Tyrosinase: a comprehensive review of its mechanism

4-Coumaroyl coenzyme A 3-hydroxylase activity from cell cultures of Lithospermum erythrorhizon and its relationship to polyphenol oxidase

A 4-coumaroyl-CoA 3-hydroxylase activity was purified 4600-fold from cell cultures of Lithospermum erythrorhizon. The enzyme showed a molecular mass of 42,400 +/- 1700 Da in gel chromatography and required ascorbate, NADH, or NADPH as cofactors. 4-Coumaroyl-CoA, 4-coumarate, p-cresol, and several other phenolic substances, but not tyrosine, were accepted as substrates for the hydroxylation. Besides hydroxylase activity, the enzyme showed diphenol oxidase activity. Both activities were inhibited by diethyldithiocarbamate or beta-mercaptoethanol, although at different concentrations. The enzyme showed striking similarity to a 4-coumaroyl-glucose 3-hydroxylase from sweet potato (Ipomoe batatas) roots, which has reportedly been purified to homogeneity and identified as a specific enzyme of chlorogenic acid biosynthesis. Close examination and comparison to a commercially available polyphenol oxidase, however, suggest that the enzyme activities purified from both Lithospermum and sweet potato are polyphenol oxidases rather than specific enzymes of secondary metabolism.

Chemical characterization of pheomelanogenesis starting from dihydroxyphenylalanine or tyrosine and cysteine. Effects of tyrosinase and cysteine concentrations and reaction time

Two types of melanin pigment are produced in mammals; the brown-to-black eumelanins and the yellow-to-reddish-brown pheomelanins. The switch from one type of melanin to the other appears to be regulated by the levels of tyrosinase and thiols, such as cysteine and glutathione. This study examines the process of pheomelanin formation starting from dihydroxyphenylalanine (dopa) or tyrosine and cysteine. We prepared pheomelanins by tyrosinase oxidation of dopa or tyrosine in the presence of cysteine. Experimental variables were reaction time, tyrosinase concentration, and dopa or tyrosine to cysteine ratio. Following the reactions, we measured concentrations of tyrosine, dopa, cysteine and cysteinyldopas, amounts of total melanin (TM) by Soluene-350 solubilization and aminohydroxyphenylalanine (AHP), a specific indicator of pheomelanin, formed by hydriodic acid hydrolysis, and absorbance ratio, A650/A500. It was found that (1) mixed melanogenesis is a heterogeneous process in which pheomelanogenesis proceeds first, followed by eumelanogenesis, as shown by changes in the tyrosine and cysteinyldopa concentrations, the AHP/TM ratio, and the A650/A500 ratio during the course of melanogenesis and (2) lower tyrosinase concentration favors pheomelanogenesis even when the availability of cysteine is limited, as shown by AHP/TM ratios that were higher than the corresponding tyrosine to cysteine ratios. These results indicate that the switch from eumelanogenesis to pheomelanogenesis can be achieved by lowering the tyrosinase activity, which conforms to our proposal that tyrosinase activity is the major factor controlling the course of melanogenesis.

Determination of the L-DOPA/L-tyrosine ratio in human plasma by high-performance liquid chromatography. Usefulness as a marker in metastatic malignant melanoma

A first procedure was devised for determining 3,4-dihydroxyphenylalanine (L-DOPA) in human plasma by isocratic RP-HPLC coupled with electrochemical detection. A second procedure was devised for determining 3-hydroxyphenylalanine (L-tyrosine) in human plasma by isocratic RP-HPLC coupled with fluorescence detection. These methods were used to ascertain the L-DOPA/L-tyrosine ratio in plasma of patients with melanoma. Reference values were established by analysis of the L-DOPA/L-tyrosine ratio in the plasma of 35 normal healthy subjects. For 29 patients diagnosed as having melanoma without metastasis, the L-DOPA/L-tyrosine (11.96 x 10(-5) +/- 2.69 x 10(-5)) level was not significantly different from that of 35 normal controls (11.20 x 10(-5) +/- 2.92 x 10(-5)). However, this level was significantly increased (p < 0.05) in the plasma of 17 patients with developing metastasis (21.02 x 10(-5) +/- 4.68 x 10(-5)).

Trypsin-induced follicular papilla apoptosis results in delayed hair growth and pigmentation

Programmed cell death is a controlled process that leads to the elimination of single cells via apoptosis. Programmed cell death is fundamental to development, morphogenesis, and homeostasis. Proteases play a major role in the death process. We have previously shown that a serine protease, secreted by a keratinocyte cell line, can induce apoptosis in numerous cell lines. Here we show that serine proteases can induce cell death in vivo as well. Using a synchronized hair growth mouse model, we show that topical trypsin treatment following depilation induces cell death at the follicular papilla. This results in delaying hair growth and pigmentation. We speculate that trypsin might affect a receptor-mediated signaling pathway that leads to follicular papilla cell death.

Tyrosinase mRNA is expressed in human substantia nigra

Dopamine acts, under appropriate conditions, as a selective neurotoxin. This toxicity is attributed to the autoxidation of the neurotransmitter into a reactive quinone that covalently modifies cellular macromolecules (i.e. proteins and nucleic acids). The oxidation of the catecholamine to a quinone is greatly accelerated by the enzyme tyrosinase. There is controversy, however, as to whether or not tyrosinase is expressed in human brain. In the present study, RT-PCR was utilized to demonstrate the presence of tyrosinase mRNA in post-mortem human brain tissues. Using gene-specific amplification primers, specific tyrosinase amplicons were detected following analysis of RNA from substantia nigra of four individuals. Analysis of cerebellar RNA from the same individuals produced no amplification products. Control reactions performed in the absence of reverse transcriptase failed to generate PCR products for any tissue tested. Three amplicons were subjected to direct DNA sequencing and all proved to be identical with tyrosinase sequences, thus obviating the possibility of amplification of a related gene. It is clear, therefore, that the tyrosinase gene is expressed in the human substantia nigra, lending support to previous studies describing tyrosinase-like activity and immunoreactive protein in the brain. This enzyme could be central to dopamine neurotoxicity as well as contribute to the neurodegeneration associated with Parkinson's disease.

Melanogenesis, tyrosinase expression, and reproductive differentiation in black and white truffles (Ascomycotina)

White and black truffles of the genus Tuber are Ascomycotina as well as Neurospora crassa, which expresses tyrosinase dependently on the reproductive cycle. Tyrosinase expression dependent on reproductive differentiation has been also described in black truffles. We present novel and comparative work on melanogenic activities in black and white truffles that both express true tyrosinases. L-tyrosine 3-monooxygenase and L-DOPA oxidase activities colocalize as histochemically detected and are similarly located in white and black truffles, from the hypothecium through the sporogenic hyphae to asci and spores. Sulfur components of truffle flavours reversibly inhibit tyrosinase. The respiratory phenotype of truffle mitochondria is discussed in relation to reproductive differentiation and melanogenesis.

Hydrogen peroxide-dependent 4-t-butylphenol hydroxylation by tyrosinase--a new catalytic activity

The aim of this work was to study the hydroxylation by tyrosinase of 4-t-butylphenol to 4-t-butylcatechol, in the presence of hydrogen peroxide. This hydroxylation reaction does not take place without the addition of hydrogen peroxide. Some properties of this new hydroxylating activity have been analysed. The kinetic parameters of mushroom tyrosinase for hydrogen peroxide (K(m) = 4.9 mM, V(m) = 48.1 microM/min) and 4-t-butylphenol (K(m) = 16 microM/min, V(m) = 6.7 microM/min) were evaluated. A lag period appeared, which was similar to the characteristic lag of monophenolase activity at the expense of molecular oxygen. The length of the lag phase decreased with increasing hydrogen peroxide concentrations but was longer with higher 4-t-butylphenol concentrations. The pH optimum for this hydroxylating activity was close to 5.5. The lag also varied with pH, reaching its highest value at pH 4.8. The lag was shortened by the addition of increasing amounts of 4-t-butylcatechol, and was abolished at 24.5 microM of 4-t-butylcatechol. 4-t-Butylphenol was oxidized by mushroom tyrosinase in the presence of 24.5 microM 4-t-butylcatechol and in the absence of hydrogen peroxide although the enzymatic activity tailed off. The presence of hydrogen peroxide is necessary to maintain a constant steady-state rate of 4-t-butylphenol oxidation by tyrosinase.

Contribution of phenols, quinones and reactive oxygen species to the mutagenicity of white grape juice in the Ames test

The purpose of this study was to evaluate the role of phenols, quinones and reactive oxygen species in the mutagenicity of white grape juice in the Ames mutagenicity test. Mutagenicity was markedly suppressed by reduced glutathione but was not influenced by superoxide dismutase or catalase. In the presence of grape polyphenol oxidase, the mutagenicity of grape juice was markedly increased. When hepatic cytosol from Aroclor 1254-induced rats, supplemented with a reduced nicotinamide adenine dinucleotide phosphate-generating system. served as an activation system, an increase in the mutagenicity of grape juice was observed. The cytosol-induced mutagenicity of grape juice was attenuated in the presence of superoxide dismutase, catalase and glutathione. It is concluded that polyphenol oxidase-catalysed oxidation of phenolic compounds generates genotoxic species that are, at least partly, responsible for the mutagenicity of grape juice. In the presence of hepatic cytosol, one-electron reduction of grape juice quinones leads to the production of reactive oxygen species resulting in an increase in the mutagenic response.

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.

Oxidation of phenols, anilines, and benzenethiols by fungal laccases: correlation between activity and redox potentials as well as halide inhibition

A comparative study has been performed with several fungal laccases for the oxidation of a series of phenols, anilines, and benzenethiols and for the inhibition by halides. The observed K(m) and kcat were correlated to the structure of substrate. The change in log (kcat/K(m)) was found to be proportional to the one-electron redox potential difference between laccase's type 1 copper site and substrate. This correlation indicated that the first electron transfer from substrate to laccase was governed by the "outersphere" mechanism. Compared to the electronic factor, the steric effect of small o-substituents (such as methyl and methoxy groups) was found to be unimportant. The depth of the laccase's type 1 copper site was estimated as approximately 10 A by comparing the steric effect among five 2-methoxyphenols whose 4-substituents ranged from 0.1 to 14 kDa in mass. The observed inhibition potency order of F- > Cl- > Br- was attributed to limited accessibility of laccase's type 2/type 3 trinuclear copper cluster site. Although the enzymes studied have homologous primary sequences and predicted similar backbone structures, the difference exhibited by each enzyme (in interacting with individual substrate or inhibitor) suggested the structural variation in their functional domains.

Triton X-114-aided purification of latent tyrosinase

Mushroom tyrosinase was partially purified using an aqueous two-phase system with Triton X-114. The purification achieved was 5.5-fold from a crude extract of mushroom pileus, with a high recovery of 84%. The phenols were reduced to 8% of the original content, avoiding pre- and post-purification tanning of the enzyme. The enzyme obtained was latent and was activated 3-fold by trypsin, 2.7-fold by changes in the pH and to different extents by cationic and anionic detergents, the latter being the more effective. There was also a synergistic effect between trypsin and detergent, at low detergent concentrations. When kinetically characterized, latent enzyme showed both monophenolase and diphenolase activities, the latter activity displaying an unexpected lag period before reaching the steady-state rate. This behaviour is characteristic of a hysteretic enzyme, and has not been previously described for this enzyme. In addition, inhibition studies with substrate analogues were carried out, tropolone being found to be the most effective inhibitor.

Reversible cleavage and formation of the dioxygen O-O bond within a dicopper complex

A key step in dioxygen evolution during photosynthesis is the oxidative generation of the O-O bond from water by a manganese cluster consisting of M2(mu-O)2 units (where M is manganese). The reverse reaction, reductive cleavage of the dioxygen O-O bond, is performed at a variety of dicopper and di-iron active sites in enzymes that catalyze important organic oxidations. Both processes can be envisioned to involve the interconversion of dimetal-dioxygen adducts, M2(O2), and isomers having M2(mu-O)2 cores. The viability of this notion has been demonstrated by the identification of an equilibrium between synthetic complexes having [Cu2(mu-eta2:eta2-O2)]2+ and [Cu2(mu-O)2]2+ cores through kinetic, spectroscopic, and crystallographic studies.

Hydroperoxidase activity of lipoxygenase: a kinetic study of isoproterenol oxidation

The hydroperoxidase activity of soybean lipoxygenase, a non-heme protein, oxidized isoproterenol using H2O2 at pH 6.0. This oxidation was enzymatic, since neither heat-denaturated enzyme or iron ions in the presence of H2O2 produced an increase in absorbance. The initial rate was not linear and showed a characteristic lag period whose length depended on the enzyme and substrate concentration. The lag was decreased if the enzyme and isoproterenol concentration were increased, whereas it increased if the H2O2 concentration was increased. Lipoxygenase showed the typical low specificity for electron donor characteristic of this hydroperoxidase activity (26 mM), but a high affinity for H2O2 (94 microM), although with substrate inhibition (ksi = 3.6 mM). The chemical intermediates produced during the oxidation of isoproterenol were characterized in order to determine the origin of the lag period. A plausible kinetic mechanism is proposed to explain the observed lag period and inhibition by high concentrations of H2O2.