Steady-state study of the mechanism of dopa-oxidase activity of tyrosinase

Synthesis, Kinetic Study, and Spectroscopic Analysis of Peroxidase-like Pinch-Porphyrin Fe(III) Complexes

In the present manuscript, we report the kinetic and spectroscopic analysis of six new pinch-porphyrins: protoporphyrin-picpenta 1, mesoporphyrin-picpenta 2, deuteroporphyrin-picpenta 3, protoporphyrin-picocta 4, mesoporphyrin-picocta 5, and deuteroporphyrin-picocta 6. The Michaelis-Menten enzymatic pathway and the guaiacol test confirmed the ability of the compounds to function like new peroxidase models. UV-vis, ¹H NMR, and electron spin resonance studies are in accordance with porphyrin-Fe(III) molecules with the quantum phenomena called quantum mixed spin (qms, s = 3/2, s = 5/2). Importantly, the influence of the presence of the s = 3/2 spin state in the compounds and its critical role for the catalytic capacity is proven here, which was the original hypothesis in our research group. The compounds with higher populations of the s = 3/2 spin state have increased peroxidase activity.

Stability, structural and suicide inactivation changes of Mushroom tyrosinase after acetylation by N-acetylimidazole

Modification (acetylation) of Tyr residues with N-acetylimidazole protects outstandingly mushroom tyrosinase (MT) from the suicide inactivation in the presence of its catecholic substrate, 4-[(4-methylbenzo) azo]-1,2-benzenediol. UV spectrophotometric experiments and differential scanning calorimetry (DSC) studies indicated a decrease in kinetic stability of the enzyme alongside with increase in its thermal stability as well as its stability against n-dodecyl trimethylammonium bromide as a denaturizing agent. Pace analysis resulted in standard Gibbs free energy values of 46.54 and 52.09 kJ/mol in the absence of denaturant for native and modified enzyme, respectively. Structural studies by circular dichroism (CD) spectrophotometry showed that modification did not have major impact on the secondary structure of MT; however, induced some changes in its tertiary structure. The near-UV CD results revealed that the modification had enhanced intramolecular van der Waals interactions in the enzyme structure, which was in coincidence with its thermodynamic stability.

Application of a normalised plot to the study of ter ter enzyme systems

The kinetic characterisation of multisubstrate systems is not a trivial task. Common approaches simplify the experimental procedures by sequentially fixing saturating concentrations of different substrates/products, thereby attempting to isolate the influence of the varying molecule. Even after such tedious work, only apparent Km values can be determined, preventing serious comparison among differential substrate behaviours. Moreover, the choice among rival kinetic models is not statistically guaranteed; instead, classical tools such as re-plots continue to be used. Here, we report the application of a normalisation of kinetic data, formerly applied to simpler systems, to the description of ter ter systems. This data treatment is able to provide true Km values and a reliable description of the system, at the same time reducing the experimental work and statistically supporting the choice of kinetic schemes.

Tyrosinase action on monophenols: evidence for direct enzymatic release of o-diphenol

Using gas chromatography-mass spectrometry, the direct enzymatic release of o-diphenol (4-tert-butylcatechol) during the action of tyrosinase on a monophenol (4-tert-butylphenol) has been demonstrated for the first time in the literature. The findings confirm the previously proposed mechanism to explain the action of tyrosinase on monophenols (J.N. Rodríguez-López, J. Tudela, R. Varón, F. García-Carmona, F. García-Cánovas, J. Biol. Chem. 267 (1992)). Oxytyrosinase, the oxidized form of the enzyme with a peroxide group, is the only form capable of catalysing the transformation of monophenols into diphenols, giving rise to an enzyme-substrate complex in the process. The o-diphenol formed is then released from the enzyme-substrate complex or oxidized to the corresponding o-quinone. In order to detect the enzymatic release of o-diphenol, the non-enzymatic evolution of the o-quinone to generate o-diphenol by weak nucleophilic attack reactions and subsequent oxidation-reduction was blocked by the nucleophilic attack of an excess of cysteine. Furthermore, the addition of catalytic quantities of an auxiliary o-diphenol (e.g. catechol) considerably increases the accumulation of 4-tert-butylcatechol. The enzyme acting on 4-tert-butylphenol generates the enzyme-4-tert-butylcatechol complex and 4-tert-butylcatechol is then released (with k(-2)) generating mettyrosinase. The auxiliary o-diphenol added (catechol) and the 4-tert-butylcatechol generated by the enzyme then enter into competition. When [catechol] >> [4-tert-butylcatechol], the enzyme preferentially binds with the catechol to close the catalytic cycle, while 4-tert-butylcatechol is accumulated in the medium. In conclusion, we demonstrate that the enzyme produces 4-tert-butylcatechol from 4-tert-butylphenol, the concentration of which increases considerably in the presence of an auxiliary o-diphenol such as catechol.

Dimethyl sulfide, a volatile flavor constituent, is a slow-binding inhibitor of tyrosinase

In this paper, the inhibition of tyrosinase by a volatile compound is kinetically analyzed for the first time. The results obtained show that the volatile flavor constituent dimethyl sulfide (DMS) inhibits the catecholase activity of tyrosinase in a nonclassical manner. A decrease in the initial velocity to a inhibited steady-state velocity can be observed within a few minutes. This time dependence, which is unaltered by prior incubation of the enzyme with the inhibitor, is consistent with a first-order transition. Both the initial and the constant rates decreased with increasing concentrations of inhibitor. The kinetic data obtained correspond to those for a postulated mechanism involving rapid formation of an enzyme-inhibitor complex that subsequently undergoes a relatively slow reversible reaction. These results, together with the high levels of DMS precursor in certain organisms, suggest a physiological role for this compound within plant tissues.

Oxymetric and spectrophotometric study of the ascorbate oxidase activity shown by frog epidermis tyrosinase

Many studies concerning the effect of ascorbic acid on the action of tyrosinase on several substrates have been carried out with contradictory results. The results shown in this work comprise a hypothetical reaction mechanism, which explains the ascorbate oxidase activity of frog epidermis tyrosinase. The reaction between frog epidermis tyrosinase and L-ascorbic acid was studied by oxymetric and spectrophotometric assays. The activity was linearly related to enzyme concentration, with a Michaelis constant for L-ascorbic acid of 0.160 +/- 0.009 mM and Vmax of 90 +/- 4 nM/s. Maximum activity was obtained at pH 7.5. The stoichiometry of the reaction was calculated by measuring the substrate (O2 and L-ascorbic acid) consumption as well as the initial rates of the consumption of oxygen and the disappearance of L-ascorbic acid. The stoichiometry was found to be 1:2 (O2:L-ascorbic acid). The action of the tyrosinase inhibitor tropolone was also studied. All the results present evidence concerning the ascorbate oxidase activity of frog epidermis tyrosinase and a possible reaction mechanism based on the different enzymatic forms of tyrosinase to explain such activity.

Kojic acid, a cosmetic skin whitening agent, is a slow-binding inhibitor of catecholase activity of tyrosinase

It was found that kojic acid, which is used in cosmetics for its excellent whitening effect, inhibits catecholase activity of tyrosinase in a non-classical manner. A decrease in the initial velocity to a steady-state inhibited velocity can be observed over a few minutes. This time-dependence, which is unaltered by prior incubation of the enzyme with the inhibitor, is consistent with a first-order transition. The kinetic data obtained correspond to those for a postulated mechanism that involves the rapid formation of an enzyme inhibitor complex that subsequently undergoes a relatively slow reversible reaction. Kinetic parameters characterizing this type of inhibition were evaluated by means of nonlinear regression of product accumulation curves.

The effect of pH on the suicide inactivation of frog epidermis tyrosinase

This paper presents a new reaction mechanism for the effect of the pH on the suicide inactivation of the diphenolase activity of tyrosinase. The applicability of the mechanism is supported by the experimental characterization of the kinetic behaviour of the frog epidermis enzyme acting on catechol, L-dopa and alpha-methyldopa at several pH values. Two enzyme froms 'met-' and 'oxy-' tyrosinase, but no their corresponding enzyme-diphenol complexes, present one ionizable group with very similar value of Ka which has been determined. The highest values of catalytic and inactivation efficiencies correspond to alpha-methyldopa and catechol, respectively. These kinetic studies have been carried out by using the transient phase approach previously developed, with negligible substrate consumption during the assay time. That illustrate the usefulness of the method for multisubstrate enzyme reactions.

Reverse vesicles as a new system for studying enzymes in organic solvents

A new approach to the study of enzyme activity in organic solvents has been developed by using optically transparent reverse vesicles. Polyphenol oxidase was incorporated in an active form into the above ternary system formed by the non-ionic surfactant tetra(ethylene glycol) dodecyl ether/n-dodecane/water. The enzyme in this microenvironment, surprisingly, showed an apparent positive co-operativity which has never before been described either in aqueous solution or in reverse micelles. In addition, the Vmax. expressed was similar to that in water and twice that displayed in reverse micelles.

Calibration of a Clark-Type oxygen electrode by tyrosinase-catalyzed oxidation of 4-tert-butylcatechol

A procedure for calibrating a Clark-type oxygen electrode is described. This method is based on the oxidation of 4-tert-butylcatechol (TBC) by O2 catalyzed by tyrosinase, to yield 4-tert-butyl-o-benzoquinone (TBCQ). This reaction consumes known amounts of oxygen in accordance with the stoichiometry: 2TBC + O2----2TBCQ + 2H2O and can be used to determine the relation between the oxygen concentration and the oxygen electrode response. TBCQ is very stable in the reaction medium for more than 30 min and shows no significant breakdown, which makes the calibration possible. A kinetic study of the oxidation of 3,4-dihydroxyphenylalanine by tyrosinase using the oxygen electrode is shown to confirm the validity of the calibration method.

A kinetic study of irreversible enzyme inhibition by an inhibitor that is rendered unstable by enzymic catalysis. The inhibition of polyphenol oxidase by L-cysteine

A kinetic study of the irreversible inhibition of an enzyme by an inhibitor that is depleted in the medium by its reaction with the product of enzymic analysis was made. The model is illustrated by the study of the inhibition of catecholase activity of polyphenol oxidase by L-cysteine. The inhibition is characterized by an initial lag period followed by a concomitant decrease in enzymic activity expressed when the steady state is reached, both kinetic parameters being modulated by enzyme, substrate and inhibitor concentrations. There is no analytical solution to the non-linear differential-equation system that describes the kinetics of the reaction, and so computer simulations of this dynamic behaviour are presented. The results obtained show that the system here studied presents kinetic co-operativity for a target enzyme that follows the simple Michaelis-Menten mechanism in its action on the substrate.

Effect of pH on the oxidation pathway of dopamine catalyzed by tyrosinase

The oxidation of 3,4-dihydroxyphenylethylamine (dopamine) by O2 catalyzed by tyrosinase yields 4-(2-aminoethyl)-1, 2-benzoquinone (o-dopaminequinone), which evolves nonenzymatically through two branches or sequences of reactions, whose respective operations are determined by the pH of the medium. The cyclization branch of o-dopaminequinone takes place in the entire range of pH and is the only significant branch at pH greater than or equal to 6. The hydroxylation branch of o-dopaminequinone only operates significantly at pH less than 6, and involves the accumulation of 2,4,5-trihydroxyphenylethylamine (6-hydroxydopamine) and 5-(2-aminoethyl)-2-hydroxy-1,4-benzoquinone (p-topaminequinone), identified from cyclic voltammetry assays. The kinetic characterization of the hydroxylation branch of o-dopaminequinone has been carried out by spectrophotometric and oxymetric assays. The successful fitting of data to the kinetic behavior predicted by the kinetic analysis at both pH greater than or equal to 6 and pH less than 6 confirms the overall oxidation pathway proposed for the dopamine oxidation catalyzed by tyrosinase. The antitumoral power of dopamine is possibly enhanced by the high cytotoxicity of 6-hydroxydopamine and p-topaminequinone, accumulated at the acidic pH characteristic of melanosomes and melanome cells.

A continuous spectrophotometric method for the determination of diphenolase activity of tyrosinase using 3,4-dihydroxymandelic acid

A continuous spectrophotometric method for the rapid determination of diphenolase activity of tyrosinase is described. It uses 3,4-dihydroxymandelic acid (DOMA) as the substrate of tyrosinase and measures the final product, 3,4-dihydroxybenzaldehyde (DOBA). The spectrum of this product shows a bathochromic displacement of its absorbance maximum when the pH increases. The optimization of the method is described by using tyrosinase from several biological sources, whose enzymatic activities show different optimal pH. Thus, the enzymatic activity of mushroom tyrosinase was assayed at pH 7.5 and monitored at 350 nm (epsilon 350 pH 7.5 (DOBA) = 15,200 M-1 cm-1), whereas the spectrophotometric experiments with grape tyrosinase were carried out at pH 3.0 and monitored at 310 nm (epsilon 310 pH 3.0 (DOBA) = 9200 M-1 cm-1). The method for mushroom tyrosinase was found to be 50-fold more sensitive than the commonly used dopachrome assay, whereas for grape tyrosinase the method was found to be threefold more sensitive than the commonly used o-quinone production assay. The great solubility and stability of the chromophoric product, DOBA, as well as its high molar absorptivities at any pH, enable the method to be employed to determine the diphenolase activity of tyrosinase from different biological sources.

Photoprotection by melanin

This paper is an attempt to summarize the current state of information on melanin and epidermal melanin pigmentation (EMP) as photoprotective agents. The chemistry and biochemistry of melanin (the particle) and its interaction, in its various forms, with UV radiation are considered. Methods of attenuation of UV radiation are discussed in terms of structure and chemical constituents. Photoprotection by constitutive and facultative pigmentation is reviewed with minimum erythema dose (MED) as the end point. The issue of acclimatization to UV radiation is discussed in terms of UVB phototherapy for psoriasis. Finally, skin cancer is considered as an end point and the reduction of its incidence with pigment level is discussed. It is concluded that whilst EMP provides protection, its extent depends on the end point chosen for evaluation. MED is a convenient photobiological end point but is rather insensitive, whereas skin cancer is sensitive but impractical for laboratory studies. Our current state of knowledge of melanin lacks information on its absorption and scattering coefficients and its refractive index. Methods for the quantitative measurement of EMP are also urgently required.

Kinetic study on the effect of pH on the melanin biosynthesis pathway

This paper deals with the quantitative description of the regulatory effect of pH on the oxidation pathway of L-dopa to yield melanins. Tyrosinase catalyzes the oxidation by molecular oxygen of L-dopa to o-dopaquinone, which evolves non-enzymatically through a branched pathway with cyclization or hydroxylation reactions. The production of several quinones and semiquinones in the pathway has also been reported. The intermediates of the hydroxylation branch have been identified and the corresponding rate constants have been determined. These compounds, such as have been detected in melanosomes and in tumoral cells, have great cytotoxic power and could have physiological significance in acidic media.

Effect of pH on the oxidation pathway of alpha-methyldopa catalysed by tyrosinase

This paper deals with the quantitative description of the effect of pH on the oxidation pathway of alpha-methyldopa. Tyrosinase catalyses the oxidation by molecular oxygen of alpha-methyldopa to o-alpha-methyldopaquinone, which evolves non-enzymically through a branched pathway with cyclization or hydroxylation reactions. The intermediates of the hydroxylation branch have been identified, and the corresponding rate constants have been determined. These compounds, which have been detected in melanosomes and in tumour cells, have great cytotoxic power and could have physiological significance in acidic media.

Kinetic study on the suicide inactivation of tyrosinase induced by catechol

Tyrosinase has a suicide inactivation reaction when it acts on omicron-diphenols. In the present paper, this reaction has been studied using a transient phase approach. Explicit equations of product vs. time have been developed for the multisubstrate mechanism of tyrosinase, and the kinetic parameters which characterize the enzyme acting on the suicide substrate catechol have been determined. The effect of pH has also been considered.

A kinetic study of the melanization pathway between L-tyrosine and dopachrome

In the pathway of melanin biosynthesis originating from L-tyrosine, the dopachrome accumulation at physiological pH is produced with a pronounced lag period, during which the level of L-dopa increases, following a sigmoidal kinetics to reach a steady-state. A kinetic model has been proposed for the overall pathway of melanization from L-tyrosine to dopachrome; it explains the lag period present during the dopachrome accumulation as well as the influence of L-tyrosine and tyrosinase over this lag period. Use of this model is also valid to explain the kinetics of L-dopa accumulation in the reaction medium, as has been tested by simulation.

Phosphonic analogues of tyrosine and 3,4-dihydroxyphenylalanine (dopa) influence mushroom tyrosinase activity

A series of phosphonic analogues of tyrosine and 3,4-dihydroxyphenylalanine (dopa) were synthesized in order to study their interaction with mushroom tyrosinase. 1-Amino-2-(3,4-dihydroxyphenyl)ethylphosphonic acid and 1-amino-2-(3,4-dimethoxyphenyl)ethylphosphonic acid turned out to be substrates for mushroom tyrosinase with Km values of 3.3 mM and 9.3 mM respectively. Shortening of the alkyl chain by one methylene group gave amino-(3,4-dihydroxyphenyl)methylphosphonic acid, one of the most powerful known inhibitors of this enzyme. This compound, racemic as well as in its optically active forms, exerts a mixed type of inhibition with an affinity for the enzyme one order of magnitude greater than that of the natural substrate.

Kinetic study on the slow inhibition of epidermis tyrosinase by m-coumaric acid

The inhibition by m-coumaric acid of oxidation of L-dopa by epidermis tyrosinase (monophenol,dihydroxy-L-phenylalanine:oxygen oxidoreductase, EC 1.14.18.1) is characterized by a prolonged transient phase. Kinetic data correspond to that for a postulated mechanism that involves rapid formation of a reduced enzyme-m-coumaric acid complex that subsequently undergoes a relatively slow reversible reaction. An overall inhibition constant for m-coumaric acid of 0.05 mM was calculated. The value of the Ki for the dissociation of m-coumaric acid from the rapidly formed complex was calculated as 0.53 mM. The first-order rate constants for the slow isomerization of the enzyme-inhibitor complex were calculated as 3.0 +/- 0.1 min-1 for the forward step and 0.31 +/- 0.06 min-1 for the reverse step.