Impact of Plasma-Activated Water Treatment on Quality and Shelf-Life of Fresh Spinach Leaves Evaluated by Comprehensive Metabolomic Analysis

Fresh baby spinach leaves are popular in salads and are sold as chilled and plastic-packed products. They are of high nutritional value but very perishable due to microbial contamination and enzymatic browning resulting from leaf senescence. Therefore, innovative food processing methods such as plasma-activated water (PAW) treatment are being explored regarding their applicability for ensuring food safety. PAW’s impact on food quality and shelf-life extension has, however, not been investigated extensively in vegetables so far. In the present study, a comprehensive metabolomic analysis was performed to determine possible changes in the metabolite contents of spinach leaves stored in a refrigerated state for eight days. Liquid chromatography high-resolution mass spectrometry, followed by stringent biostatistics, was used to compare the metabolomes in control, tap-water-rinsed or PAW-rinsed samples. No significant differences were discernible between the treatment groups at the beginning or end of the storage period. The observed loss of nutrients and activation of catabolic pathways were characteristic of a transition into the senescent state. Nonetheless, the presence of several polyphenolic antioxidants and γ-linolenic acid in the PAW-treated leaves indicated a significant increase in stress resistance and health-promoting antioxidant capacity in the sample. Furthermore, the enhancement of carbohydrate-related metabolisms indicated a delay in the senescence development. These findings demonstrated the potential of PAW to benefit food quality and the shelf-life of fresh spinach leaves.

Chemical Reactivities of ortho-Quinones Produced in Living Organisms: Fate of Quinonoid Products Formed by Tyrosinase and Phenoloxidase Action on Phenols and Catechols

Tyrosinase catalyzes the oxidation of phenols and catechols (o-diphenols) to o-quinones. The reactivities of o-quinones thus generated are responsible for oxidative browning of plant products, sclerotization of insect cuticle, defense reaction in arthropods, tunichrome biochemistry in tunicates, production of mussel glue, and most importantly melanin biosynthesis in all organisms. These reactions also form a set of major reactions that are of nonenzymatic origin in nature. In this review, we summarized the chemical fates of o-quinones. Many of the reactions of o-quinones proceed extremely fast with a half-life of less than a second. As a result, the corresponding quinone production can only be detected through rapid scanning spectrophotometry. Michael-1,6-addition with thiols, intramolecular cyclization reaction with side chain amino groups, and the redox regeneration to original catechol represent some of the fast reactions exhibited by o-quinones, while, nucleophilic addition of carboxyl group, alcoholic group, and water are mostly slow reactions. A variety of catecholamines also exhibit side chain desaturation through tautomeric quinone methide formation. Therefore, quinone methide tautomers also play a pivotal role in the fate of numerous o-quinones. Armed with such wide and dangerous reactivity, o-quinones are capable of modifying the structure of important cellular components especially proteins and DNA and causing severe cytotoxicity and carcinogenic effects. The reactivities of different o-quinones involved in these processes along with special emphasis on mechanism of melanogenesis are discussed.

Inhibitory effect of GB-2a (I3-naringenin-II8-eriodictyol) on melanogenesis

ETHNOPHARMACOLOGY RELEVANCE

GB-2a is a I3-naringenin-II8-eriodictyol compound isolated from Garcinia gardneriana (Planchon & Triana) Zappi, a plant used in folk medicine for the treatment of skin disorders.

AIM OF STUDY

In the search for new depigmenting agents, this study was carried out to investigate the in vitro effects of GB-2a isolated from G. gardneriana (Planchon & Triana) Zappi in B16F10 melanoma cells.

MATERIALS AND METHODS

The effects of GB-2a were evaluated through determination of melanin biosynthesis in B16F10 melanoma cells in comparison with the reference drug kojic acid (500µM). In parallel, the GB-2a effect was assessed in a cell viability assay. Mushroom tyrosinase activity assays were conducted to verify the effect of this enzyme. In order to ascertain the nature of enzyme inhibition on tyrosinase, kinetics analysis of the GB-2a was performed with L-tyrosine and L-3,4-dihydroxyphenylalanine (L-DOPA) substrates.

RESULTS

The results showed that GB-2a biflavonoid significantly inhibited the melanin content, without reducing cell viability. GB-2a also showed a strong antityrosinase activity in the mushroom tyrosinase assay. GB-2a inhibited the tyrosinase activity, exerting a mixed inhibition. For the L-tyrosine substrate the inhibition was in non-competitive mode and for L-DOPA it was in uncompetitive mode.

CONCLUSION

GB-2a biflavonoid promoted inhibition on tyrosinase activity and reduced melanin biosynthesis in B16F10 cells, which suggests great potential for medical and cosmetic uses as a depigmenting agent.

A novel adamantyl benzylbenzamide derivative, AP736, inhibits melanogenesis in B16F10 mouse melanoma cells via glycogen synthase kinase 3β phosphorylation

Recently, much effort has been made to develop effective dermatological depigmenting compounds. In this study, we investigated the novel candidate compound, AP736 (an adamantyl benzylbenzamide derivative), and its effects on melanogenesis in B16F10 melanoma cells, as well as the mechanisms involved. AP736 has been reported to exert anti-melanogenic effects in melanocytes in vitro and in artificial skin equivalents through the inhibition of key melanogenic enzymes and the suppression of the cAMP-protein kinase A (PKA)-cAMP response element‑binding protein (CREB) signaling pathway. Thus, we examined another pathway of melanogenesis involving the effects of AP736 on the glycogen synthesis kinase 3β (GSK3β) pathway. Melanin content and tyrosinase activity were measured using a spectrophotometer after the cells were treated with AP736. The AP736-induced activation of signaling pathways was examined by western blot analysis. We confirmed that AP736 decreased melanin production in a dose-dependent manner; however, it did not directly inhibit tyrosinase, the rate-limiting melanogenic enzyme. The expression of microphthalmia-associated transcription factor, tyrosinase, and related signal transduction pathways was also investigated. The Wnt signaling pathway is deeply involved in melanogenesis; therefore, phosphorylation by GSK3β was assessed following treatment with AP736. AP736 induced GSK3β phosphorylation (inactivation), but it did not alter the level of β-catenin. Furthermore, the expression of α-melanocyte-stimulating hormone-induced tyrosinase was downregulated by AP736. Our data suggest that AP736 exerts hypopigmentary effects through the downregulation of tyrosinase via GSK3β phosphorylation.

Design and discovery of mushroom tyrosinase inhibitors and their therapeutic applications

INTRODUCTION

Tyrosinase inhibitors could have a huge importance in medicine, cosmetics and agriculture. Although many tyrosinase inhibitors are available, they have demonstrated only mild efficacy and safety concerns. This has led to the discovery of novel tyrosinase inhibitors that are more safe, potent and efficacious.

AREAS COVERED

The authors provide an overview of the recent scientific accounts describing the design of new molecules. These compounds belong to different chemical families. The review emphasizes the rationale behind the discovery, the study of structure-activity relationships, the study of the mechanism and kinetic of inhibition and the cellular effect of the inhibitors. The article is based on the literature published from 2007 onward related with the development of synthetic tyrosinase inhibitors.

EXPERT OPINION

Although a great number of tyrosinase inhibitors have been published in the literature, none, as of yet, have reached the potency and safety requirements needed to enter clinical trials. The emergence of new in vitro and in vivo tests will finally allow the arrival of new compounds that are more potent and safe.

Tyrosinase: the four oxidation states of the active site and their relevance to enzymatic activation, oxidation and inactivation

Tyrosinase is an enzyme widely distributed in the biosphere. It is one of a group of proteins with a strongly conserved bicopper active centre able to bind molecular oxygen. Tyrosinase manifests two catalytic properties; monooxygenase and oxidase activity. These actions reflect the oxidation states of the active centre. Tyrosinase has four possible oxidation states and the details of their interaction are shown to give rise to the unusual kinetic behaviour of the enzyme. The resting state of the enzyme is met-tyrosinase [Cu(II)2] and activation, associated with a 'lag period', involves reduction to deoxy-tyrosinase [Cu(I)2] which is capable of binding dioxygen to form oxy-tyrosinase [Cu(II)2·O2]. Initially the conversion of met- to deoxy-tyrosinase is brought about by a catechol that is indirectly formed from an ortho-quinone product of tyrosinase action. The primary function of the enzyme is monooxygenation of phenols to ortho-quinones by oxy-tyrosinase. Inactivation of the enzyme results from monooxygenase processing of catechols which can lead to reductive elimination of one of the active-site copper ions and conversion of oxy-tyrosinase to the inactive deact-tyrosinase [Cu(II)Cu(0)]. This review describes the tyrosinase pathways and the role of each oxidation state in the enzyme's oxidative transformations of phenols and catechols.

The influence of hydroquinone on tyrosinase kinetics

In vitro studies, using combined spectrophotometry and oximetry together with hplc/ms examination of the products of tyrosinase action demonstrate that hydroquinone is not a primary substrate for the enzyme but is vicariously oxidised by a redox exchange mechanism in the presence of either catechol, L-3,4-dihydroxyphenylalanine or 4-ethylphenol. Secondary addition products formed in the presence of hydroquinone are shown to stimulate, rather than inhibit, the kinetics of substrate oxidation.

Suicide inactivation of the diphenolase and monophenolase activities of tyrosinase

The suicide inactivation mechanism of tyrosinase acting on its phenolic substrates has been studied. Kinetic analysis of the proposed mechanism during the transition phase provides explicit analytical expressions for the concentrations of o-quinone versus time. The electronic, steric, and hydrophobic effects of the phenolic substrates influence the enzymatic reaction, increasing the catalytic speed by three orders of magnitude and the inactivation by one order of magnitude. To explain this suicide inactivation, we propose a mechanism in which the enzymatic form oxy-tyrosinase is responsible for the inactivation. In this mechanism, the rate constant of the reaction would be directly related with the strength of the nucleophilic attack of the C-1 hydroxyl group, which depends on the chemical shift of the carbon C-1 (delta(1)) obtained by (13)C-NMR. The suicide inactivation would occur if the C-2 hydroxyl group transferred the proton to the protonated peroxide, which would again act as a general base. In this case, the coplanarity between the copper atom, the oxygen of the C-1 and the ring would only permit the oxidation/reduction of one copper atom, giving rise to copper (0), hydrogen peroxide, and an o-quinone, which would be released, thus inactivating the enzyme. One possible application of this property could be the use of these suicide substrates as skin depigmenting agents.

Serine–Ca2+ versus serine–Cu2+ complexes — A theoretical perspective

The association of Ca2+ and Cu2+ to serine was investigated by means of B3LYP DFT calculations. The [serine–M]2+ (M = Ca, Cu) potential energy surfaces include, as does the neutral serine, a large number of conformers, in which a drastic reorganization of the electron density of the serine moiety is observed. This leads to significant changes in the number and strength of the intramolecular hydrogen bonds existing in the neutral serine tautomers. In some cases, a proton is transferred from the carboxylic OH group to the amino group and accordingly, some of the more stable [serine–M]2+ complexes can be viewed as the result of the interaction of the zwiterionic form of serine with the doubly charged metal ion. Whereas the interaction between Ca2+ and serine is essentially electrostatic, that between Cu2+ and serine has a non-negligible covalent character, reflected in larger electron densities at the bond critical points between the metal and the base, in the negative values of the electron density between the two interacting systems, and in much larger Cu2+ than Ca2+ binding energies. More importantly, the interaction with Cu2+ is followed by a partial oxidation of the base, which is not observed when the metal ion is Ca2+. The main consequence is that in Cu2+ complexes a significant acidity enhancement of the serine moiety takes place, which strongly favors the deprotonation of the [serine–Cu]2+ complexes. This is not the case for Ca2+ complexes. Thus, [serine–Ca]2+ complexes, like those formed by urea, thiourea, selenourea, or glycine, should be detected in the gas phase. Conversely, the complexes with Cu2+ should deprotonate spontaneously and therefore only [(serine–H)–Cu]+ monocations should be experimentally accessible.

A reactive ortho-quinone generated by tyrosinase-catalyzed oxidation of the skin depigmenting agent monobenzone: self-coupling and thiol-conjugation reactions and possible implications for melanocyte toxicity

Monobenzone (hydroquinone monobenzylether, 1) is a potent skin depigmenting agent that causes irreversible loss of epidermal melanocytes by way of a tyrosinase-dependent mechanism so far little understood. Herein, we show that 1 can be oxidized by mushroom tyrosinase to an unstable o-quinone (1-quinone) that has been characterized by comparison of its properties with those of a synthetic sample obtained by o-iodoxybenzoic acid-mediated oxidation of 1. Preparative scale oxidation of 1 with tyrosinase and catalytic l-DOPA, followed by reductive workup and acetylation, led to the isolation of two main products that were identified as the acetylated catechol derivative 4 and an unusual biphenyl-type dimer of 4, acetylated 5, arising evidently by coupling of 4 with 1-quinone. In the presence of l-cysteine or N-acetyl-l-cysteine, formation of 4 and 5 was inhibited, and the reaction led instead to monoadducts (6 or 9) and diadducts (7 and 8). A similar behavior was observed when the tyrosinase-promoted oxidation of 1 was carried out in the presence of sulfhydryl-containing peptides, such as reduced glutathione, or proteins, such as bovine serum albumin (BSA), as inferred by detection of adduct 9 by high pressure liquid chromatography-electrochemical detection (HPLC-ED) after acid hydrolysis. The generation and reaction chemistry of 1-quinone described in this article may bear relevance to the etiopathogenetic mechanisms of monobenzone-induced leukoderma as well as to the recently proposed haptenation hypothesis of vitiligo, a disabling pigmentary disorder characterized by irreversible melanocyte loss.

The hunt for natural skin whitening agents

Skin whitening products are commercially available for cosmetic purposes in order to obtain a lighter skin appearance. They are also utilized for clinical treatment of pigmentary disorders such as melasma or postinflammatory hyperpigmentation. Whitening agents act at various levels of melanin production in the skin. Many of them are known as competitive inhibitors of tyrosinase, the key enzyme in melanogenesis. Others inhibit the maturation of this enzyme or the transport of pigment granules (melanosomes) from melanocytes to surrounding keratinocytes. In this review we present an overview of (natural) whitening products that may decrease skin pigmentation by their interference with the pigmentary processes.

An updated review of tyrosinase inhibitors

Tyrosinase is a multifunctional, glycosylated, and copper-containing oxidase, which catalyzes the first two steps in mammalian melanogenesis and is responsible for enzymatic browning reactions in damaged fruits during post-harvest handling and processing. Neither hyperpigmentation in human skin nor enzymatic browning in fruits are desirable. These phenomena have encouraged researchers to seek new potent tyrosinase inhibitors for use in foods and cosmetics. This article surveys tyrosinase inhibitors newly discovered from natural and synthetic sources. The inhibitory strength is compared with that of a standard inhibitor, kojic acid, and their inhibitory mechanisms are discussed.