an organism responsible for fermenting soy milk, contained a metabolite exhibiting anti-melanogenesis in B16F0 mouse melanoma cells, where the melanogenic inhibitor was identified as uracil. Its activity was found to be due to the downregulation of a transcription gene encoding melanocortin 1 receptor, decreasing phosphorylation of cAMP response element-binding protein, and repressing the expression of MITF (Chang et al. 2015). Exopolysaccharides (EPS) isolated from L. sakei Probio 65 have also been reported, with tyrosinase inhibiting activity in the range 13.17–62.85% (Bajpai et al. 2016). Wang et al. (2016) reported tyrosinase inhibition activity in walnuts, Moutan Cortex Radicis, and asparagus root extract fermented by B. bifidum with IC₅₀ values of 420, 380, and 260 μg/mL, respectively. The study also reports the fermented extract to have low cytotoxic activity as compared to unfermented extracts.

Conclusions

Tyrosinase plays a vital role in the enzymatic browning of food and depigmentation disorders in humans. Thus, targeting tyrosinase inhibitors could be the best solution in preventing such problems. Natural product research still has an enormous unexplored potential with microorganisms representing promising sources producing anti-tyrosinase metabolites in high yields with feasible extraction methods at a reasonable cost. Thousands of bacterial metabolites have been reported with wide application in varied sectors. However, the chemical diversity in the metabolites produced by microorganisms remains an unparalleled resource for the discovery of new compounds for application in the agriculture, cosmetics, and pharmaceutical industries. This review, therefore, compiles an updated database of tyrosinase or melanogenesis inhibitors reported from microbial sources. Tyrosinase inhibitors isolated from natural sources comprise a small group, with the majority of the compounds identified from plant sources and marginally from microbial sources. Although tyrosinase inhibitors isolated from plant sources are diverse, belonging to the family of polyphenol, benzaldehyde derivatives, anthraquinones, lipids, and steroids, inhibitors isolated from fungi are structurally comparable to those from plant sources. Tyrosinase inhibitors from fungi are derivatives of isoflavones and pyrones, along with terpenes, steroids, and alkaloids, which may reversibly or irreversibly inactivate the enzyme. In contrast, tyrosinase inhibitors from bacteria comprise a smaller group, belonging to alkaloids, macrolides, and polyphenols, which competitively inhibit the enzyme. However, profound work on the mechanism of these compounds needs to be established. To conclude, the information provided could serve as leads in the search for new inhibitors from microorganisms with increased efficiency and safety in the food and cosmetics industries.