The Mechanism of Suicide-Inactivation of Tyrosinase: A Substrate Structure Investigation

Tyrosinase Inhibition and Antimelanogenic Effects of Resorcinol-Containing Compounds

Tyrosinases (TYRs) are copper-containing metalloenzymes present in a large diversity of species. In human, hTYR is responsible for pivotal steps in melanogenesis, catalysing the oxidation of l-tyrosine to l-DOPA and further to dopaquinone. While numerous TYR inhibitors have been reported, polyphenolic compounds tend to dominate the literature. However, many of these compounds, particularly monophenols and catechols, have been identified as alternative substrates rather than true inhibitors, given their structural similarity to natural substrates. Resorcinol-containing compounds have emerged as promising candidates to address this challenge, as the meta-dihydroxy moiety in resorcinol demonstrates resistance to TYR-mediated oxidation, while retaining the favourable interactions with copper ions provided by the hydroxy groups. Although their precise mechanism of action remains debated, resorcinol derivatives have yielded some of the most active compounds against isolated mushroom and human TYRs, as well as clinically used dermocosmetic agents like rucinol and thiamidol, which exhibited very promising effects in patients with facial melasma. This review outlines the development of resorcinol-containing TYR inhibitors, categorized by scaffold type, ranging from simple alkyl analogues to intricate synthetic derivatives. Mechanistic insights about the resorcinol-TYR interaction are also presented and debated.

Tyrosinases: a family of copper-containing metalloenzymes

Tyrosinases (TYRs) are a family of copper-containing metalloenzymes that are present in all domains of life. TYRs catalyze the reactions that start the biosynthesis of melanin, the main pigment of the animal kingdom, and are also involved in the formation of the bright colors seen on the caps of mushrooms and in the petals of flowers. TYRs catalyze the ortho-hydroxylation and oxidation of phenols and the oxidation of catechols to the respective o-quinones. They only need molecular oxygen to do that, and the products of TYRs-o-quinones-are highly reactive and will usually react with the next available nucleophile. This reactivity can be harnessed for pharmaceutical applications as well as in environmental and food biotechnology. The majority of both basic and applied research on TYRs utilizes "mushroom tyrosinase", a crude enzyme preparation derived from button mushroom (Agaricus bisporus) fruiting bodies. Access to pure TYR preparations comes almost exclusively from the production of recombinant TYRs as the purification of these enzymes from the natural source is usually very laborious and plagued by low yields. In this text an introduction into the biochemistry of the enzyme TYR will be given, followed by an overview of available structural data of TYRs, the current model for the catalytic mechanism, a survey of reports on the recombinant production of this important metalloenzyme family, and a review of the applications of TYRs for the synthesis of catechols, as biosensors, in bioremediation, for the cross-linking of proteins and medical hydrogels as well as for melanoma treatment.

Graphical Abstract

Plant growth promoting endophyte Burkholderia contaminans NZ antagonizes phytopathogen Macrophomina phaseolina through melanin synthesis and pyrrolnitrin inhibition

The endophytic bacterium Burkholderia contaminans NZ was isolated from jute, which is an important fiber-producing plant. This bacterium exhibits significant growth promotion activity in in vivo pot experiments, and like other plant growth-promoting (PGP) bacteria fixes nitrogen, produces indole acetic acid (IAA), siderophore, and 1-aminocyclopropane-1-carboxylate (ACC) deaminase activity. B. contaminans NZ is considered to exert a promising growth inhibitory effect on Macrophomina phaseolina, a phytopathogen responsible for infecting hundreds of crops worldwide. This study aimed to identify the possibility of B. contaminans NZ as a safe biocontrol agent and assess its effectiveness in suppressing phytopathogenic fungi, especially M. phaseolina. Co-culture of M. phaseolina with B. contaminans NZ on both solid and liquid media revealed appreciable growth suppression of M. phaseolina and its chromogenic aberration in liquid culture. Genome mining of B. contaminans NZ using NaPDoS and antiSMASH revealed gene clusters that displayed 100% similarity for cytotoxic and antifungal substances, such as pyrrolnitrin. GC-MS analysis of B. contaminans NZ culture extracts revealed various bioactive compounds, including catechol; 9,10-dihydro-12'-hydroxy-2'-methyl-5'-(phenylmethyl)- ergotaman 3',6',18-trione; 2,3-dihydro-3,5- dihydroxy-6-methyl-4H-pyran-4-one; 1-(1,6-Dioxooctadecyl)- pyrrolidine; 9-Octadecenamide; and 2- methoxy- phenol. These compounds reportedly exhibit tyrosinase inhibitory, antifungal, and antibiotic activities. Using a more targeted approach, an RP-HPLC purified fraction was analyzed by LC-MS, confirming the existence of pyrrolnitrin in the B. contaminans NZ extract. Secondary metabolites, such as catechol and ergotaman, have been predicted to inhibit melanin synthesis in M. phaseolina. Thus, B. contaminans NZ appears to inhibit phytopathogens by apparently impairing melanin synthesis and other potential biochemical pathways, exhibiting considerable fungistatic activity.

Polyphenol-Hydroxylating Tyrosinase Activity under Acidic pH Enables Efficient Synthesis of Plant Catechols and Gallols

Tyrosinase is generally known as a melanin-forming enzyme, facilitating monooxygenation of phenols, oxidation of catechols into quinones, and finally generating biological melanin. As a homologous form of tyrosinase in plants, plant polyphenol oxidases perform the same oxidation reactions specifically toward plant polyphenols. Recent studies reported synthetic strategies for large scale preparation of hydroxylated plant polyphenols, using bacterial tyrosinases rather than plant polyphenol oxidase or other monooxygenases, by leveraging its robust monophenolase activity and broad substrate specificity. Herein, we report a novel synthesis of functional plant polyphenols, especially quercetin and myricetin from kaempferol, using screened bacterial tyrosinases. The critical bottleneck of the biocatalysis was identified as instability of the catechol and gallol under neutral and basic conditions. To overcome such instability of the products, the tyrosinase reaction proceeded under acidic conditions. Under mild acidic conditions supplemented with reducing agents, a bacterial tyrosinase from Bacillus megaterium (BmTy) displayed efficient consecutive two-step monophenolase activities producing quercetin and myricetin from kaempferol. Furthermore, the broad substrate specificity of BmTy toward diverse polyphenols enabled us to achieve the first biosynthesis of tricetin and 3'-hydroxyeriodictyol from apigenin and naringenin, respectively. These results suggest that microbial tyrosinase is a useful biocatalyst to prepare plant polyphenolic catechols and gallols with high productivity, which were hardly achieved by using other monooxygenases such as cytochrome P450s.

Natural and synthetic flavonoid derivatives as new potential tyrosinase inhibitors: a systematic review

Tyrosinase is a multifunctional glycosylated and copper-containing oxidase that is highly prevalent in plants and animals and plays a pivotal role in catalyzing the two key steps of melanogenesis: tyrosine's hydroxylation to dihydroxyphenylalanine (DOPA), and oxidation of the latter species to dopaquinone. Melanin guards against the destructive effects of ultraviolet radiation which is known to produce considerable pathological disorders such as skin cancer, among others. Moreover, the overproduction of melanin can create aesthetic problems along with serious disorders linked to hyperpigmented spots or patches on skin. Several skin-whitening products which reduce melanogenesis activity and alleviate hyperpigmentation are commercially available. A few of them, particularly those obtained from natural sources and that incorporate a phenolic scaffold, have been exploited in the cosmetic industry. In this context, synthetic tyrosinase inhibitors (TIs) with elevated efficacy and fewer side effects are direly needed in the pharmaceutical and cosmetic industries owing to their protective effect against pigmentation and dermatological disorders. Furthermore, the biological significance of the chromone skeleton and its associated medicinal and bioactive properties has drawn immense interest and inspired many researchers to design and develop novel anti-tyrosinase agents based on the flavonoid core (2-arylchromone). This review article is oriented to provide an insight and a deeper understanding of the tyrosinase inhibitory activity of an array of natural and bioinspired phenolic compounds with special emphasis on flavonoids to demonstrate how the position of ring substituents and their interaction with tyrosinase could be correlated with their effectiveness or lack thereof against inhibiting the enzyme.

Nanoreporter of an Enzymatic Suicide Inactivation Pathway

Enzymatic suicide inactivation, a route of permanent enzyme inhibition, is the mechanism of action for a wide array of pharmaceuticals. Here, we developed the first nanosensor that selectively reports the suicide inactivation pathway of an enzyme. The sensor is based on modulation of the near-infrared fluorescence of an enzyme-bound carbon nanotube. The nanosensor responded selectively to substrate-mediated suicide inactivation of the tyrosinase enzyme via bathochromic shifting of the nanotube emission wavelength. Mechanistic investigations revealed that singlet oxygen generated by the suicide inactivation pathway induced the response. We used the nanosensor to quantify the degree of enzymatic inactivation by measuring response rates to small molecule tyrosinase modulators. This work resulted in a new capability of interrogating a specific route of enzymatic death. Potential applications include drug screening and hit-validation for compounds that elicit or inhibit enzymatic inactivation and single-molecule measurements to assess population heterogeneity in enzyme activity.

Chemical Reactivities of ortho-Quinones Produced in Living Organisms: Fate of Quinonoid Products Formed by Tyrosinase and Phenoloxidase Action on Phenols and Catechols

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of carboxyl group, alcoholic group, and water are mostly slow reactions. A variety of catecholamines also exhibit side chain desaturation through tautomeric quinone methide formation. Therefore, quinone methide tautomers also play a pivotal role in the fate of numerous o-quinones. Armed with such wide and dangerous reactivity, o-quinones are capable of modifying the structure of important cellular components especially proteins and DNA and causing severe cytotoxicity and carcinogenic effects. The reactivities of different o-quinones involved in these processes along with special emphasis on mechanism of melanogenesis are discussed.

A comprehensive review on tyrosinase inhibitors

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

Natural and Bioinspired Phenolic Compounds as Tyrosinase Inhibitors for the Treatment of Skin Hyperpigmentation: Recent Advances

One of the most common approaches for control of skin pigmentation involves the inhibition of tyrosinase, a copper-containing enzyme which catalyzes the key steps of melanogenesis. This review focuses on the tyrosinase inhibition properties of a series of natural and synthetic, bioinspired phenolic compounds that have appeared in the literature in the last five years. Both mushroom and human tyrosinase inhibitors have been considered. Among the first class, flavonoids, in particular chalcones, occupy a prominent role as natural inhibitors, followed by hydroxystilbenes (mainly resveratrol derivatives). A series of more complex phenolic compounds from a variety of sources, first of all belonging to the Moraceae family, have also been described as potent tyrosinase inhibitors. As to the synthetic compounds, hydroxycinnamic acids and chalcones again appear as the most exploited scaffolds. Several inhibition mechanisms have been reported for the described inhibitors, pointing to copper chelating and/or hydrophobic moieties as key structural requirements to achieve good inhibition properties. Emerging trends in the search for novel skin depigmenting agents, including the development of assays that could distinguish between inhibitors and potentially toxic substrates of the enzyme as well as of formulations aimed at improving the bioavailability and hence the effectiveness of well-known inhibitors, have also been addressed.

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

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

Molecular inhibitory mechanism of dihydromyricetin on mushroom tyrosinase

Tyrosinase is the rate-limiting enzyme for controlling the production of melanin in the human body, and overproduction of melanin can lead to a variety of skin disorders. In this paper, the inhibitory kinetics of Dihydromyricetin (DHM) on tyrosinase and their binding mechanism were determined using spectroscopy, molecular docking, antioxidant assays, and chromatography. The spectroscopic results indicate that DHM reversibly inhibits tyrosinase in a mixed-type manner through a multiphase kinetic process with the IC₅₀ of 849.88 μM. It is shown that DHM has a strong ability to quench the intrinsic fluorescence of tyrosinase mainly through a static quenching procedure, suggesting that a stable DHM-tyrosinase complex is generated. Molecular docking results suggest that the dominant conformation of DHM does not directly bind to the active site of tyrosinase. Moreover, the antioxidant assays demonstrate that DHM has powerful antioxidant and reducing capacity but does not have the ability to reduce dopachrome to L-DOPA. Interestingly, the results of spectroscopy and chromatography indicate that DHM is a substrate of tyrosinase but not a suicide substrate. The possible inhibitory mechanism is proposed, which will be helpful to design and search for tyrosinase inhibitors.

Oxidative discolouration in whole-head and cut lettuce: biochemical and environmental influences on a complex phenotype and potential breeding strategies to improve shelf-life

Lettuce discolouration is a key post-harvest trait. The major enzyme controlling oxidative discolouration has long been considered to be polyphenol oxidase (PPO) however, levels of PPO and subsequent development of discolouration symptoms have not always correlated. The predominance of a latent state of the enzyme in plant tissues combined with substrate activation and contemporaneous suicide inactivation mechanisms are considered as potential explanations for this phenomenon. Leaf tissue physical properties have been associated with subsequent discolouration and these may be influenced by variation in nutrient availability, especially excess nitrogen and head maturity at harvest. Mild calcium and irrigation stress has also been associated with a reduction in subsequent discolouration, although excess irrigation has been linked to increased discolouration potentially through leaf physical properties. These environmental factors, including high temperature and UV light intensities, often have impacts on levels of phenolic compounds linking the environmental responses to the biochemistry of the PPO pathway. Breeding strategies targeting the PAL and PPO pathway biochemistry and environmental response genes are discussed as a more cost-effective method of mitigating oxidative discolouration then either modified atmosphere packaging or post-harvest treatments, although current understanding of the biochemistry means that such programs are likely to be limited in nature and it is likely that they will need to be deployed alongside other methods for the foreseeable future.

Heterologous expression and characterization of functional mushroom tyrosinase (AbPPO4)

Tyrosinases are an ubiquitous group of copper containing metalloenzymes that hydroxylate and oxidize phenolic molecules. In an application context the term 'tyrosinase' usually refers to 'mushroom tyrosinase' consisting of a mixture of isoenzymes and containing a number of enzymatic side-activities. We describe a protocol for the efficient heterologous production of tyrosinase 4 from Agaricus bisporus in Escherichia coli. Applying this procedure a pure preparation of a single isoform of latent tyrosinase can be achieved at a yield of 140 mg per liter of autoinducing culture medium. This recombinant protein possesses the same fold as the enzyme purified from the natural source as evidenced by single crystal X-ray diffraction. The latent enzyme can be activated by limited proteolysis with proteinase K which cleaves the polypeptide chain after K382, only one The latent enzyme can amino acid before the main in-vivo activation site. Latent tyrosinase can be used as obtained and enzymatic activity may be induced in the reaction mixture by the addition of an ionic detergent (e.g. 2 mM SDS). The proteolytically activated mushroom tyrosinase shows >50% of its maximal activity in the range of pH 5 to 10 and accepts a wide range of substrates including mono- and diphenols, flavonols and chalcones.

Mechanism and kinetics of tyrosinase inhibition by glycolic acid: a study using conventional spectroscopy methods and hydrogen/deuterium exchange coupling with mass spectrometry

Tyrosinase is an enzyme that promotes enzymatic browning of fruits and vegetables, thereby reducing product quality.

Effective L-Tyrosine Hydroxylation by Native and Immobilized Tyrosinase

Hydroxylation of L-tyrosine to 3,4-dihydroxyphenylalanine (L-DOPA) by immobilized tyrosinase in the presence of ascorbic acid (AH₂), which reduces DOPA-quinone to L-DOPA, is characterized by low reaction yields that are mainly caused by the suicide inactivation of tyrosinase by L-DOPA and AH₂. The main aim of this work was to compare processes with native and immobilized tyrosinase to identify the conditions that limit suicide inactivation and produce substrate conversions to L-DOPA of above 50% using HPLC analysis. It was shown that immobilized tyrosinase does not suffer from partitioning and diffusion effects, allowing a direct comparison of the reactions performed with both forms of the enzyme. In typical processes, additional aeration was applied and boron ions to produce the L-DOPA and AH₂ complex and hydroxylamine to close the cycle of enzyme active center transformations. It was shown that the commonly used pH 9 buffer increased enzyme stability, with concomitant reduced reactivity of 76%, and that under these conditions, the maximal substrate conversion was approximately 25 (native) to 30% (immobilized enzyme). To increase reaction yield, the pH of the reaction mixture was reduced to 8 and 7, producing L-DOPA yields of approximately 95% (native enzyme) and 70% (immobilized). A three-fold increase in the bound enzyme load achieved 95% conversion in two successive runs, but in the third one, tyrosinase lost its activity due to strong suicide inactivation caused by L-DOPA processing. In this case, the cost of the immobilized enzyme preparation is not overcome by its reuse over time, and native tyrosinase may be more economically feasible for a single use in L-DOPA production. The practical importance of the obtained results is that highly efficient hydroxylation of monophenols by tyrosinase can be obtained by selecting the proper reaction pH and is a compromise between complexation and enzyme reactivity.

Dual effects of alpha-arbutin on monophenolase and diphenolase activities of mushroom tyrosinase

The effects of α-arbutin on the monophenolase and diphenolase activities of mushroom tyrosinase were investigated. The results showed that α-arbutin inhibited monophenolase activity but it activated diphenolase activity. For monophenolase activity, IC₅₀ value was 4.5 mmol·L⁻¹ and 4.18 mmol·L⁻¹ of α-arbutin could extend the lag time from 40.5 s to 167.3 s. Alpha- arbutin is proposed to be regarded as a triphenolic substrate by the enzyme during catalyzation, leading to the suicide inactivation of the active site of tyrosinase. For diphenolase activity, α-arbutin acted as an activator and its activation mechanism was mixed type activation. To reveal such activation, it should be mainly refered to the conformational changes in tyrosinase caused by the interaction of α-arbutin with residues located at the entrance to the active site, and the decrease of the effect of suicide inactivation.

Mechanistic aspects of the tyrosinase oxidation of hydroquinone

Contradictory reports on the behaviour of hydroquinone as a tyrosinase substrate are reconciled in terms of the ability of the initially formed ortho-quinone to tautomerise to the thermodynamically more stable para-quinone isomer. Oxidation of phenols by native tyrosinase requires activation by in situ formation of a catechol formed via an enzyme generated ortho-quinone. In the special case of hydroquinone, catechol formation is precluded by rapid tautomerisation of the ortho-quinone precursor to catechol formation.

Design and discovery of mushroom tyrosinase inhibitors and their therapeutic applications

INTRODUCTION

Tyrosinase inhibitors could have a huge importance in medicine, cosmetics and agriculture. Although many tyrosinase inhibitors are available, they have demonstrated only mild efficacy and safety concerns. This has led to the discovery of novel tyrosinase inhibitors that are more safe, potent and efficacious.

AREAS COVERED

The authors provide an overview of the recent scientific accounts describing the design of new molecules. These compounds belong to different chemical families. The review emphasizes the rationale behind the discovery, the study of structure-activity relationships, the study of the mechanism and kinetic of inhibition and the cellular effect of the inhibitors. The article is based on the literature published from 2007 onward related with the development of synthetic tyrosinase inhibitors.

EXPERT OPINION

Although a great number of tyrosinase inhibitors have been published in the literature, none, as of yet, have reached the potency and safety requirements needed to enter clinical trials. The emergence of new in vitro and in vivo tests will finally allow the arrival of new compounds that are more potent and safe.

Tyrosinase: the four oxidation states of the active site and their relevance to enzymatic activation, oxidation and inactivation

Tyrosinase is an enzyme widely distributed in the biosphere. It is one of a group of proteins with a strongly conserved bicopper active centre able to bind molecular oxygen. Tyrosinase manifests two catalytic properties; monooxygenase and oxidase activity. These actions reflect the oxidation states of the active centre. Tyrosinase has four possible oxidation states and the details of their interaction are shown to give rise to the unusual kinetic behaviour of the enzyme. The resting state of the enzyme is met-tyrosinase [Cu(II)2] and activation, associated with a 'lag period', involves reduction to deoxy-tyrosinase [Cu(I)2] which is capable of binding dioxygen to form oxy-tyrosinase [Cu(II)2·O2]. Initially the conversion of met- to deoxy-tyrosinase is brought about by a catechol that is indirectly formed from an ortho-quinone product of tyrosinase action. The primary function of the enzyme is monooxygenation of phenols to ortho-quinones by oxy-tyrosinase. Inactivation of the enzyme results from monooxygenase processing of catechols which can lead to reductive elimination of one of the active-site copper ions and conversion of oxy-tyrosinase to the inactive deact-tyrosinase [Cu(II)Cu(0)]. This review describes the tyrosinase pathways and the role of each oxidation state in the enzyme's oxidative transformations of phenols and catechols.

Catalysis and inactivation of tyrosinase in its action on hydroxyhydroquinone

Hydroxyhydroquinone (HHQ) was characterized kinetically as a tyrosinase substrate. A kinetic mechanism is proposed, in which HHQ is considered as a monophenol or as an o-diphenol, depending on the part of the molecule that interacts with the enzyme. The kinetic parameters obtained from an analysis of the measurements of the initial steady state rate of 2-hydroxy p-benzoquinone formation were kcatapp=229.0±7.7 s(-1) and KMapp,HHQ=0.40±0.05 mM. Furthermore, the action of tyrosinase on HHQ led to the enzyme's inactivation through a suicide inactivation mechanism. This suicide inactivation process was characterized kinetically by λmaxapp (the apparent maximum inactivation constant) and r, the number of turnovers made by 1 mol of enzyme before being inactivated. The values of λmaxapp and r were (8.2±0.1)×10(-3) s(-1) and 35,740±2,548, respectively.

Deuterium isotope effect on the suicide inactivation of tyrosinase in its action on o-diphenols

A solvent deuterium isotope effect on the inactivation suicide of tyrosinase in its action on o-diphenols (catechol, 4-methylcatechol, and 4-tert-butylcatechol) was observed. This isotope effect, observed during kinetic studies in the transition phase, was higher than that described previously in the steady state, indicating that there is an additional slow step in the suicide inactivation mechanism, which we believe to be responsible for the inactivation. In a proton inventory study of oxidation of o-diphenols, the representation of λmax(D,fn)/λmax(D,f0) versus n (atom fractions of deuterium), where λmax(D,fn) is the maximum apparent inactivation constant for a molar fraction of deuterium (n) and λmax(D,f0) is the corresponding kinetic parameter in a water solution, was linear for all substrates. This suggests that only one of the protons transferred from the two hydroxyl groups of the substrate, which are oxidized in one turnover, is responsible for the isotope effects. We propose that this proton could be the proton transferred from the hydroxyl group of C-2 to the hydroperoxide of the oxytyrosinase form (Eox ) and that it probably causes enzyme inactivation through the reduction of the Cu(2+) A to Cu(0) and its subsequent release from the active site.

Melanins and melanogenesis: methods, standards, protocols

Despite considerable advances in the past decade, melanin research still suffers from the lack of universally accepted and shared nomenclature, methodologies, and structural models. This paper stems from the joint efforts of chemists, biochemists, physicists, biologists, and physicians with recognized and consolidated expertise in the field of melanins and melanogenesis, who critically reviewed and experimentally revisited methods, standards, and protocols to provide for the first time a consensus set of recommended procedures to be adopted and shared by researchers involved in pigment cell research. The aim of the paper was to define an unprecedented frame of reference built on cutting-edge knowledge and state-of-the-art methodology, to enable reliable comparison of results among laboratories and new progress in the field based on standardized methods and shared information.

Indirect inactivation of tyrosinase in its action on 4-tert-butylphenol

Under anaerobic conditions, the o-diphenol 4-tert-butylcatechol (TBC) irreversibly inactivates met and deoxytyrosinase enzymatic forms of tyrosinase. However, the monophenol 4-tert-butylphenol (TBF) protects the enzyme from this inactivation. Under aerobic conditions, the enzyme suffers suicide inactivation when it acts on TBC. We suggest that TBF does not directly cause the suicide inactivation of the enzyme in the hydroxylase activity, but that the o-diphenol, which is necessary for the system to reach the steady state, is responsible for the process. Therefore, monophenols do not induce the suicide inactivation of tyrosinase in its hydroxylase activity, and there is a great difference between the monophenols that give rise to unstable o-quinones such as L-tyrosine, which rapidly accumulate L-dopa in the medium and those like TBF, after oxidation, give rise to a very stable o-quinone.

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

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

The influence of hydroquinone on tyrosinase kinetics

In vitro studies, using combined spectrophotometry and oximetry together with hplc/ms examination of the products of tyrosinase action demonstrate that hydroquinone is not a primary substrate for the enzyme but is vicariously oxidised by a redox exchange mechanism in the presence of either catechol, L-3,4-dihydroxyphenylalanine or 4-ethylphenol. Secondary addition products formed in the presence of hydroquinone are shown to stimulate, rather than inhibit, the kinetics of substrate oxidation.

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

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

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

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

Tetrahydrofolic Acid is a potent suicide substrate of mushroom tyrosinase

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

Suicide inactivation of the diphenolase and monophenolase activities of tyrosinase

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

A reactive ortho-quinone generated by tyrosinase-catalyzed oxidation of the skin depigmenting agent monobenzone: self-coupling and thiol-conjugation reactions and possible implications for melanocyte toxicity

Monobenzone (hydroquinone monobenzylether, 1) is a potent skin depigmenting agent that causes irreversible loss of epidermal melanocytes by way of a tyrosinase-dependent mechanism so far little understood. Herein, we show that 1 can be oxidized by mushroom tyrosinase to an unstable o-quinone (1-quinone) that has been characterized by comparison of its properties with those of a synthetic sample obtained by o-iodoxybenzoic acid-mediated oxidation of 1. Preparative scale oxidation of 1 with tyrosinase and catalytic l-DOPA, followed by reductive workup and acetylation, led to the isolation of two main products that were identified as the acetylated catechol derivative 4 and an unusual biphenyl-type dimer of 4, acetylated 5, arising evidently by coupling of 4 with 1-quinone. In the presence of l-cysteine or N-acetyl-l-cysteine, formation of 4 and 5 was inhibited, and the reaction led instead to monoadducts (6 or 9) and diadducts (7 and 8). A similar behavior was observed when the tyrosinase-promoted oxidation of 1 was carried out in the presence of sulfhydryl-containing peptides, such as reduced glutathione, or proteins, such as bovine serum albumin (BSA), as inferred by detection of adduct 9 by high pressure liquid chromatography-electrochemical detection (HPLC-ED) after acid hydrolysis. The generation and reaction chemistry of 1-quinone described in this article may bear relevance to the etiopathogenetic mechanisms of monobenzone-induced leukoderma as well as to the recently proposed haptenation hypothesis of vitiligo, a disabling pigmentary disorder characterized by irreversible melanocyte loss.

Tyrosinase inactivation in its action on dopa

Under aerobic or anaerobic conditions, tyrosinase undergoes a process of irreversible inactivation induced by its physiological substrate L-dopa. Under aerobic conditions, this inactivation occurs through a process of suicide inactivation involving the form oxy-tyrosinase. Under anaerobic conditions, both the met- and deoxy-tyrosinase forms undergo irreversible inactivation. Suicide inactivation in aerobic conditions is slower than the irreversible inactivation under anaerobic conditions. The enzyme has less affinity for the isomer D-dopa than for L-dopa but the velocity of inactivation is the same. We propose mechanisms to explain these processes.

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

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

An updated review of tyrosinase inhibitors

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