The inhibitory effect of some new synthesized xanthates on mushroom tyrosinase activities

Abstract

Three iso-alkyldithiocarbonates (xanthates), as sodium salts, C₃H₇OCS₂Na (I), C₄H₉OCS₂Na (II) and C₅H₁₁OCS₂Na (III), were synthesized, by the reaction between CS₂ with the corresponding iso-alcohol in the presence of NaOH, and examined for inhibition of both cresolase and catecholase activities of mushroom tyrosinase (MT) from a commercial source of Agricus bisporus. 4-[(4-methylbenzo)azo]-1,2-benzendiol (MeBACat) and 4-[(4-methylphenyl)azo]-phenol (MePAPh) were used as synthetic substrates for the enzyme for the catecholase and cresolase reactions, respectively. Lineweaver-Burk plots showed different patterns of mixed and competitive inhibition for the three xanthates and also for cresolase and catecholase activities of MT. For cresolase activity, I and II showed a mixed inhibition pattern but III showed a competitive inhibition pattern. For catecholase activity, I showed mixed inhibition but II and III showed competitive inhibition. These new synthesized compounds are potent inhibitors of MT with K_i values of 9.8, 7.2 and 6.1 μM for cresolase inhibitory activity, and also 12.9, 21.8 and 42.2 μM for catecholase inhibitory activity for I, II and III, respectively. They showed a greater inhibitory potency towards the cresolase activity of MT. Both substrate and inhibitor can be bound to the enzyme with negative cooperativity between the binding sites (α>1) and this negative cooperativity increases with increasing length of the aliphatic tail in these compounds in both cresolase and catecholase activities. The cresolase inhibition is related to the chelating of the copper ions at the active site by a negative head group (S⁻ ) of the anion xanthate, which leads to similar values of K_i for all three xanthates. Different K_i values for catecholase inhibition are related to different interactions of the aliphatic chains of I, II and III with hydrophobic pockets in the active site of the enzyme.

Keywords::

• Mushroom tyrosinase
• n-alkyl xanthate
• mixed inhibition
• competitive inhibition
• inhibition constant

Introduction

Tyrosinase (MT) (monophenol mono-oxygenase; polyphenol oxidase; catechol oxidase; and oxygen oxidoreductase; EC 1.14.18.1) is a bifunctional enzyme, which catalyzes ortho-hydroxylation of monophenols (cresolase activity) and oxidation of catechols to the corresponding ortho-quinones (catecholase activity). Tyrosinase is a copper-containing enzyme, responsible for the formation of pigment in the skin, hair, and eye 1-7. o-Quinones follow some other enzymatic and nonenzymatic reactions, which result in formation of biopolymers like melanin [8,9]. Tyrosinases are widely distributed among animals, plants and fungi [3,10]. They are responsible for many biologically essential functions, such as pigmentation, sclerotization, primary immune response and host defense [11,12]. In mushroom (Agaricus bisporus), as well as in fruits and vegetables, the enzyme is responsible for browning, a commercially undesirable phenomenon [10,13,14]. Common mushroom tyrosinase (MT) from the species Agraricus bisporus has a molecular mass of 120 kD, is composed of two H subunits (43 kD) and two L subunits (13 kD) and contains two active sites [15,16]. Its active site has a di-copper center, resembling that of hemocyanins 12-17 but not identical [18]. Each copper ion in the active site is coordinated by three nitrogen atoms coming from three adjacent histidine residues and the enzyme can exist in three forms; met, oxy, and deoxy [19,20]. Mettyrosinase contains two tetragonal Cu(II) ions antiferromagnetically coupled through an endogenous bridge, although hydroxide exogenous ligands other than peroxide are bound to the copper site. This species can be converted by addition of peroxide to oxytyrosinase, which in turn decays back to mettyrosinase when the peroxide is lost. Oxytyrosinase also consists of two tetragonal Cu(II) atoms, each coordinated by two strong equatorial and one weaker axial ligands. The exogenous oxygen molecule is bound as peroxide and bridges the two Cu centers. Deoxytyrosinase has a bicuprous structure (Cu(I))-Cu(I)). Structural models for their active site have been proposed [12,21,22]. In the food industry, tyrosinase is a very important enzyme in controlling the quality and economics of fruits and vegetables [23,24]. Tyrosinase is responsible for the enzymatic browning of fruits and vegetables. In addition to the undesirable color and flavor, the quinone compounds produced in the browning reaction may irreversibly react with the amino and sulfhydryl groups of proteins. The quinone-protein reaction decreases the digestibility of the protein and the bioavailability of essential amino acids, including lysine and cysteine. Therefore, development of high-performance tyrosinase inhibitors is much needed in the agricultural and food fields [25]. Tyrosinase inhibitors have attracted interest recently due to undesired browning in vegetables and fruits in post-harvest handling [26]. Additionally, tyrosinase inhibitors may be clinically used for treatment of some skin disorders associated with melanin hyper-pigmentation and are also important in cosmetics for skin whitening effects 27-30. Also tyrosinase may play a role in neuromelanin formation in the human brain and is central to dopamine neurotoxicity as well as contribute to the neurodegeneration associated with Parkinson's disease [31]. There is a vast variety of natural and synthetic inhibitors known against catecholase, cresolase or both reactions of tyrosinase. Polyphenols, aldehydes and their derivatives are the most important inhibitors from plant natural sources 32-36. Besides higher plants, some compounds from fungal sources have also been identified, e.g. metallothionein from Aspergillus niger has strong avidity to chelate copper at the active site of MT, thereby acting as a strong inhibitor [37]. Kojic acid, an antibiotic, produced by species of Aspergillus and Penicillium in the aerobic process acts as a potent, “slow-binding”, competitive inhibitor of tyrosinase 38-42, and is widely used as a cosmetic whitening agent 43-45. Synthetic tyrosinase inhibitors currently used as drugs are captopril, an antihypertensive drug, and methimazole [46,47]. Simple chemical species capable of binding to copper, such as cyanide, azide, and halide ions behave, as expected, as purely competitive inhibitors towards dioxygen binding, even if sharp differences have been seen among polyphenoloxidases from different sources [48]. Sulfur-containing compounds such as tiron, thiol and sulfites are among the most important tyrosinase inhibitors, the most commonly applied inhibitor of the discoloration process currently being sulfite [26].

To understand the mechanism of the enzyme's action and inhibition, we have attempted to obtain additional information about the structure, function and relationship of MT 49-53. After introducing two new bi-pyridine synthetic compounds as potent uncompetitive MT inhibitors [54], the inhibitory effects of three synthetic n-alkyl dithiocarbamates, with different tails, were elucidated [55]. The binding process for catecholase inhibition by benzenethiol showed the predominance of hydrophobic interaction in the active site of the enzyme, whereas electrostatic interaction can be important for cresolase inhibition [56]. Understanding the role of hydrophobic and electrostatic interactions of inhibitor binding to the active site of the enzyme can lead to the design of new potent MT inhibitors. Hence, in the present investigation the inhibitory effects of three new synthesized alkyl xanthates, sodium salts, with different aliphatic tails, of C3, C4 and C5, are described and the kinetics of their inhibition has been elucidated for both cresolase and catecholase activities.

Materials and methods

Materials

Mushroom tyrosinase (MT; EC 1.14.18.1; specific activity 3400 units/mg) was purchased from Sigma. 4-[(4-methylbenzo)azo]-1,2-benzendiol (MeBACat) (Figure 1a) and 4-[(4-methylphenyl)azo]-phenol (MePAPh) (Figure 1b), as synthetic substrates for the enzyme for catecholase and cresolase reactions, respectively, were prepared as previously described [57].

Figure 1 (a) 4-[(4-methylbenzo)azo]-1,2-benzendiol (MeBACat) and (b) 4-[(4-methylphenyl)azo]-phenol (MePAPh) (b), as synthetic substrates of MT for catecholase and cresolase reactions, respectively. (c) Iso-propyl xanthate (I), iso-butyl xanthate (II) and iso-pentyl xanthate (III), sodium salts (c) as three new MT inhibitors.

Iso-propyl xanthate (I), iso-butyl xanthate (II) and iso-pentyl xanthate (III), sodium salts (Figure 1c), were synthesized. iso-Propanol, iso-butanol, iso-pentanol, carbon disulfide and sodium hydroxide were purchased from Merck Chemical Co. Germany and used as received. Infrared spectra were obtained on a Nicolet 5-DXB FT-IR spectrophotometer in the range 4000-400 cm^{− 1} in KBr pellets. Microchemical analysis of carbon and hydrogen were carried out on CHN Rapid Herause. ¹H NMR spectra were recorded on a Brucker DRX-500 Avance spectrophotometer at 500 MHz in DMSO-d₆ using sodium-3-trimethylpropionate as internal reference. ¹H NMR data are expressed in part per million (ppm) and are reported as chemical shift position (δH), multiplicity (s = singlet, d = doublet, t = triplet, q = quartet, m = multiple) and assignment. Melting points were measured on a Unimelt capillary melting point apparatus and are reported uncorrected.

Phosphate buffer (10 mM, pH 6.8) was used throughout this work and the corresponding salts were obtained from Merck. All experiments were carried out at a temperature of 20°C.

Methods

Synthesis of sodium iso-propyl xanthate (I)

This compound was prepared by an improved procedure as compared to that given in the literature [58]. 4 g (100 mmol) NaOH and 20.3 mL (100 mmol) iso-propyl alcohol were mixed together and stirred to get homogenous curdy solution. The mixture was kept in an ice bath and 20 mL of CS₂ (200 mmol) was added dropwise with constant stirring over a period of 30 min, the solution becoming cloudy yellow. Then the mouth of the reaction vessel was closed using a proper stopper and the mixture left to stir for 1 h in the ice bath and 2 h at room temperature. This crude product was completely dried at 35°C, powdered in a mortar and the powder stirred with 30 mL acetone for 15 min, and then filtered to remove undissolved particles. To the filtrate 40 mL diethylether was added and the mixture kept in the refrigerator overnight. The bright yellow crystals obtained were filtered and washed twice with ether and dried at 35°C. (yield 11.85 g, 75 mmol, 75%; decomposes at 127°C). ¹H NMR (500 MHz, DMSO-d₆, ppm): 5.42 (m, O-CH), 1.14 (d, O-CH(CH₃)₂). Analysis Calculated for C₄H₇OS₂Na: C, 30.38; H, 4.31. Found: C, 30.41; H, 4.26%. Solid-state IR spectroscopy of xanthates shows two characteristic bands between 1189 and 1070 cm^{− 1} 58-60 assigned to ν_c-o and ν_c-s modes. Sodium iso-propyl xanthate shows similar bands at 1187 and 1035 cm^{− 1}.

Synthesis of sodium iso-butyl xanthate (II)

This compound was prepared by following the same procedure as described for (CH₃)₂CHOCSSNa except that iso-butanol 21.50 ml (100 mmol) was used instead of iso-propanol. (yield 14.79 g, 86 mmol, 86%; decomposes at 105°C). ¹H NMR (500 MHz, DMSO-d₆, ppm): 3.95 (m, O-CH₂), 1.88 (m, O-CH₂-CH), 0.86 (d, O-CH₂-CH(CH₃)₂). Analysis Calculated for C₅H₉OS₂Na: C, 34.88; H, 5.23. Found: C, 34.52; H, 5.12%. Solid-state IR spectroscopy of showed bands at 1158 and 1076 cm^{− 1}.

Synthesis of sodium iso-pentyl xanthate (III)

This compound was prepared by following the same procedure as described for (CH₃)₂CHOCSSNa except that iso-pentanol (22.68 ml, 100 mmol) was used instead of iso-propanol (yield 15.63 g, 84 mmol, 84%; decomposes at 88°C). ¹H NMR (500 MHz, DMSO-d₆, ppm): 4.20 (t, O-CH₂), 1.46 (m, O-CH₂-CH₂), 1.63 (m, O-CH₂-CH₂-CH), 0.86 (d, O-CH₂-CH₂-CH(CH₃)₂). Analysis Calculated for C₆H₁₁OS₂Na: C, 38.71; H, 5.1. Found: C, 38.82; H, 5.2%. Solid state IR spectroscopy showsed two characteristic bands at 1158 and 1074 cm^{− 1}.

Kinetic measurements

Kinetic assay of catecholase and cresolase activities was carried out through depletion of MeBACat and MePAPh, respectively, for 1 and 2 min, with enzyme concentrations of 11.11 and 112.68 μg/mL, at wavelengths of 473 nm and 352 nm using a Cary spectrophotometer, 100 Bio-model, with jacketed cell holders. Freshly prepared enzyme, substrate, I, II, and III were used in this work. All enzymatic reactions were run in phosphate buffer (10 mM) at pH 6.8 in a conventional quartz cell, thermostated to maintain the temperature at 20 ± 0.1°C. Substrate addition followed after incubation of enzyme with different concentrations of the n-alkyl xanthate.

Results and discussion

The inhibitory effects of three different ligands on both MT activities were examined at pH 6.8 and a temperature of 20°C.

Kinetic parameters of cresolase activity of MT in the presence of I, II and III:

Double reciprocal Lineweaver-Burk plots for the cresolase activity of MT on hydroxylation of MePAPh, as the substrate, in the presence of different fixed concentrations of I, II and III are shown in Figures 2a, 4, respectively. These plots show a set of straight lines, which intersect on the left hand side of the vertical axis, close to the horizontal axis for I (see Figure 2a) and a little further from the horizontal axis for II (Figure 3a), which confirms mixed inhibition. The apparent maximum velocity (V_max′) and apparent Michaelis constant (K_m′) values as well as the slope values of these straight lines (K_m′/V_max′) can be obtained at different fixed concentrations of each inhibitors (I and II). A secondary plot of the slope against the concentration of inhibitor gives a straight line with an abscissa-intercept of –K_i (see Figures 2b and 3b for I and II, respectively) and also another secondary plot of the reciprocal apparent maximum velocity against the concentration of inhibitor gives a straight line with an abscissa-intercept of -αK_i (see Figures 2c and 3c for I and II, respectively), where K_i is the inhibition constant and α is the interaction factor between the substrate and inhibitor sites. Double reciprocal Lineweaver-Burk plot for III gives a set of straight lines intersecting exactly on the vertical axis, the value of the maximum velocity (V_max) is unchanged by the inhibitor but the K_m′ value is increased, confirming competitive inhibition for III (see Figure 4). The inset on Figure 4 shows the secondary plot of the K_m′ at any concentration of inhibitor (III) versus the concentration of inhibitor, which gives the inhibition constant ( − K_i) from the abscissa-intercepts.

Figure 2 (a) Double reciprocal Lineweaver-Burk plots of MT kinetics assays for cresolase reactions of MePAPh in 10 mM phosphate buffer, pH 6.8, at temperature of 20°C and 112.68 μg/mL enzyme concentration, in the presence of different concentrations of I: 0 mM (▪), 0.02 mM (◊), 0.0225 mM (▴), 0.025 mM (○), 0.03 mM (•). (b) Secondary plot of the slope against the concentration of inhibitor, which gives the − K_i from the abscissa-intercept. (c) Secondary plot of 1/V_max′ versus concentration of inhibitor, which gives − αK_i from the abscissa-intercept.

Figure 3 (a) Double reciprocal Lineweaver-Burk plots of MT kinetic assay for cresolase reactions of MePAPh in 10 mM phosphate buffer, pH 6.8, at temperature of 20°C and 112.68 μg/mL enzyme concentration, in the presence of different concentrations of II: 0 mM (▪), 0.0175 mM (◊), 0.02 mM (▴), 0.0225 mM (○), 0.025 mM (•). (b) Secondary plot of the slope against the concentration of inhibitor, which gives the − K_i from the abscissa-intercept. (c) Secondary plot of 1/V_max′ versus concentration of inhibitor, which gives − αK_i from the abscissa-intercept.

Figure 4 Double reciprocal Lineweaver-Burk plots of MT kinetic assay for cresolase reactions of MePAPh in 10 mM phosphate buffer, pH 6.8, at temperature of 20°C and 112.68 μg/mL enzyme concentration, in the presence of different concentrations of III: 0 mM (▪), 0.0125 mM (◊), 0.015 mM (▴), 0.0175 mM (○), 0.02 mM (•). Inset: secondary plot of the slope against different concentrations of inhibitor, which gives the inhibition constant ( − K_i) from the abscissa-intercept.

Results for K_i and α values for the cresolase activity of MT in the presence of I, II and III have been summarized in Table I. The α value for I is 1.2. An α value equal to one means that the mode of inhibition is noncompetitive. In noncompetitive inhibition there is no interaction between substrate and inhibitor binding sites. Hence, there is a weak interaction, negative cooperativity, between substrate and I binding sites causing some deviation for α value from one (i.e. it is equal to 1.2). The α value for II is 4.1 which means that the negative cooperativity between substrate and inhibitor has been increased due to the addition of a –CH₂– group to the molecular structure of the ligand. Addition of a further –CH₂– group has led to competitive inhibition for III with an infinite value for α which means that the interaction between substrate and inhibitor binding sites is so high that only substrate or inhibitor can bind to the enzyme. As shown in Table I, the largest value of K_i is for I (9.8 μM) and the smallest is for III (6.1 μM). So the affinity of inhibitor binding to the enzyme does not change largely from I to III; it changes slightly in the order of I < II < III. Although extending the size of the inhibitor molecule by adding a –CH₂–group causes more negative cooperativity in the binding sites of substrate and inhibitor, the affinity of binding is slightly increased.

Table I.  Thermodynamic parameters of binding 2-Propanol dithioxantate (I), iso-butanol dithioxantate (II) and iso-pentanol dithioxantate (III), sodium salts, on mushroom tyrosinase at temperature 20°C and pH 6.8.

Reaction type	Ligands	Ka (M)^− 1	Ki (μM)	ΔG°(kJ mol)^− 1	α
Catecholase activity	I	7.8 × 10^4	12.9	− 27.4	2.0
	II	4.6 × 10^4	21.8	− 26.1	∞
	III	2.3 × 10^4	42.2	− 24.6	∞
Cresolase activity	I	1.0 × 10^5	9.8	− 28.1	1.2
	II	1.4 × 10^5	7.2	− 28.9	4.1
	III	1.6 × 10^5	6.1	− 29.2	∞

K_a: association constant; K_i : inhibition constant; α: interaction factor

Kinetic parameters of catecholase activity of MT in the presence of I, II and III

Double reciprocal Lineweaver-Burk plots for the catecholase activity of MT on oxidation of MeBACat as substrate, in the presence of different fixed concentrations of I, II and III at pH 6.8 and temperature 20°C are shown in Figures 5a, 7, respectively. This plot for I gives a set of straight lines, which intersect on the left hand side of the vertical axis, close to the horizontal axis (see Figure 5a), which confirms mixed inhibition. Secondary plots of the slope against the concentration of inhibitor and the reciprocal of apparent maximum velocity against the concentration of inhibitor give straight lines with abscissa-intercepts of –K_i (see Figure 5b) and − αK_i (see Figure 5c), respectively. Double reciprocal Lineweaver-Burk plots for II and III give a set of straight lines intersecting exactly on the vertical axis, confirming competitive inhibition (see Figures 6 and 7 for II and III, respectively). The insets on Figures 6 and 7 show the secondary plots, which give the inhibition constants ( − K_i) from the abscissa-intercepts.

Figure 5 (a) Double reciprocal Lineweaver-Burk plots of MT kinetic assay for catecholase reactions of MeBACat in 10 mM phosphate buffer, pH 6.8, at temperature of 20°C and 11.11 μg/mL enzyme concentration, in the presence of different concentrations of I: O mM (▪), 0.01 mM (◊), 0.0175 mM (▴), 0.025 mM (○), 0.0325 mM (•). (b) Secondary plot of the slope against the concentration of inhibitor, which gives the − K_i from the abscissa-intercept. (c) Secondary plot of 1/V_max′ versus concentration of inhibitor, which gives − αK_i from the abscissa-intercept.

Figure 6 Double reciprocal Lineweaver-Burk plots of MT kinetic assay for catecholase reactions of MeBACat in 10 mM phosphate buffer, pH 6.8, at temperature of 20°C and 11.11 μg/mL enzyme concentration, in the presence of different concentrations of II: 0 mM (▪), 0.0075 mM (◊), 0.015 mM (▴), 0.0225 mM (○), 0.03 mM (•). Inset: secondary plot of the slope against different concentrations of inhibitor, which gives the inhibition constant ( − K_i) from the abscissa-intercept.

Figure 7 Double reciprocal Lineweaver-Burk plots of MT kinetic assay for catecholase reactions of MeBACat in 10 mM phosphate buffer, pH 6.8, at temperature of 20°C and 11.11 μg/mL enzyme concentration, in the presence of different concentrations of III: 0 mM (▪), 0.0025 mM (◊), 0.01 mM (▴), 0.0175 mM (○), 0.025 mM (•). Inset: secondary plot of the slope against different concentrations of inhibitor, which gives the inhibition constant ( − K_i) from the abscissa-intercept.

Results for K_i and α-values of catecholase activity of MT in the presence of I, II and III have been summarized in Table I. The α-value for I is 2 which means that there is negative cooperativity between substrate and inhibitor binding sites but this interaction is not very strong. Infinite α-values for II and III mean that by adding one or two –CH₂– groups to the molecular structure of ligand the interaction between substrate and inhibitor binding sites becomes so high that only substrate or inhibitor can bind to the enzyme. As shown in Table I, the greatest value of K_i is for III (42.2 μM) and the smallest for I (12.9 μM). The affinity of inhibitor binding to the enzyme is in the order III < II < I. Although extending the size of the inhibitor molecule by adding a –CH₂–group causes more negative cooperativity in the binding sites of substrate and inhibitor, the affinity of binding is decreased.

The change of the standard Gibbs free energy of binding (ΔG°) for each inhibitor was calculated using the association binding constant (K_a), obtained from the inverse of the K_i value, in the equation ΔG° = − RT ln K_a; where R is the gas constant and T is the absolute temperature [61]. The calculated ΔG° values of the three ligands for cresolase and catecholase activities are summarized in Table I. The inhibitor binding process is spontaneous (ΔG˚ < 0) in all cases, although for cresolase inhibition the binding process occurs more favorably.

Some thiol compounds acts as inhibitors of tyrosinase due to their ability to chelate Cu⁺² [62]. All of our new synthesized compounds also act as inhibitors. Each n-alkyl xanthate (sodium salt) produces an anion with a head S⁻ group and a hydrophobic tail. The comparison of K_i and ΔG° values in Table I reveals that these three ligands have inhibited the cresolase activity more strongly than the catecholase activity of MT. As reported previously [56], here our results also show that the type of inhibitor binding process is different in the two types of MT activities. The predominant interaction in the active site of the enzyme for the binding process of catecholase inhibition is hydrophobic, whereas the electrostatic interaction can be important for cresolase inhibition [56]. Hence, we expect all three xanthates to have approximately identical K_i values for cresolase activity due to the same charged head group. However, different values of K_i for catecholase activity are expected due to different tails for these three xanthates. Moreover, K_i values increase in magnitude as the length of the hydrophobic tail increases for these compounds, which means that a shorter tail gives a more potent inhibitor. This result is the same as our previous result [55]. Hydrophobic interaction between the tail of the ligand and hydrophobic pockets in the active site of the enzyme reduces the affinity of inhibitor binding in the catecholase inhibition. The cresolase activity occurs when enzyme is in the oxy-form and the catecholase activity occurs when the enzyme is in the met-form, so maybe the set of hydrophobic pockets in the oxy-form are different from these in the met-form. The structure of hydrophobic pockets of the enzyme is energetically favorable for inhibitor binding with a shorter hydrophobic chain in the catecholase activity. Our results here may assist future aims in the design of inhibitors to prevent undesirable fruit browning in vegetables or as color skin modulators in mammals.

Acknowledgements

The financial support given by the University of Tehran and the Iran National Science Foundation (INSF) are gratefully acknowledged.