4-Hexylresorcinol, a Potent Inhibitor of Mushroom Tyrosinase

New insights and advances on pyomelanin production: from microbial synthesis to applications

Pyomelanin is a brown-black phenolic polymer and results from the oxidation of homogentisic acid (HGA) in the L-tyrosine pathway. As part of the research for natural and active ingredients issued from realistic bioprocesses, this work re-evaluates the HGA pigment and makes an updated inventory of its syntheses, microbial pathways, and properties, with tracks and recent advances for its large-scale production. The mechanism of the HGA polymerization is also well documented. In alkaptonuria, pyomelanin formation leads to connective tissue damages and arthritis, most probably due to the ROS issued from HGA oxidation. While UV radiation on human melanin may generate degradation products, pyomelanin is not photodegradable, is hyperthermostable, and has other properties better than the L-Dopa melanin. This review aims to raise awareness about the potential of this pigment for various applications, not only for skin coloring and protection but also for other cells, materials, and as a promising (semi)conductor for bioelectronics and energy.

Tyrosinase-Catalyzed Hydroxylation of 4-Hexylresorcinol, an Antibrowning and Depigmenting Agent: A Kinetic Study

4-Hexylresorcinol (HR) is a compound used in the food and cosmetic industries as an antibrowning and lightening agent. Its use is mainly attributed to its inhibitory effect on the enzyme tyrosinase. However, the enzyme hydroxylates HR to an o-diphenol, which it then oxidizes to an o-quinone, which rapidly isomerizes to p-quinone. For tyrosinase to act in this way, the Eox form (oxy-tyrosinase) must be present in the reaction medium, which can be brought about by (a) hydrogen peroxide, (b) ascorbic acid, or (c) catalytic concentrations of o-diphenol and a reductant (NADH) to maintain it constant. This work demonstrates that HR is a substrate of tyrosinase and proposes a mechanism for its action. Its kinetic characterization provides a catalytic constant of 0.85 ± 0.04 s(-1) and a Michaelis constant of 60.31 ± 6.73 μM.

Study on the stability of deoxyArbutin in an anhydrous emulsion system

The skin-whitening agent, deoxyArbutin, is a potent tyrosinase inhibitor that is safer than hydroquinone and arbutin. However, it is thermolabile in aqueous solutions, where it decomposes to hydroquinone. Pharmaceutical and cosmetic emulsions are normally oil-in-water (o/w) or water-in-oil (w/o) systems; however, emulsions can be formulated with no aqueous phase to produce an anhydrous emulsion system. An anhydrous emulsion system could offer a stable vehicle for compounds that are sensitive to hydrolysis or oxidation. Therefore, to enhance the stability of deoxyArbutin in formulations, we chose the polyol-in-silicone, anhydrous emulsion system as the basic formulation for investigation. The quantity of deoxyArbutin and the accumulation of hydroquinone in both hydrous and anhydrous emulsions at various temperatures were analyzed through an established high performance liquid chromatographic (HPLC) method. The results indicated that water increased the decomposition of deoxyArbutin in the formulations and that the polyol-in-silicone, oil-based, anhydrous emulsion system provided a relatively stable surrounding for the deoxyArbutin that delayed its degradation at 25 °C and 45 °C. Moreover, the composition of the inner hydrophilic phase, containing different amounts of glycerin and propylene glycol, affected the stability of deoxyArbutin. Thus, these results will be beneficial when using deoxyArbutin in cosmetics and medicines in the future.

Interaction of sesamol (3,4-methylenedioxyphenol) with tyrosinase and its effect on melanin synthesis

Sesamin, sesamolin (lignans) and sesamol--from sesame seed (Sesamum indicum L.)--are known for their health promoting properties. We examined the inhibition effect of sesamol, a phenolic degradation product of sesamolin, on the key enzyme in melanin synthesis, viz. tyrosinase, in vitro. Sesamol inhibits both diphenolase and monophenolase activities with midpoint concentrations of 1.9 μM and 3.2 μM, respectively. It is a competitive inhibitor of diphenolase activity with a K(i) of 0.57 μM and a non-competitive inhibitor of monophenolase activity with a K(i) of 1.4 μM. Sesamol inhibits melanin synthesis in mouse melanoma B16F10 cells in a concentration dependant manner with 63% decrease in cells exposed to 100 μg/mL sesamol. Apoptosis is induced by sesamol, limiting proliferation. This study of the chemistry and biology of lignans, in relation to the mode of action of bioactive components, may open the door for drug applications targeting enzymes.

N-(3,4-Difluoro-phen-yl)-N'-(2,5-di-methoxy-phen-yl)urea

In the title compound, C₁₅H₁₄F₂N₂O₃, the dihedral angle between the benzene rings is 64.5 (1)°. One F atom is disordered over two meta positions, with occupancy factors of 0.72 and 0.28. In the crystal, mol­ecules are linked by N—H⋯O hydrogen bonds involving two N—H and one C=O groups of the urea central fragment, leading to a supra­molecular chain along [011].

N-(3,4-Difluoro-phen-yl)-3,4,5-trimethoxy-benzamide

In the title amide, C(16)H(15)F(2)NO(4), the dihedral angle between the benzene rings is 2.33 (15)°. Mol-ecules are linked in the crystal structure by N-H⋯O hydrogen bonding involving N-H and C=O groups of the amide function, leading to a supra-molecular chain along [100].

4-Hydroxyphenyl 4-fluorobenzoate

In the title compound, C₁₃H₉FO₃, the dihedral angle between the two benzene rings is 59.86 (4)°. In the crystal, inter­molecular O—H⋯H hydrogen bonds lead to molecular chains propagating in [010].

N-(3,4-Difluoro-phen-yl)-3,4-dimethoxy-benzene-sulfonamide

In the title sulfonamide derivative, C(14)H(13)F(2)NO(4)S, the dihedral angle between the benzene rings is 66.05 (9)°. The crystal structure is stabilized by weak inter-molecular N-H⋯O hydrogen bonds involving the amine and meth-oxy groups, which link the mol-ecules into a one-dimensional chain. No significant inter-chain contacts are observed.

N-(3,4-Difluoro-phen-yl)-2-(3,4-dimethoxy-phen-yl)acetamide

In the title amide, C(16)H(15)F(2)NO(3), the dihedral angle between the benzene rings is 53.7 (1)°. Mol-ecules are linked in the crystal structure by an inter-molecular N-H⋯O hydrogen bond involving N-H and C=O functionalities of the amide group. A one-dimensional network is thus formed along the [001] direction. No significant inter-chain contacts are observed.

3,4-Dihydroxy-phenyl 3,4,5-trimethoxy-benzoate

In the title compound, C(16)H(16)O(7), the dihedral angle between the two benzene rings is 82.02 (7)°. The crystal structure is stabilized by inter-molecular O-H⋯O hydrogen bonds, which link the mol-ecules into a two-dimensional network.

Dietary alkylresorcinols: absorption, bioactivities, and possible use as biomarkers of whole-grain wheat- and rye-rich foods

The biologic and chemical properties of alkylresorcinols (ARs) have been reviewed previously, but there has been relatively little research or focus on the importance of ARs in food and diet. ARs represent a significant proportion of the phytochemicals present in wheat and rye, in which they normally exist in concentrations between 300 and 1500 microg/g. ARs are concentrated in the bran fraction of these cereals, and are therefore a significant component of food products rich in whole grain wheat and rye but not in products containing only refined cereal flour. In this review, we discuss the presence of ARs in food, methods of analysis, their absorption and role in the diet in light of their in vitro bioactivities, and their possible use as biomarkers of whole-grain wheat and rye intake.

A catechol antioxidant protocatechuic acid potentiates inflammatory leukocyte-derived oxidative stress in mouse skin via a tyrosinase bioactivation pathway

The modifying effects of topical application of a catechol antioxidant protocatechuic acid (PA) on 12-O-tetradecanoylphorbol-13-acetate (TPA)-induced inflammatory responses in mouse skin were investigated. Treatment with a high dose (20,000 nmol) of PA, based on time of application, modifies inflammatory responses in the skin of the B6C3F(1) mouse, a resistant strain to inflammatory response induction by TPA, but shows much higher tyrosinase expression than that of an albino mouse. The responsibility of a large amount of PA-induced leukocyte infiltration to an inflamed region in a B6C3F(1) mouse is more sensitive than that of an ICR albino mouse. When ICR mice were treated with TPA (1.6 nmol) twice weekly for 5 weeks to induce chronic inflammatory responses, pretreatment with 1600 nmol PA 30 min prior to each TPA treatment significantly enhanced the inflammatory responses including edema formation, leukocyte infiltration, and the level of thiobarbituric acid-reacting substances. The dose-dependency was closely parallel to the results of a tumor promotion study of PA previously reported. Further, the treatment of PA alone resulted in tyrosinase-dependent contact hypersensitivity in ICR mouse skin. In addition, the in vitro study of cytotoxicity demonstrated that bioactivation by tyrosinase but not myeloperoxidase of PA significantly enhanced cytotoxicity and intracellular glutathione consumption. We conclude that the tyrosinase-derived reactive quinone intermediate(s) of PA, which binds nucleophilic residues of proteins including sulfhydryl group and conjugates of which are recognized as haptens, was partially involved in alteration of the cellular immune functions including oxygen radical-generating leukocytes migration to inflamed regions.

Effect of wounding on phenolic enzymes in six minimally processed lettuce cultivars upon storage

The effect of wounding on polyphenol oxidase (PPO), peroxidase (POD), and phenylalanine ammonia-lyase (PAL) was studied in six minimally processed lettuce (Lactuca sativa L.) cultivars upon storage for 7 days at 5 degrees C (Iceberg Mikonos (IM), I. Green Queen (IGQ), I. Asdrúbal (IA), Little Gem Sandra (LGS), Romaine Cazorla (RC), and R. Modelo (RM)). Wounding of lettuce tissue midribs (because of minimal processing) caused an exponential increase in PPO activity due to the activation process from latent to fully active PPO by following first order kinetics in the time range from 3.7 days (LGS) to 6.3 days (RC). However, total PPO activity (active plus latent) remained constant. Isoform pattern of PPO changed upon storage probably because of posttranslational processes. POD activity linearly increased with induction of new POD isoenzymes. PAL activity presented a typical bell-shaped induction pattern in four cultivars. Only IM and IGQ showed a second induction response which has not been previously described in the literature. IM was the cultivar most susceptible to browning and RC was the cultivar least susceptible. However, no clear correlation was observed between browning and any of the biochemical and physiological attributes investigated (PPO, PAL, and POD activities, total and individual phenols accumulation, and ascorbic acid content).

Kinetic study of oxalic acid inhibition on enzymatic browning

Oxalic acid has a strong antibrowning activity. The inhibitory pattern on catechol-PPO model system appeared to be competitive, with a K(i) value of 2.0 mM. When the PPO was incubated with oxalic acid, the activity was not recovered via dialysis, but the inactivated enzyme partially recovered its activity when cupric ion was added. Comparing the relative antibrowning effectiveness of oxalic acid with other common antibrowning agents, oxalic acid with I(50) value of 1.1 mM is as effective as kojic acid and more potent than cysteine and glutathione.

Kinetics of activation of latent mushroom (Agaricus bisporus) tyrosinase by benzyl alcohol

A latent isoform of Agaricus bisporus tyrosinase has been isolated and activated by benzyl alcohol, one of the major volatile compounds in mushrooms of this genus. The progress curve that describes the activation process reached the steady-state rate (V(ss)) after a lag period (tau). The rate of active tyrosinase formation was calculated by coupling the oxidation of o-diphenols to the activation process. V(ss) depended on benzyl alcohol, o-diphenol, and latent tyrosinase concentrations. The lag period depended on benzyl alcohol concentrations but not on o-diphenol and enzyme concentrations. The size of the latent mushroom tyrosinase was 67 kDa, determined by SDS-PAGE and Western blotting assays. This size was not modified after activation by benzyl alcohol. The presence of a lag period and the lack of change of the molecular mass of the protein after activation could indicate a slow conformational change of the protein to render the final active form. The values of the kinetic constants V(max) and K(m) on the o-diphenols 4-tert-butylcatechol, L-DOPA, and dopamine were different between the latent tyrosinase activated by benzyl alcohol and the commercial tyrosinase. They might indicate that a different final active tyrosinase, depending on the activator used, could arise.