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.

Computer program for the equations describing the steady state of enzyme reactions

MOTIVATION

The derivation of steady-state equations is frequently carried out in enzyme kinetic studies. Done manually, this becomes tedious and prone to human error. The computer programs now available which are able to accept reaction mechanisms of some complexity are focused only on the strict steady-state approach.

RESULTS

Here we present a computer program called REFERASS, with a short computation time and a user-friendly format for the input and output files, able to derive the strict steady-state equations and/or those corresponding to the usual assumption that one ore more of the reversible steps are in rapid equilibrium. This program handles enzyme-catalysed reactions with mechanisms involving up to 255 enzyme species connected by up to 255 reaction steps, subject to limits imposed by the memory and disk space available.

Kinetic behaviour of zymogen activation processes in the presence of an inhibitor

A global kinetic analysis of a general zymogen activation model, where not only the activating but also the activated enzyme suffer an irreversible inhibition is presented. A reaction in which the enzyme acts upon a substrate is coupled to monitor the process. In addition, we determined the corresponding kinetic equations for a number of particular cases of the general model studied. Finally, a kinetic data analysis and a procedure to discriminate among the different mechanisms considered, which are based on the kinetic equations obtained, are suggested.

A kinetic study of simultaneous suicide inactivation and irreversible inhibition of an enzyme. Application to 1-aminocyclopropane-1-carboxylate (ACC) synthase inactivation by its substrate S-adenosylmethionine

This paper deals with the development of an experimental method for the kinetic study of the inactivation of an enzyme by a racemic mixture of an inhibitor, whose isomers operate as suicide substrate and irreversible inhibitor respectively. The ratio between the isomer concentration in the biological or commercial source must be determined, but no separation of them is required. The method involves a kinetic analysis and an experimental design that enables the affinity (1/Km), rate of catalysis (kcat), rate of inactivation (lambda max), efficiency of catalysis (kcat/Km) and efficiency of inactivation (lambda max/Km) to be determined. The method has been applied to the kinetic characterization of the inactivation of 1-aminocyclopropane-1-carboxylate (ACC) synthase from tomato fruits by its substrate, S-adenosylmethionine (AdoMet). The ratio between AdoMet isomers with respect to its sulfonium centre, namely (-)-AdoMet and (+)-AdoMet, present in the commercial sample used, has been determined by 1H nuclear magnetic resonance.

A kinetic study of the suicide inactivation of an enzyme measured through coupling reactions. Application to the suicide inactivation of tyrosinase

A systematic procedure for the kinetic study of reaction mechanisms with enzyme inactivation induced by a suicide substrate in the presence or in the absence of an auxiliary substrate, when the enzyme activity is measured through coupling reactions, enzymically catalysed or not, was developed and analysed by using the transient-phase approach. The methodology is established to determine the parameters and kinetic constants corresponding to the enzyme suicide inactivation and the coupling reactions. This approach is illustrated by a study of the suicide inactivation of tyrosinase by catechol in the presence of L-proline. Treatment of the experimental data was carried out by non-linear regression.

Experimental methods for kinetic study of suicide substrates

The kinetic study of the enzymatic inactivation originated by suicide substrates can be carried out by means of two alternative approaches. One method considers the substrate concentration as practically constant during the assay time and provides explicit equations of product concentration vs. time. The other method involves the significant consumption of the substrate, yielding implicit equations of time vs. product concentration. The utility of both methods is discussed and adequate experimental conditions for their correct application are established.

A kinetic study of hypoxanthine oxidation by milk xanthine oxidase

The course of the reaction sequence hypoxanthine----xanthine----uric acid catalysed by xanthine:oxygen oxidoreductase from milk was investigated on the basis of u.v. spectra taken during the course of hypoxanthine and xanthine oxidations. It was found that xanthine accumulated in the reaction mixture when hypoxanthine was used as a substrate. The time course of the concentrations of hypoxanthine, xanthine intermediate and uric acid product was simulated numerically. The mathematical model takes into account the competition of substrate, intermediate and product and the accumulation of the intermediate at the enzyme. This type of analysis permits the kinetic parameters of the enzyme for hypoxanthine and xanthine to be obtained.

Kinetic study of sinephrine oxidation by mushroom tyrosinase

Mushroom tyrosinase catalyzes the oxidation of sinephrine showing a marked lag period during appearance of adrenochrome and simultaneously adrenaline accumulation in the reaction medium can be detected. The adrenaline accumulation follows a sigmoidal curve until a constant level of adrenaline is reached when the system is in the steady-state. These experimental results agree with a model of enzymatic catalysis that includes the chemical evolution of adrenoquinone and permit us to explain these phenomenon as well as the influence that enzyme and sinephrine concentration present on the lag period and the level of adrenaline accumulated in the steady-state.

Influence of NaCl on the kinetic behaviour of mammalian muscle acetylcholinesterase

Acetylcholinesterase was solubilized from rabbit white muscle by means of dilute buffer and Triton X-100 (0.5%). About 50% of total activity was brought into solution with buffer, the rest being solubilized by extracting the tissue with buffer and Triton X-100. The enzyme activity recovered in the supernatants was 170% of that found in the homogenate in the absence of Triton X-100 indicating that, to some extent, the enzyme could be found in an occluded form in muscle. At suboptimum substrate concentration the Triton-solubilized acetylcholinesterase displayed a negative cooperativity, this phenomenon being greatly modified in the presence of NaCl. As the salt concentration increased (0-400 mM) the enzyme activity decreased, the Km values being linearly-dependent on the NaCl concentration in the assay medium. We propose a kinetic pattern to explain both the negative cooperativity produced by the substrate and the effect of NaCl on the kinetic behaviour on this enzyme. Our data are consistent with the hypothesis of binding of substrate to both the catalytic anionic site and a peripheral anionic site, the salt showing the capacity to compete with the substrate for these two binding sites.

Kinetic characterization of dopamine as a suicide substrate of tyrosinase

A kinetic study of the inactivation of frog epidermis tyrosinase by a suicide substrate dopamine hydrochloride is described. The kinetic parameters and constants which characterize this reaction have been determined and the effects of pH and the stoichiometric inhibition by chloride have been considered.

Irreversible inhibition of trypsin by TLCK. A continuous method for kinetic study of irreversible enzymatic inhibitors in the presence of substrate

This work presents a kinetic study on irreversible inhibition of trypsin by TLCK, using a new experimental approach. The method consists of the incubation of the enzyme with an irreversible inhibitor in the presence of a substrate which allows enzyme turnover as well as continuous measurement of the appearance of the product, a simultaneous change in the initial concentrations of the irreversible inhibitor and enzyme being undertaken, though a constant ratio between the latter, is maintained. This new approach enables the kinetic constants for TLCK, k2 and K1, to be determined.

Study of alpha-methyldopa oxidation by tyrosinase

The pathway for alpha-methyldopa oxidation to alpha-methyldopachrome, by mushroom tyrosinase, is proposed. Characterization of intermediates in this oxidative reaction and stoichiometry determination have both been undertaken. The steps for alpha-methyldopa transformation into its aminochrome would be: alpha-methyldopa----o-alpha-methyldopaquinone-H+----o-alpha- methyldopaquinone----leuko-alpha-methyldopachrome----alpha- methyldopachrome. The stoichiometry for this conversion corresponded to the equation: 2 o-alpha-methyldopaquinone-H+----alpha-methyldopa + alpha-methyldopachrome. At very acid pH values, another route implying the addition of water to the quinonic ring, competes with the first one. Two chemical pathways can be proposed from alpha-methyldopaquinone-H+, the relative importance of which is determined by the pH. A theoretical and experimental kinetic approach was applied to this oxidative reaction. Rate constants and thermodynamic activation parameters of the chemical steps, have been evaluated. The results obtained confirmed that alpha-methyldopa oxidation by tyrosinase followed a scheme similar to that established for L-dopa and alpha-methylnoradrenaline.

Chemical intermediates in dopamine oxidation by tyrosinase, and kinetic studies of the process

A minor pathway for dopamine oxidation to dopaminochrome, by tyrosinase, is proposed. Characterization of intermediates in this oxidative reaction and stoichiometric determination have both been undertaken. After oxidizing dopamine with mushroom tyrosinase or sodium periodate in a pH range from 6.0 to 7.0, it was spectrophotometrically possible to detect o-dopaminoquinone-H+ as the first intermediate in this pathway. The steps for dopamine transformation to dopaminochrome are as follows: dopamine----o-dopaminequinone-H+----o-dopaminequinone---- leukodopaminochrome--- - dopaminochrome. No participation of oxygen was detected in the conversion of leukodopaminochrome to dopaminochrome. Scanning spectroscopy and graphical analysis of the obtained spectra also verified that dopaminequinone-H+ was transformed into aminochrome in a constant ratio. The stoichiometry equation for this conversion is 2 o-dopaminequinone-H+----dopamine + dopaminochrome. The pathway for dopamine oxidation to dopaminochrome by tyrosinase has been studied as a system of various chemical reactions coupled to an enzymatic reaction. A theoretical and experimental kinetic approach is proposed for such a system; this type of mechanism has been named "Enzymatic-chemical-chemical" (EzCC). Rate constants for the implied chemical steps at different pH and temperature values have been evaluated from the measurement of the lag period arising from the accumulation of dopaminochrome that took place when dopamine was oxidized at acid pH. The thermodynamic activation parameters of the chemical steps, the deprotonation of dopaminequinone-H+ to dopaminequinone, and the internal cyclization of dopaminequinone to leukodopaminochrome have been calculated.

Kinetic study and intermediates identification of noradrenaline oxidation by tyrosinase

Characterization of intermediates formed in the noradrenaline oxidation by mushroom tyrosinase and sodium periodate has been performed by rapid scanning spectrophotometry and graphical analysis of obtained spectra. In a pH range from 5.0 to 6.0, it has been possible to detect o-noradrenalinequinone-H+ as the first intermediate in these oxidations. The following steps for noradrenaline transformation into noradrenochrome would be: noradrenaline----o-noradrenalinequinone-H+----o- noradrenalinequinone----leukonoradrenochrome----noradreno chrome. It has been also verified that o-noradrenalinequinone-H+ is transformed into noradrenochrome at a constant ratio. The stoichiometry for this converstion followed the equation: 2-noradrenalinequinone-H+----noradrenaline + noradrenochrome. The pathway between noradrenaline and noradrenochrome has been studied as a system of various chemical reactions coupled to an enzymatic reaction. We have denominated this type of mechanism as an enzymatic-chemical-chemical mechanism, (E2CC). Whole rate constants for the implicated chemical steps at different pH and temperature values have been evaluated from measurement of the lag period arising from the accumulation of noradrenochrome that takes place when noradrenaline was oxidized at pH 5-6. The lag period was independent on enzyme concentration, but was increased when pH and/or temperature were increased. Rate constants pH independent for the deprotonation of noradrenalinequinone-H+ into noradrenalinequinone and for the internal cyclization of noradrenalinequinone into leukonoradrenochrome have been obtained. We conclude that this minor pathway of noradrenaline oxidation by tyrosinase follows a scheme similar to that established for L-dopa.