Tyrosinase kinetics: discrimination between two models to explain the oxidation mechanism of monophenol and diphenol substrates

Electrooxidation of Phenol on Polyelectrolyte Modified Carbon Electrodes for Use in Insulin Pump Infusion Sets

BACKGROUND

Many type 1 diabetes patients using continuous subcutaneous insulin infusion (CSII) suffer from the phenomenon of unexplained hypoglycemia or "site loss." Site loss is hypothesized to be caused by toxic excipients, for example, phenolic compounds within insulin formulations that are used as preservatives and stabilizers. Here, we develop a bioinspired polyelectrolyte-modified carbon electrode for effective electrooxidative removal of phenol from insulin and eventual incorporations into an infusion set of a CSII device.

METHODS

We modified a carbon screen printed electrode (SPE) with poly-L-lysine (PLL) to avoid passivation due to polyphenol deposition while still removing phenolic compounds from insulin injections. We characterized these electrodes using scanning electron microscopy (SEM) and electrochemical impedance spectroscopy (EIS) and compared their data with data from bare SPEs. Furthermore, we performed electrochemical measurements to determine the extent of passivation, and high-performance liquid chromatography (HPLC) measurements to confirm both the removal of phenol and the integrity of insulin after phenol removal.

RESULTS

Voltammetry measurements show that electrode passivation due to polyphenol deposition is reduced by a factor of 2X. HPLC measurements confirm a 10x greater removal of phenol by our modified electrodes relative to bare electrodes.

CONCLUSION

Using bioinspired polyelectrolytes to modify a carbon electrode surface aids in the electrooxidation of phenolic compounds from insulin and is a step toward integration within an infusion set for mitigating site loss.

Synthesis, in silico modelling, and in vitro biological evaluation of substituted pyrazole derivatives as potential anti-skin cancer, anti-tyrosinase, and antioxidant agents

Twenty-five azole compounds (P1-P25) were synthesised using regioselective base-metal catalysed and microwave-assisted approaches, fully characterised by high-resolution mass spectrometry (HRMS), nuclear magnetic resonance (NMR), and infrared spectra (IR) analyses, and evaluated for anticancer, anti-tyrosinase, and anti-oxidant activities in silico and in vitro. P25 exhibited potent anticancer activity against cells of four skin cancer (SC) lines, with selectivity for melanoma (A375, SK-Mel-28) or non-melanoma (A431, SCC-12) SC cells over non-cancerous HaCaT-keratinocytes. Clonogenic, scratch-wound, and immunoblotting assay data were consistent with anti-proliferative results, expression profiling therewith implicating intrinsic and extrinsic apoptosis activation. In a mushroom tyrosinase inhibition assay, P14 was most potent among the compounds (half-maximal inhibitory concentration where 50% of cells are dead, IC50 15.9 μM), with activity greater than arbutin and kojic acid. Also, P6 exhibited noteworthy free radical-scavenging activity. Furthermore, in silico docking and absorption, distribution, metabolism, excretion, and toxicity (ADMET) simulations predicted prominent-phenotypic actives to engage diverse cancer/hyperpigmentation-related targets with relatively high affinities. Altogether, promising early-stage hits were identified - some with multiple activities - warranting further hit-to-lead optimisation chemistry with further biological evaluations, towards identifying new skin-cancer and skin-pigmentation renormalising agents.

The Crystal Structure of Tyrosinase from Verrucomicrobium spinosum Reveals It to Be an Atypical Bacterial Tyrosinase

Tyrosinases belong to the type-III copper enzyme family, which is involved in melanin production in a wide range of organisms. Despite similar overall characteristics and functions, their structures, activities, substrate specificities and regulation vary. The tyrosinase from the bacterium Verrucomicrobium spinosum (vsTyr) is produced as a pre-pro-enzyme in which a C-terminal extension serves as an inactivation domain. It does not require a caddie protein for copper ion incorporation, which makes it similar to eukaryotic tyrosinases. To gain an understanding of the catalytic machinery and regulation of vsTyr activity, we determined the structure of the catalytically active "core domain" of vsTyr by X-ray crystallography. The analysis showed that vsTyr is an atypical bacterial tyrosinase not only because it is independent of a caddie protein but also because it shows the highest structural (and sequence) similarity to plant-derived members of the type-III copper enzyme family and is more closely related to fungal tyrosinases regarding active site features. By modelling the structure of the pre-pro-enzyme using AlphaFold, we observed that Phe453, located in the C-terminal extension, is appropriately positioned to function as a "gatekeeper" residue. Our findings raise questions concerning the evolutionary origin of vsTyr.

Fast screening of tyrosinase inhibitors from traditional Chinese medicinal plants by ligand fishing in combination with in situ fluorescent assay

A simple and rapid method for screening of tyrosinase (TYR) inhibitors present in traditional Chinese medicines (TCMs) was developed by combining ligand fishing and the fluorescent enzymatic assay based on dopamine-functionalized carbon quantum dots (CQDs-Dopa). Ligands of the enzyme present in the TCM extractions were firstly adsorbed on the enzyme-modified magnetic beads, and then the beads were magnetically separated and subjected directly to the CQDs-Dopa-based fluorescent assay. Finally, compounds were desorbed from the "active" beads and identified with ultra-performance liquid chromatography-triple quadrupole mass spectrometry. A known natural TYR inhibitor quercetin was selected to assess the feasibility and quantification performance of this method, and good linearity in the range of 0.01-0.16 mM (R² = 0.992) with a low detection limit of 0.004 mM was obtained. This method was then applied to screen TYR inhibitors present in Scutellaria baicalensis and Sophora flavescens. Six TYR inhibitors including baicalin (1), baicalein (2), wogonin (3), oroxylin A (4), kurarinone (5), and sophoraflavanone G (6) were found, among which 1-4 were firstly discovered in this work. This is the first report on the in situ assessment of the target compounds obtained by ligand fishing in the form of a mixture, which exhibited the combined advantages of specific extraction ability of ligand fishing and the high sensitivity of CQDs-based fluorescent assay, showing great potential for fast screening of enzyme inhibitors from TCMs.

Determination of tyrosinase-cyanidin-3-O-glucoside and (-/+)-catechin binding modes reveal mechanistic differences in tyrosinase inhibition

Tyrosinase, exquisitely catalyzes the phenolic compounds into brown or black pigment, inhibition is used as a treatment for dermatological or neurodegenerative disorders. Natural products, such as cyanidin-3-O-glucoside and (-/+)-catechin, are considered safe and non-toxic food additives in tyrosinase inhibition but their ambiguous inhibitory mechanism against tyrosinase is still elusive. Thus, we presented the mechanistic insights into tyrosinase with cyanidin-3-O-glucoside and (-/+)-catechin using computational simulations and in vitro assessment. Initial molecular docking results predicted ideal docked poses (- 9.346 to - 5.795 kcal/mol) for tyrosinase with selected flavonoids. Furthermore, 100 ns molecular dynamics simulations and post-simulation analysis of docked poses established their stability and oxidation of flavonoids as substrate by tyrosinase. Particularly, metal chelation via catechol group linked with the free 3-OH group on the unconjugated dihydropyran heterocycle chain was elucidated to contribute to tyrosinase inhibition by (-/+)-catechin against cyanidin-3-O-glucoside. Also, predicted binding free energy using molecular mechanics/generalized Born surface area for each docked pose was consistent with in vitro enzyme inhibition for both mushroom and murine tyrosinases. Conclusively, (-/+)-catechin was observed for substantial tyrosinase inhibition and advocated for further investigation for drug development against tyrosinase-associated diseases.

Synergistic effects and antityrosinase mechanism of four plant polyphenols from Morus and Hulless Barley

Sanggenone C, oxyresveratrol, catechin and l-epicatechin exist in Morus and Hulless Barley as natural polyphenols with antityrosinase activity. Little research on their synergistic and structure-function relationships of them has been reported in recent years. In this paper, the inhibition mechanisms of these four plant polyphenols were investigated by enzyme kinetics, HPLC, fluorescence spectra, and molecular docking methods. The results showed that oxyresveratrol (IC₅₀ = 1.096 ± 0.048 μg/mL), sanggenone C (IC₅₀ = 13.360 ± 1.029 μg/mL), l-epicatechin (IC₅₀ = 55.730 ± 1.762 μg/mL), and catechin (IC₅₀ = 148.500 ± 3.355 μg/mL) exhibited tyrosinase inhibition activity. When sangenone C (14 μg/mL) was mixed with l-epicatechin (56 μg/mL) at 4:1 (40 μL + 10 μL), the highest tyrosinase inhibition was achieved. Molecular docking showed that the number and position of phenolic hydroxyls of polyphenols were the key for tyrosinase inhibition activity. This study provided new ideas for the application of these four plant polyphenols from Hulless Barley and Morus as tyrosinase inhibitors in food preservation.

Spectrophotometric Assays for Sensing Tyrosinase Activity and Their Applications

Tyrosinase (TYR, E.C. 1.14.18.1), a critical enzyme participating in melanogenesis, catalyzes the first two steps in melanin biosynthesis including the ortho-hydroxylation of L-tyrosine and the oxidation of L-DOPA. Previous pharmacological investigations have revealed that an abnormal level of TYR is tightly associated with various dermatoses, including albinism, age spots, and malignant melanoma. TYR inhibitors can partially block the formation of pigment, which are always used for improving skin tone and treating dermatoses. The practical and reliable assays for monitoring TYR activity levels are very useful for both disease diagnosis and drug discovery. This review comprehensively summarizes structural and enzymatic characteristics, catalytic mechanism and substrate preference of TYR, as well as the recent advances in biochemical assays for sensing TYR activity and their biomedical applications. The design strategies of various TYR substrates, alongside with several lists of all reported biochemical assays for sensing TYR including analytical conditions and kinetic parameters, are presented for the first time. Additionally, the biomedical applications and future perspectives of these optical assays are also highlighted. The information and knowledge presented in this review offer a group of practical and reliable assays and imaging tools for sensing TYR activities in complex biological systems, which strongly facilitates high-throughput screening TYR inhibitors and further investigations on the relevance of TYR to human diseases.

Tel-Cu-NPs Catalyst: Synthesis of Naphtho[2,3-g]phthalazine Derivatives as Potential Inhibiters of Tyrosinase Enzymes and Their Investigation in Kinetic, Molecular Docking, and Cytotoxicity Studies

Novel one-pot synthesis naphtho[2,3-g]phthalazine (1a–1k) of Mannich base derivatives can be achieved via grindstone chemistry using a Tel-Cu-NPs (telmisartan-copper nanoparticles) catalyst. This method offers efficient mild reaction conditions and high yields. Tyrosinase inhibitory activity was evaluated for all synthesized compounds, along with analysis of kinetic behavior and molecular docking studies. The synthesized compound, 1c was (IC50 = 11.5 µM) more active than kojic acid (IC50 = 78.0 µM). Lineweaver Burk plots were used to analyze the kinetic behavior of the most active compound 1c, it was reversible and competitive behavior. Compound 1c and kojic acid occurred in the presence of 2-hydroxyketone, which has the same inhibitory mechanism. The molecular docking of compound 1c and the control kojic acid were docked against 2Y9X protein via the Schrodinger Suite. The compound 1c showed a respectable dock score (−5.6 kcal/mol) compared to kojic acid with a dock score of (−5.2 kcal/mol) in the 2Y9X protein. Cytotoxicity activity was also evaluated by using HepG2 (liver), MCF-7 (breast), and HeLa (cervical) cancer cell lines, and high activity for 1c (GI50 = 0.01, 0.03, and 0.04 µM, respectively) against all cell lines was found compared to standard and other compounds. Therefore, this study succeeded in testing a few promising molecules as potential antityrosinase agents.

Hydroxylated biphenyls as tyrosinase inhibitor: A spectrophotometric and electrochemical study

A small collection of C₂-symmetry hydroxylated biphenyls was prepared by straightforward methods and the capability to act as inhibitors of tyrosinase has been evaluated by both spectrophotometric and electrochemical assays. Our attention was focused on the diphenolase activity of this enzyme characterized by the absence of the characteristic lag time of enzymatic reaction of its monophenolase activity. To this purpose, we evaluated the capability of tyrosinase to oxidize a natural o-diphenol substrate to o-quinone analyzing the changes in the UV-Vis spectrum of a solution of caffeic acid and the reduction of the cathodic current in a tyrosinase-biosensor, respectively. Results of both the methods were comparable. Most of the compounds possessed higher inhibitory activity compared to compound 1, a known hydroxylated biphenyl inhibitor of tyrosinase.

Kinetic characterization of substrate-analogous inhibitors of tyrosinase

The development of effective tyrosinase inhibitors has become increasingly important in the cosmetic, medicinal, and agricultural industries for application as antibrowning and depigmenting agents. The kinetic mechanisms of action of tyrosinase on monophenols and o-diphenols are complex, particularly in the case of monophenols because of the lag period that occurs at the beginning of the reaction. When enzyme inhibitors are studied, the problem becomes more complicated because the lag period increases, which has led to erroneous identification of the type of inhibition that many compounds exert on the monophenolase activity and the inaccurate determination of their inhibition constants. When the degrees of inhibition of an inhibitor which is analogous to tyrosinase substrates are the same for both monophenolase and diphenolase activities, this means that the inhibitor binds to the same enzymatic species and so the inhibition constants should be similar for both activities. In this study, we demonstrate this typical behavior of substrate-analogous inhibitors and propose a methodology for determining the type of inhibition and the inhibition constants for the monophenolase and diphenolase activities of the enzyme. Benzoic acid and cinnamic acid were used as inhibitors and the monophenol/o-diphenol pairs l-tyrosine/l-dopa and α-methyl-L-tyrosine/α-methyl-L-dopa as substrates.

Structure-activity relationships of the thujaplicins for inhibition of human tyrosinase

Tyrosinase inhibitors have become increasingly critical agents in cosmetic, agricultural, and medicinal products. Although a large number of tyrosinase inhibitors have been reported, almost all the inhibitors were unfortunately evaluated by using commercial available mushroom tyrosinase. Here, we examined the inhibitory effects of three isomers of thujaplicin (α, β, and γ) on human tyrosinase and analyzed their binding modes using homology model and docking studies. As the results, γ-thujaplicin was found to strongly inhibit human tyrosinase with the IC50 of 1.15 μM, extremely superior to a well-known tyrosinase inhibitor kojic acid (IC50 = 571.17 μM). MM-GB/SA binding free energy decomposition analyses suggested that the potent inhibitory activity of γ-thujaplicin may be due to the interactions with His367, Ile368, and Val377 (hot spot amino acid residues) in human tyrosinase. Furthermore, the binding mode of α-thujaplicin indicated that Val377 and Ser380 may cause van der Waals clashes with the isopropyl group of α-thujaplicin. These results provide a novel structural insight into the hot spot of human tyrosinase for the specific binding of γ-thujaplicin and a way to optimize not only thujaplicins but also other lead compounds as specific inhibitors for human tyrosinase in a rational manner.

Microbial tyrosinases: promising enzymes for pharmaceutical, food bioprocessing, and environmental industry

Tyrosinase is a natural enzyme and is often purified to only a low degree and it is involved in a variety of functions which mainly catalyse the o-hydroxylation of monophenols into their corresponding o-diphenols and the oxidation of o-diphenols to o-quinones using molecular oxygen, which then polymerizes to form brown or black pigments. The synthesis of o-diphenols is a potentially valuable catalytic ability and thus tyrosinase has attracted a lot of attention with respect to industrial applications. In environmental technology it is used for the detoxification of phenol-containing wastewaters and contaminated soils, as biosensors for phenol monitoring, and for the production of L-DOPA in pharmaceutical industries, and is also used in cosmetic and food industries as important catalytic enzyme. Melanin pigment synthesized by tyrosinase has found applications for protection against radiation cation exchangers, drug carriers, antioxidants, antiviral agents, or immunogen. The recombinant V. spinosum tryosinase protein can be used to produce tailor-made melanin and other polyphenolic materials using various phenols and catechols as starting materials. This review compiles the recent data on biochemical and molecular properties of microbial tyrosinases, underlining their importance in the industrial use of these enzymes. After that, their most promising applications in pharmaceutical, food processing, and environmental fields are presented.

On the interpretation of tyrosinase inhibition kinetics

Abstract Tyrosinases (monophenol monooxygenases, EC 1.14.18.1) utilize a reaction mechanism that involves two intertwined catalytic cycles and at least three different ligand-binding enzyme forms. Therefore a variety of different inhibition types may arise in inhibition experiments, depending on the binding mode of the compound studied. Here we discuss a steady-state equation that describes inhibition of the diphenolase cycle of tyrosinase catalysis in a general way. In addition, we employ numerical simulations to explore the kinetic outcome of various binding schemes. As the full equation is far too complex to be applicable for data evaluation by curve fitting, we propose to use the general modifier scheme of Botts-Morales for fitting and demonstrate that especially the value of parameter α of the equation allows conclusions about the binding mode of the inhibitor. The approach is exemplified by selected data describing the inhibition of human tyrosinase by typical inhibitors.

First structures of an active bacterial tyrosinase reveal copper plasticity

Tyrosinase is a member of the type 3 copper enzyme family that is involved in the production of melanin in a wide range of organisms. The crystal structures of a tyrosinase from Bacillus megaterium were determined at a resolution of 2.0-2.3 Å. The enzyme crystallized as a dimer in the asymmetric unit and was shown to be active in crystal. The overall monomeric structure is similar to that of the monomer of the previously determined tyrosinase from Streptomyces castaneoglobisporus, but it does not contain an accessory Cu-binding "caddie" protein. Two Cu(II) ions, serving as the major cofactors within the active site, are coordinated by six conserved histidine residues. However, determination of structures under different conditions shows varying occupancies and positions of the copper ions. This apparent mobility in copper binding modes indicates that there is a pathway by which copper is accumulated or lost by the enzyme. Additionally, we suggest that residues R209 and V218, situated in a second shell of residues surrounding the active site, play a role in substrate binding orientation based on their flexibility and position. The determination of a structure with the inhibitor kojic acid, the first tyrosinase structure with a bound ligand, revealed additional residues involved in the positioning of substrates in the active site. Comparison of wild-type structures with the structure of the site-specific variant R209H, which possesses a higher monophenolase/diphenolase activity ratio, lends further support to a previously suggested mechanism by which monophenolic substrates dock mainly to CuA.

Inhibitory effects on mushroom tyrosinase by flavones from the stem barks of Morus lhou (S.) Koidz

Five flavones displaying tyrosinase inhibitory activity were isolated from the stem barks of Morus lhou (S.) Koidz., a cultivated edible plant. The isolated compounds were identified as mormin (1), cyclomorusin (2), morusin (3), kuwanon C (4), and norartocarpetin (5). Mormin (1) was characterized as a new flavone possesing a 3-hydroxymethyl-2-butenyl at C-3. The inhibitory potencies of these flavonoids toward monophenolase activity of mushroom tyrosinase were investigated. The IC50 values of compounds 1-5 for monophenolase activity were determined to be 0.088, 0.092, 0.250, 0.135 mM, and 1.2 microM, respectively. Mormin (1), cyclomorusin (2), kuwanon C (4) and norartocarpetin (5) exhibited competitive inhibition characteristics. Interestingly norartocarpetin (5) showed a time-dependent inhibition against oxidation of L-tyrosine: it also operated under the enzyme isomerization model (k5 = 0.8424 min(-1), k6 = 0.0576 min(-1), K(app)(i) = 1.354 microM).

Effect of tetrahydropteridines on the monophenolase and diphenolase activities of tyrosinase

This study explains the action of compounds such as 6-tetrahydrobiopterin, (6BH4) and 6,7-dimethyltetrahydrobiopterin (6,7-di-CH3BH4) on the monophenolase and diphenolase activities of tyrosinase. These reductants basically act by reducing the o-quinones, the reaction products, to o-diphenol. In the case of the diphenolase activity a lag period is observed until the reductant is depleted; then the system reaches the steady-state. In the action of the enzyme on monophenol substrates, when the reductant concentration is less than that of the o-diphenol necessary for the steady-state to be reached, the system undergoes an apparent activation since, in this way, the necessary concentration of o-diphenol will be reached more rapidly. However, when the reductant concentration is greater than that of the o-diphenol necessary for the steady-state to be reached, the lag period lengthens and is followed by a burst, by means of which the excess o-diphenol is consumed, the steady-state thus taking longer to be reached. Moreover, in the present kinetic study, we show that tyrosinase is not inhibited by an excess of monophenol, although, to confirm this, the system must be allowed to pass from the transition state and enter the steady-state, which is attained when a given amount of o-diphenol has accumulated in the medium.

Some kinetic properties of polyphenol oxidase obtained from dill (Anethum graveolens)

Polyphenol oxidase (PPO) was partially purified from dill by (NH4)(2)SO4 precipitation followed by dialysis and gel filtration chromatography. Polyphenol oxidase activity was measured spectrophotometrically at 420 nm using catechol, dopamine and chlorogenic acid as substrates. Optimum pH, temperature, and ionic strength were determined with three substrates. The best substrate of dill PPO was found to be chlorogenic acid. Some kinetic properties of the enzyme such as V(max,) K(M) and V(max)/K(M) were determined for all three substrates. The effects of various inhibitors on the reaction catalysed by the enzyme were tested and I(50) values calculated. The most effective inhibitor was L-cysteine. Activation energies, E(a), were determined from the Arrhenius equation. In addition, activation enthalpy, DeltaH(a), and Q(10) values of the enzyme were also calculated.

A review on spectrophotometric methods for measuring the monophenolase and diphenolase activities of tyrosinase

Tyrosinase is a copper enzyme with broad substrate specifity toward a lot of phenols with different biotechnological applications. The availability of quick and reliable measurement methods of the enzymatic activity of tyrosinase is of outstanding interest. A series of spectrophotometric methods for determining the monophenolase and diphenolase activities of tyrosinase are discussed. The product of both reactions is the o-quinone of the corresponding monophenol/diphenol. According to the stability and properties of the o-quinone, the substrate is classified as four substrate types. For each of these substrate types, we indicate the best method for measuring diphenolase activity (among eight methods) and, when applicable, for measuring monophenolase activity (among four methods). The analytical and numerical solutions to the system of differential equations corresponding to the reaction mechanism of each case confirm the underlying validity of the different spectrophotometric methods proposed for the kinetic characterization of tyrosinase in its action on different substrates.

Immobilization of tyrosinase and alcohol oxidase in conducting copolymers of thiophene functionalized poly(vinyl alcohol) with pyrrole

Immobilization of tyrosinase and alcohol oxidase is achieved in the copolymer of pyrrole with vinyl alcohol with thiophene side groups (PVATh-co-PPy) which is a newly synthesized conducting polymer. PVATh-co-PPy/alcohol oxidase and PVATh-co-PPy/tyrosinase electrodes are constructed by the entrapment of enzyme in conducting copolymer matrix during electrochemical copolymerization. For tyrosinase and alcohol oxidase enzymes, catechol and ethanol are used as the substrates, respectively. Kinetic parameters: maximum reaction rates (V(max)) and Michaelis-Menten constants (K(m)) are obtained. V(max) and K(m) are found as 2.75 micromol/(minelectrode) and 18 mM, respectively, for PVATh-co-PPy/alcohol oxidase electrode and as 0.0091micromol/(minelectrode) and 40 mM, respectively, for PVATh-co-PPy/tyrosinase electrode. Maximum temperature and pH values are investigated and found that both electrodes have a wide working range with respect to both temperature and pH. Operational and storage stabilities show that although they have limited storage stabilities, the enzyme electrodes are useful with respect to operational stabilities.

An approximate analytical solution to the lag period of monophenolase activity of tyrosinase

Tyrosinase shows a lag period in its action on monophenols (l-tyrosine). We propose an approximate analytical solution for the lag period, which fulfils the dependences with regard to initial enzyme concentration, and initial monophenol concentration. Furthermore, from a study of the dependences of the lag period on these variables, we can determine experimentally the o-diphenol concentration in the steady state. The Michaelis constant of the o-diphenol in the presence of the monophenol can be determined from the relationship between the o-diphenol concentration in the steady state and the initial monophenol concentration, taking into consideration the experimentally calculated Michaelis constant for the monophenol substrate. Although this Michaelis constant is much lower than the Michaelis constant for diphenol in the absence of monophenol, the binding site is the same. A kinetic analysis of the action mechanism of tyrosinase explains this difference in the values of the Michaelis constants.

Mechanistic studies on the tyrosinase-catalyzed formation of the anachelin chromophore

The complex secondary metabolite anachelin, isolated from the freshwater cyanobacterium Anabaena cylindrica, is believed to act as siderophore, facilitating iron uptake. Its structure is characterized by a fascinating blend of polyketide, peptide, and alkaloid fragments. In particular, the tetrahydroquinolinium-derived chromophore is unique among natural products, and its biosynthesis is unknown. We propose a hypothesis for the biogenesis of the anachelin chromophore starting from a C-terminally bound L-Tyr residue. It is proposed that this amino acid is reductively aminated, methylated, and hydroxylated. Oxidation of this catechol diamine substrate by a tyrosinase would lead to an o-quinone, which would react by intramolecular aza-annulation and tautomerization to give the anachelin chromophore. In order to evaluate this hypothesis, a model substrate related to the proposed biogenetic precursor was prepared. It was shown that the enzyme tyrosinase is able to transform this substrate into an anachelin chromophore derivative, which corroborates the biogenetic hypothesis. In order to gain further insight into the mechanism of this transformation, we performed spectrophotometric reaction monitoring, allowing the formation of the expected product to be observed. In addition, a rise in absorption at around 250 nm might be due to the presence of a spiro five-membered ring intermediate resulting from an alternative 1,4-addition to the o-quinone. Lastly, we were able to show that the action of tyrosinase on this substrate follows Michaelis-Menten kinetics (k(cat)=123 s(-1) and K(m)=8.66 mM). Interestingly, the catalytic efficiency is decreased only by a factor of 30 relative to the natural substrate L-DOPA.

A Novel o-Aminophenol Oxidase Responsible for Formation of the Phenoxazinone Chromophore of Grixazone

Grixazone contains a phenoxazinone chromophore and is a secondary metabolite produced by Streptomyces griseus. In the grixazone biosynthesis gene cluster, griF (encoding a tyrosinase homolog) and griE (encoding a protein similar to copper chaperons for tyrosinases) are encoded. An expression study of GriE and GriF in Escherichia coli showed that GriE activated GriF by transferring copper ions to GriF, as has been observed for a Streptomyces melanogenesis system in which the MelC1 copper chaperon transfers copper ions to MelC2 tyrosinase. In contrast with tyrosinases, GriF showed no monophenolase activity, although it oxidized various o-aminophenols as preferable substrates rather than catechol-type substrates. Deletion of the griEF locus on the chromosome resulted in accumulation of 3-amino-4-hydroxybenzaldehyde (3,4-AHBAL) and its acetylated compound, 3-acetylamino-4-hydroxybenzaldehyde. GriF oxidized 3,4-AHBAL to yield an o-quinone imine derivative, which was then non-enzymatically coupled with another molecule of the o-quinone imine to form a phenoxazinone. The coexistence of N-acetylcysteine in the in vitro oxidation of 3,4-AH-BAL by GriF resulted in the formation of grixazone A, suggesting that the -SH group of N-acetylcysteine is conjugated to the o-quinone imine formed from 3,4-AHBAL and that the conjugate is presumably coupled with another molecule of the o-quinone imine. GriF is thus a novel o-aminophenol oxidase that is responsible for the formation of the phenoxazinone chromophore in the grixazone biosynthetic pathway.

Opposite effects of peroxidase in the initial stages of tyrosinase-catalysed melanin biosynthesis

The tyrosinase/oxygen enzymatic system catalyses the orthohydroxylation of L-tyrosine to L-dopa and the oxidation of this to dopaquinone, which evolves non-enzymatically towards to form melanins. The literature has demonstrated and revised the existence of peroxidase/hydrogen peroxide in the melanosomas of skin melanocytes, but points to controversy concerning the effects on melanogenesis. Some authors have recently proposed a new physiological function for tyrosinase, namely the direct scavenging of tyrosyl radicals, which are toxic oxidants of melanocytes. In this contribution, we describe and interpret four effects of peroxidase/hydrogen peroxide on melanogenesis. Two of these effects are its antagonism and synergy as regards the monophenolase and diphenolase activities, respectively, of tyrosinase/oxygen in the initial steps that trigger melanogenesis. Another effect concerns the increase in the oxidant character of the medium in the melanosome by increasing the synthesis of oxidising quinones (o-dopaquinone, p-topaquinone, dopachrome) and the consumption of antioxidant diphenols (L-dopa), which are intermediate biomolecules in melanogenesis. Lastly, we demonstrate that the tyrosyl radicals generated by light or by the peroxidase/hydrogen peroxide system are not directly trapped by the tyrosinase but by the antioxidant orthodiphenol, L-dopa, accumulated in the steady-state of melanogenesis. In conclusion, peroxidase/hydrogen peroxide may help regulate the development of melanogenesis and the oxidant environment within the melanosome. This enzyme deserves further study for its possible antitumoral and depigmentation capacities in skin cancer and hyperpigmentation.

Human melanocyte biology, toxicology, and pathology

The human melanocytes of the skin, hair, eyes, inner ears, and covering of the brain provide physiologic functions important in organ development and maintenance. Melanocytes develop from embryonic neural crest progenitors and share certain traits with other neural crest derivatives found in the adrenal medulla and peripheral nervous system. The distinctive metabolic feature of melanocytes is the synthesis of melanin pigments from tyrosine and cysteine precursors involving over 100 gene products. These complex biochemical mechanisms create inherent liabilities for melanocytic cells if intracellular systems necessary for compartmentalization, detoxification, or repair are compromised. Melanocyte disorders may involve pigmentation, sensory functions, autoimmunity, or malignancy. Environmental factors such as ultraviolet radiation and chemical exposures, combined with heritable traits, represent the principal hazards associated with melanocyte disorders.

Substrate share in the suicide inactivation of mushroom tyrosinase

To address the real cause of the suicide inactivation of mushroom tyrosinase (MT), under in vitro conditions, cresolase and catecholase reactions of this enzyme were investigated in the presence of three different pairs of substrates, which had been selected for their structural specifications. It was showed that the cresolase activity is more vulnerable to the inactivation. Acetylation of the free tyrosyl residues of MT did not cure susceptibility of the cresolase activity, but clearly decreased the inactivation rate of MT in the presence of 4-[(4-methylbenzo)azo]-1,2-benzenediol (MeBACat) as a catecholase substrate. Considering the results of the previous works and this research, some different possible reasons for the suicide inactivation of MT have been discussed. Accordingly, it was proposed that the interruption in the conformational changes in the tertiary and quaternary structures of MT, triggered by the substrate then mediated by the solvent molecules, might be the real reason for the suicide inactivation of the enzyme. However, minor causes like the toxic effect of the ortho-quinones on the protein body of the enzyme or the oxidation of some free tyrosyl residues on the surface of the enzyme by itself, which could boost the inactivation rate, should not be ignored.