A chromogenic assay for catecholoxidases based on the addition of L-proline to quinones

Synthesis, in silico modelling, and in vitro biological evaluation of substituted pyrazole derivatives as potential anti-skin cancer, anti-tyrosinase, and antioxidant agents

Twenty-five azole compounds (P1-P25) were synthesised using regioselective base-metal catalysed and microwave-assisted approaches, fully characterised by high-resolution mass spectrometry (HRMS), nuclear magnetic resonance (NMR), and infrared spectra (IR) analyses, and evaluated for anticancer, anti-tyrosinase, and anti-oxidant activities in silico and in vitro. P25 exhibited potent anticancer activity against cells of four skin cancer (SC) lines, with selectivity for melanoma (A375, SK-Mel-28) or non-melanoma (A431, SCC-12) SC cells over non-cancerous HaCaT-keratinocytes. Clonogenic, scratch-wound, and immunoblotting assay data were consistent with anti-proliferative results, expression profiling therewith implicating intrinsic and extrinsic apoptosis activation. In a mushroom tyrosinase inhibition assay, P14 was most potent among the compounds (half-maximal inhibitory concentration where 50% of cells are dead, IC50 15.9 μM), with activity greater than arbutin and kojic acid. Also, P6 exhibited noteworthy free radical-scavenging activity. Furthermore, in silico docking and absorption, distribution, metabolism, excretion, and toxicity (ADMET) simulations predicted prominent-phenotypic actives to engage diverse cancer/hyperpigmentation-related targets with relatively high affinities. Altogether, promising early-stage hits were identified - some with multiple activities - warranting further hit-to-lead optimisation chemistry with further biological evaluations, towards identifying new skin-cancer and skin-pigmentation renormalising agents.

Considerations about the Continuous Assay Methods, Spectrophotometric and Spectrofluorometric, of the Monophenolase Activity of Tyrosinase

With the purpose to obtain the more useful tyrosinase assay for the monophenolase activity of tyrosinase between the spectrofluorometric and spectrophotometric continuous assays, simulated assays were made by means of numerical integration of the equations that characterize the mechanism of monophenolase activity. These assays showed that the rate of disappearance of monophenol (VssM,M) is equal to the rate of accumulation of dopachrome (VssM,DC) or to the rate of accumulation of its oxidized adduct, originated by the nucleophilic attack on o-quinone by a nucleophile such as 3-methyl-2-benzothiazolinone (MBTH), (VssM, A−ox), despite the existence of coupled reactions. It is shown that the spectrophotometric methods that use MBTH are more useful, as they do not have the restrictions of the L-tyrosine disappearance measurement method, of working at pH = 8 and not having a linear response from 100 μM of L-tyrosine. It is possible to obtain low LOD^M (limit of detection of the monophenolase activity) values with spectrophotometric methods. The spectrofluorimetric methods had a lower LOD^M than spectrophotometric methods. In the case of 4-hydroxyphenil-propionic acid, the LOD^M obtained by us was 0.25 U/mL. Considering the relative sensitivities of 4-hydroxyanisole, compared with 4-hydroxyphenil-propionic acid, LOD^M values like those obtained by fluorescent methods would be expected.

Spectrophotometric Assays for Sensing Tyrosinase Activity and Their Applications

Tyrosinase (TYR, E.C. 1.14.18.1), a critical enzyme participating in melanogenesis, catalyzes the first two steps in melanin biosynthesis including the ortho-hydroxylation of L-tyrosine and the oxidation of L-DOPA. Previous pharmacological investigations have revealed that an abnormal level of TYR is tightly associated with various dermatoses, including albinism, age spots, and malignant melanoma. TYR inhibitors can partially block the formation of pigment, which are always used for improving skin tone and treating dermatoses. The practical and reliable assays for monitoring TYR activity levels are very useful for both disease diagnosis and drug discovery. This review comprehensively summarizes structural and enzymatic characteristics, catalytic mechanism and substrate preference of TYR, as well as the recent advances in biochemical assays for sensing TYR activity and their biomedical applications. The design strategies of various TYR substrates, alongside with several lists of all reported biochemical assays for sensing TYR including analytical conditions and kinetic parameters, are presented for the first time. Additionally, the biomedical applications and future perspectives of these optical assays are also highlighted. The information and knowledge presented in this review offer a group of practical and reliable assays and imaging tools for sensing TYR activities in complex biological systems, which strongly facilitates high-throughput screening TYR inhibitors and further investigations on the relevance of TYR to human diseases.

Effects of Extraction Conditions on Banana Peel Polyphenol Oxidase Activity and Insights into Inactivation Kinetics Using Thermal and Cold Plasma Treatment

The objective of this work was to characterize banana peel polyphenol oxidase (PPO) and to study the inactivation kinetics during thermal and cold atmospheric pressure plasma treatment. Since varietal differences in enzyme characteristics are a well-known phenomenon, 'Prata' banana peel PPO was characterized, and PPO activity and thermal stability of the peel PPO of the two dessert banana cultivars 'Cavendish' and 'Prata' were compared to identify the cultivar better suited for industrial food applications. A crude extract obtained from the peels of the Brazilian banana variety 'Prata' revealed highest PPO activities (46.0-55.2 nkat/mL) at 30-40 °C in a range of pH 6.0-6.5 after addition of 0.5 g/g_sample polyvinylpyrrolidone and 0.5% (v/v) Triton X-100 during extraction. 'Cavendish' PPO activity was four times higher. Banana peel PPO exhibited the highest affinity towards dopamine (K_M = 0.94 mM). Thermal inactivation of 'Prata' and 'Cavendish' PPO was achieved at 90 °C after 5 and 15 min, respectively, whereas cold plasma treatment did not decrease PPO activity below 46% of the initial enzyme activity. The inactivation behavior of PPO could successfully be described by a two-fraction model indicating at least two types of isoenzymes with different thermal stability. The overall high thermal stability was mainly attributed to membrane-bound PPO. The results may help to prevent enzymatic browning of banana peels and thereby facilitate their valorization as food ingredients.

Halophytes as a potential source of melanosis-inhibiting compounds. Mechanism of inhibition of a characterized polyphenol extract of purslane (Portulaca oleracea)

The market value of crustaceans depreciates during storage due to the appearance of melanosis caused by polyphenol oxidases. Sulfite derivatives are used as melanosis-inhibiting agents, but their unhealthy effects make it preferable to replace them with natural preservatives. In this work, a crude enzymatic extract from whiteleg shrimp (Penaeus vannamei) was characterized and used to test the diphenol oxidase-inhibiting activity of polyphenol extracts of five underutilized halophyte plants, namely crystalline ice plant, seaside arrowgrass, purslane, sea fennel, and seashore aster. The extracts inhibited diphenol oxidase activity more efficiently than sodium sulfite. The purslane extract was rich in isoorientins, isovitexin, and apigenin, and showed the highest inhibiting effect, being this classified as mixed or non-competitive. Hydroxyl groups in the phenyl B ring could be responsible for the inhibitory activity of the extract. The polyphenol extracts tested in this work could be promising melanosis-inhibiting agents of interest for seafood industries.

Using A Spin-Coater to Capture Adhesive Species during Polydopamine Thin-Film Fabrication

Spin-coating was evaluated as a technique to study events that occur during polydopamine (PDA) thin-film formation. The reaction variants studied included type of oxidant, dopamine (DA) concentration, pH, adhesion time prior to spin, substrate chemistry, and notably, DA solution aging time. A strong oxidant, sodium periodate (SP), and a weak oxidant, atmospheric oxygen were chosen. It was found that reactions in solution were much faster and produced much thicker PDA films with SP than with oxygen. PDA thickness correlated positively with DA concentration, SP solution pH, and adhesion time. DA oxidation and aggregation is a dynamic process, which is reflected in the DA aging-time parameter. PDA film thickness reached a maximum value as DA solution aged. Color photography, UV-vis spectroscopy, and dynamic light scattering indicated that the optimal DA aging time for PDA adhesion is the result of the evolution of PDA particle size and chemistry over time. The capture of the optimal aging-time window was identified as the critical parameter for preparing PDA films with continuity and appreciable thickness. When these conditions were applied in a modified dip-coating method, comparable PDA films were fabricated as those obtained from spin-coating. Native silicon wafers (SiO₂) as well as wafers that were modified with polydimethylsiloxane (PDMS) and amine-containing polydimethylsiloxane (PADMS) were chosen to represent a wide range of substrates with different substrate-PDA interactions. The main effect of substrate structural difference was on PDA film morphology. "Island" morphologies were obtained on PDMS where only hydrophobic interactions are responsible for PDA adhesion, while "speck" morphologies were observed on SiO₂ and PADMS. The stabilities of the fabricated PDA films were tested in 0.1 M HCl and DMSO. The SP-derived PDA films exhibited very little mass loss compared to those fabricated using either the conventional dip-coating method or oxygen as an oxidant. Choosing a strong oxidant, understanding the DA reaction dynamics, and taking advantage of the optimal DA aging time are important in the fabrication of stable PDA films on a variety of substrates.

Electrochemical-Mediated Gelation Of Catechol-Bearing Hydrogels Based On Multimodal Crosslinking

Catechol-bearing polymers form hydrogel networks through cooperative oxidative crosslinking and coordination chemistry. Here we describe the kinetics of cation-dependent electrochemical-mediated gelation of precursor solutions composed of catechol functionalized four-arm poly(ethylene glycol) combined with select metal cations. The gelation kinetics, mechanical properties, crosslink composition, and self-healing capacity is a strong function of the valency and redox potential of metal ions in the precursor solution. Catechol-bearing hydrogels exhibit highly compliant mechanical properties with storage moduli ranging from G' = 0.1-5 kPa depending on the choice of redox active metal ions in the precursor solution. The gelation kinetics is informed by the net cell potential of redox active components in the precursor solution. Finally, redox potential of the metal ion precursor can differentially alter the effective density of crosslinks in networks and confer properties to hydrogels such as self-healing capacity. Taken together, this parametric study generates new insight to inform the design of catechol-bearing hydrogel networks formed by electrochemical-mediated multimodal crosslinking.

Biomimetic recyclable microgels for on-demand generation of hydrogen peroxide and antipathogenic application

Microgels that can generate antipathogenic levels of hydrogen peroxide (H₂O₂) through simple rehydration in solutions with physiological pH are described herein. H₂O₂ is a widely used disinfectant but the oxidant is hazardous to store and transport. Catechol, an adhesive moiety found in mussel adhesive proteins, was incorporated into microgels, which generated 1-5 mM of H₂O₂ for up to four days as catechol autoxidized. The sustained release of low concentrations of H₂O₂ was antimicrobial against both gram-positive (Staphylococcus epidermidis) and gram-negative (Escherichia coli) bacteria and antiviral against both non-enveloped porcine parvovirus (PPV) and enveloped bovine viral diarrhea virus (BVDV). The amount of released H₂O₂ is several orders of magnitude lower than H₂O₂ concentration previously reported for antipathogenic activity. Most notably, these microgels reduced the infectivity of the more biocide resistant non-envelope virus by 3 log reduction value (99.9% reduction in infectivity). By controlling the oxidation state of catechol, microgels can be repeatedly activated and deactivated for H₂O₂ generation. These microgels do not contain a reservoir for storing the reactive H₂O₂ and can potentially function as a lightweight and portable dried powder source for the disinfectant for a wide range of applications. STATEMENT OF SIGNIFICANCE: Researchers have designed bioadhesives and coatings using the adhesive moiety catechol to mimic the strong adhesion capability of mussel adhesive proteins. During catechol autoxidation, hydrogen peroxide (H₂O₂) is generated as a byproduct. Here, catechol was incorporated into microgels, which can generate millimolar levels of H₂O₂ by simply hydrating the microgels in a solution with physiological pH. The sustained release of H₂O₂ was both antimicrobial and antiviral, inactivating even the more biocide resistant non-enveloped virus. These microgels can be repeatedly activated and deactivated for H₂O₂ generation by incubating them in solutions with different pH. This simplicity and recyclability will enable this biomaterial to function as a lightweight and portable source for the disinfectant for a wide range of applications.

Correlating enzymatic browning inhibition and antioxidant ability of Maillard reaction products derived from different amino acids

BACKGROUND

Up to now, only limited research on enzymatic browning inhibition capacity (BIC) of Maillard reaction products (MRPs) has been reported and there are still no overall and systematic researches on MRPs derived from different amino acids. In the present study, BIC and antioxidant capacity, including 2,2-diphenyl-1-picrylhydrazyl radical scavenging activity and Fe²⁺ reducing power activity, of the MRPs derived from 12 different amino acids and three reducing sugars were investigated.

RESULTS

The MRPs of cysteine (Cys), cystine, arginine (Arg) and histidine (His) showed higher BIC compared to other amino acids. Lysine (Lys)-MRPs showed the highest absorbance value at 420 nm (A₄₂₀ ) but very limited BIC, whereas Cys-MRPs, showed the highest BIC and the lowest A₄₂₀ . The A₄₂₀ can roughly reflect the trend of BIC of MRPs from different amino acids, except Cys and Lys. MRPs from tyrosine (Tyr) showed the most potent antioxidant capacity but very limited BIC, whereas Cys-MRPs showed both higher antioxidant capacity and BIC compared to other amino acids. Partial least squares regression analysis showed positive and significant correlation between BIC and Fe²⁺ reducing power of MRPs from 12 amino acids with glucose or fructose, except Lys, Cys and Tyr. The suitable pH for generating efficient browning inhibition compounds varies depending on different amino acids: acidic pH was favorable for Cys, whereas neutral and alkaline pH were suitable for His and Arg, respectively. Increasing both heating temperature and time over a certain range could improve the BIC of MRPs of Cys, His and Arg, whereas any further increase deteriorates their browning inhibition efficiencies.

CONCLUSION

The types of amino acid, initial pH, temperature and time of the Maillard reaction were found to greatly influence the BIC and antioxidant capacity of the resulting MRPs. There is no clear relationship between BIC and the antioxidant capacity of MRPs when reactant type and processing parameters of the Maillard reaction are considered as variables. © 2017 Society of Chemical Industry.

Luminescent Metal-Organic-Framework-Based Label-Free Assay of Polyphenol Oxidase with Fluorescent Scan

Metal-organic frameworks (MOFs) are emerging in recent years as a kind of versatile fluorescent sensing materials, but their application to enzyme assays has rarely been studied. Here, the first example of a MOF-based label-free enzyme assay system is reported. A luminescent MOF was synthesized and applied to the activity analysis of polyphenol oxidase (PPO). With its distinct responses to the phenolic substrate and o-quinone product, this MOF could transduce the extent of PPO-catalyzed oxidation to fluorescence signal and enable the real-time monitoring of this reaction. Wide substrate adaptability and high sensitivity (detection limit=0.00012 U mL^-1 ) were exhibited by this method, which meets the requirement of common bioanalysis. Interestingly, by the comparison with molecular capturing reagents, the heterogeneous nature of this MOF-based assay effectively preventing the interaction with the enzyme was proven, thus ensuring the authenticity of results.

Screening of inhibitors for mushroom tyrosinase using surface plasmon resonance

Tyrosinase inhibitors have been used as whitening or antihyperpigment agents because of their ability to suppress dermal-melanin production. In the present study, screening and kinetic evaluation of various small molecules were performed on mushroom tyrosinase (MT) using surface plasmon resonance. The binding constant KD (M) values obtained for tannic acid, phloroglucinol, saffron, catechol, and pyrogallol are 1.213 × 10(-4), 7.136 × 10(-5), 3.111 × 10(-5), 1.557 × 10(-5), and 7.981 × 10(-6) M, respectively. Pyrogallol has been found to display high affinity for MT, whereas catechol, saffron, and phloroglucinol have been found to bind with low affinity. MT shows considerable changes in the secondary structure in the presence of inhibitors. The study reveals the Biacore/SPR sensor's ability in the rapid identification and characterization of inhibitors for MT. The methodology described here can be used to rapidly screen and optimize various lead compounds for other enzymes and elucidate structure function inter-relationships between various enzymes.

Effect of pH on the rate of curing and bioadhesive properties of dopamine functionalized poly(ethylene glycol) hydrogels

The remarkable underwater adhesion strategy employed by mussels has inspired bioadhesives that have demonstrated promise in connective tissue repair, wound closure, and local delivery of therapeutic cells and drugs. While the pH of oxygenated blood and internal tissues is typically around 7.4, skin and tumor tissues are significantly more acidic. Additionally, blood loss during surgery and ischemia can lead to dysoxia, which lowers pH levels of internal tissues and organs. Using 4-armed PEG end-capped with dopamine (PEG-D) as a model adhesive polymer, the effect of pH on the rate of intermolecular cross-linking and adhesion to biological substrates of catechol-containing adhesives was determined. Adhesive formulated at an acidic pH (pH 5.7-6.7) demonstrated reduced curing rate, mechanical properties, and adhesive performance to pericardium tissues. Although a faster curing rate was observed at pH 8, these adhesives also demonstrated reduced mechanical and bioadhesive properties when compared to adhesives buffered at pH 7.4. Adhesives formulated at pH 7.4 demonstrated a good balance of fast curing rate, elevated mechanical properties and interfacial binding ability. UV-vis spectroscopy evaluation revealed that the stability of the transient oxidation intermediate of dopamine was increased under acidic conditions, which likely reduced the rate of intermolecular cross-linking and bulk cohesive properties for hydrogels formulated at these pH levels. At pH 8, competing cross-linking reaction mechanisms and reduced concentration of dopamine catechol due to auto-oxidation likely reduced the degree of dopamine polymerization and adhesive strength for these hydrogels. pH plays an important role in the adhesive performance of mussel-inspired bioadhesives and the pH of the adhesive formulation needs to be adjusted for the intended application.

A review on spectrophotometric methods for measuring the monophenolase and diphenolase activities of tyrosinase

Tyrosinase is a copper enzyme with broad substrate specifity toward a lot of phenols with different biotechnological applications. The availability of quick and reliable measurement methods of the enzymatic activity of tyrosinase is of outstanding interest. A series of spectrophotometric methods for determining the monophenolase and diphenolase activities of tyrosinase are discussed. The product of both reactions is the o-quinone of the corresponding monophenol/diphenol. According to the stability and properties of the o-quinone, the substrate is classified as four substrate types. For each of these substrate types, we indicate the best method for measuring diphenolase activity (among eight methods) and, when applicable, for measuring monophenolase activity (among four methods). The analytical and numerical solutions to the system of differential equations corresponding to the reaction mechanism of each case confirm the underlying validity of the different spectrophotometric methods proposed for the kinetic characterization of tyrosinase in its action on different substrates.

Post TLC developing technique for tyrosinase inhibitor detection

Post TLC developing technique was developed to detect substances which can inhibit tyrosinase activity. The method involved spraying the TLC plate or chromatographic paper containing sample spot(s) with tyrosinase and l-tyrosine solutions successively. A positive result could be visualized directly as white spot(s) against a brownish-purple background. The method can either be used as a quick screening method for tyrosinase inhibitor detection or a guiding procedure for an isolation of tyrosinase inhibitors from mixtures or natural product extracts. The technique is sensitive enough to give a clear result in the presence of only 6 ng glabridin.

Enzymatic synthesis of arbutin undecylenic acid ester and its inhibitory effect on mushroom tyrosinase

A novel tyrosinase inhibitor, an arbutin derivative having undecylenic acid at the 6-position of its glucose moiety, was enzymatically synthesized. Its inhibitory activity was studied in vitro by using catechol and phenol as substrates. The IC(50) value of the arbutin ester on tyrosinase using catechol (4 x 10(-4) M) was 1% of that when arbutin (4 x 10(-2) M) was used. Using phenol, IC(50) of the arbutin ester (3 x 10(-4) M) as substrate was 10% of that of arbutin (3 x 10(-3) M). These results suggest that the arbutin ester inhibits the latter part of the tyrosinase reaction, which consists of hydroxylation and oxidation.

The immunostimulating effect of bacterial genomic DNA on the innate immune responses of bivalve mussel, Hyriopsis cumingii Lea

The genomic DNA of Escherichia coli, which contains the unmethylated CpG motif, was used to evaluate the immunostimulating effect of bacterial DNA on innate immune responses in the bivalve mussel Hyriopsis cumingii Lea. The results showed that the E. coli DNA had no significant effect on the production of superoxide anion (O(2-)) or acid phosphatase (AP) by haemocytes in vitro. However, the bactericidal activity of the haemocytes was significantly increased when the cells were incubated with 50 or 100mug/ml bacterial DNA for 12 and 24h. Antibacterial activity, lysozyme activity, and prophenoloxidase (proPO) production of haemolymph were also increased, when the bivalve molluscs were injected with 50 or 100mug/ml of bacterial DNA for 12 and 24h. These activities returned to the control level after 48h. This work showed the bacterial DNA with unmethylated CpG motif could activate some parameters of the immune system of bivalve molluscs in vivo and in vitro.

Calculating molar absorptivities for quinones: Application to the measurement of tyrosinase activity

The molar absorptivities of the quinones produced from different o-diphenols, triphenols, and flavonoids were calculated by generating the respective quinones through oxidation with an excess of periodate. Oxidation of these substrates by this reagent was analogous to oxidation by tyrosinase with molecular oxygen, although the procedure showed several advantages over the enzymatic method in that oxidation took place almost immediately and quinone stability was favored because no substrate remained. The o-diphenols studied were pyrocatechol, 4-methylcatechol, 4-tert-butylcatechol, 3,4-dihydroxyphenylalanine, 3,4-dihydroxyphenylethylamine, 3,4-dihydroxyphenylacetic acid, 3,4-dihydroxyphenylpropionic acid, and caffeic acid; the triphenols studied were pyrogallol, 1,2,4-benzenetriol, 6-hydroxydopa, and 6-hydroxydopamine; and the flavonoids studied were (+)catechin, (-)epicatechin, and quercetin. In addition, the stability of the quinones generated by oxidation of the compounds by [periodate]0/[substrate]0 << 1 was studied. Taking the findings into account, tyrosinase could be measured by following o-quinone formation in rapid kinetic studies using the stopped-flow method. However, measuring o-quinone formation could not be useful for steady-state studies. Therefore, several methods for following tyrosinase activity are proposed, and a kinetic characterization of the enzyme's action on these substrates is made.

Synthesis and characterization of model compounds of the lysine tyrosyl quinone cofactor of lysyl oxidase

4-n-Butylamino-5-ethyl-1,2-benzoquinone (1(ox)) has been synthesized as a model compound for the LTQ (lysine tyrosyl quinone) cofactor of lysyl oxidase (LOX). At pH 7, 1(ox) has a lambda(max) at 504 nm and exists as a neutral o-quinone in contrast to a TPQ (2,4,5-trihydroxyphenylalanine quinone) model compound, 4, which is a resonance-stabilized monoanion. Despite these structural differences 1(ox) and 4 have the same redox potential (ca. -180 mV vs SCE). The structure of the phenylhydrazine adduct of 1(ox) (2) is reported, and 2D NMR spectroscopy has been used to show that the position of nucleophilic addition is at C(1). UV-vis spectroscopic pH titration of phenylhydrazine adducts of 1(ox) and 4, 2, and 11, respectively, reveals a similar red shift in lambda(max) at alkaline pH with the same pK(a) (approximately 11.8). In contrast, the red shift in lambda(max) at acidic pH conditions yields different pK(a) values (2.12 for 2 vs -0.28 for 11), providing a means to distinguish LTQ from TPQ. Reactions between in situ generated 4-ethyl-1,2-benzoquinone and primary amines give a mixture of products, indicating that the protein environment must play an essential role in LTQ biogenesis by directing the nucleophilic addition of the epsilon-amino group of a lysine residue to the C(4) position of a putative dopaquinone intermediate. Characterization of a 1,6-adduct between an o-quinone and butylamine (3-n-butylamino-5-ethyl-1,2-benzoquinone, 13) confirms the assignment of LTQ as a 1,4-addition product.

Studies of a disposable biosensor based on the beta-cyclodextrin inclusion complex as mediator

This paper reports the use of a disposable voltammetric biosensor based on a screen-printed basal electrode and highly efficient mediators, the beta-cyclodextrin inclusion complexes tetramethylbenzidine and ferrocene. The polyphenol oxidase, contained in the crude extract of mushroom, was immobilized in the membrane of the cross-linked beta-cyclodextrin polymer. The resulting biosensor gave excellent analytical performance due to the supramolecular complexation for mediators. It responded to the concentration of dopamine with high selectively in the range from 1.0 x 10(-9) to 1.0 x 10(-6) M with a detection limit as low as 5 x 10(-10) M. The response time for reaching 90% of its steady-state value was less than 60 s, and the activity was retained for at least 15 days.

Functional Conversion of Hemocyanin to Phenoloxidase by Horseshoe Crab Antimicrobial Peptides

Arthropod hemocyanins and phenoloxidases serve different physiological functions as oxygen transporters and enzymes involved in defense reactions, respectively. However, they are equipped with a structurally similar oxygen-binding center. We have shown that the clotting enzyme of the horseshoe crab, Tachypleus tridentatus, functionally converts hemocyanin to phenoloxidase by forming a complex without proteolytic cleavage (Nagai, T., and Kawabata, S. (2000) J. Biol. Chem. 275, 35297-35301). Here we show that chitin-binding antimicrobial peptides of the horseshoe crab induce the intrinsic phenoloxidase activity of hemocyanin. Tachyplesin, a major Tachypleus antimicrobial peptide with an amphiphilic structure, converted the hemocyanin to phenoloxidase. Surface plasmon resonance analysis revealed the specific interaction of tachyplesin with hemocyanin at K(d) = 3.4 x 10(-)6 m. The chemical modification of Trp or Tyr in tachyplesin, but not Lys or Arg, dramatically reduced the affinity to hemocyanin, suggesting that the binding site is located in the hydrophobic face of tachyplesin. Hemocyanin has no affinity with chitin, but it significantly binds to tachyplesin-coated chitin, leading to the expression of phenoloxidase activity. The chitin coated with antimicrobial peptides may serve as a scaffold for the binding of hemocyanin, and the resulting phenoloxidase activity appears to function as a trigger of exoskeleton wound healing.

Influence of Sodium Periodate and Tyrosinase on Binding of Alginate to Adlayers of Mytilus edulis Foot Protein 1

Mytilus edulis foot protein 1 (Mefp-1) is the most well-characterized component of this sea mussel's adhesive plaque. The plaque is a condensed, heterogeneous mixture consisting of a large proportion of cross-linked biopolymers that bonds the mussel to a chosen mooring. Mefp-1 is densely populated with lysine and L-3,4-dihyroxyphenylalanine (L-dopa) residues incorporated into a repeating amino acid sequence motif. It has been proposed that one plaque cross-linking reaction is the nucleophilic addition of the epsilon-amino groups of the lysine residues into the oxidized catechol (o-diphenol) functionality (quinone) of the L-dopa residues. In order to determine if this reaction occurs in adlayers of Mefp-1, a previously developed assay for epsilon-amino groups was applied. Adlayers of Mefp-1 were exposed to an oxidant, either the enzyme, mushroom tyrosinase, or sodium periodate. Binding of alginate to adlayers was used to probe for accessibility of epsilon-amino groups. It was found that lysine residues lose the ability to bind alginate after exposure to sodium periodate, but that this loss is not clearly due to a reaction with L-dopa residues. There is a slight decrease of binding of alginate to adlayers of Mefp-1 exposed to either active or thermally deactivated mushroom tyrosinase, probably due to the obstruction of binding sites by bound enzyme. Adsorption kinetics of mushroom tyrosinase onto adlayers of Mefp-1 for active and thermally inactivated enzyme were nearly identical. Attenuated total reflection Fourier transform infrared spectroscopy was used to characterize these interactions at a germanium (Ge) interface. Copyright 2000 Academic Press.

A link between blood coagulation and prophenol oxidase activation in arthropod host defense

Phenol oxidase, a copper-containing enzyme, is widely distributed not only in animals but also in plants and fungi, which is responsible for initiating the biosynthesis of melanin. Activation of prophenol oxidase in arthropods is important in host defense. However, the prophenol oxidase-activating system remains poorly understood at the molecular level. Here we show that the coagulation cascade of the horseshoe crab Tachypleus tridentatus is linked to prophenol oxidase activation, with the oxygen carrier hemocyanin functioning as a substitute for prophenol oxidase. Tachypleus clotting enzyme functionally transforms hemocyanin to phenol oxidase, and the conversion reaches a plateau at 1:1 stoichiometry without proteolytic cleavage. The active site-masked clotting enzyme also has the same effect, suggesting that complex formation of the clotting enzyme with hemocyanin is critical for the conversion. The two systems of blood coagulation and prophenol oxidase activation may have evolved from a common ancestral protease cascade.

Thermal inactivation of mushroom polyphenoloxidase employing 2450 MHz microwave radiation

Browning reactions in fruits and vegetables are a serious problem for the food industry. In mushrooms, the principal enzyme responsible for the browning reaction is polyphenoloxidase (PPO). A microwave applicator has been designed and used for studying mushroom PPO inactivation. The effects of microwaves and conventional heating on the kinetics of the monophenolase and diphenolase activities of PPO were studied. Conventional and microwave treatments produce different enzyme intermediates with different stability and kinetic properties. We describe how considerable time can be saved during microwave inactivation of the enzyme compared with the time needed when conventional hot-water treatment is used, resulting in greater profitability and enhanced quality. The short exposure time required for samples irradiated with microwaves is very important for maintaining the quality of mushrooms. The fast microwave treatment used resulted in an increase in antioxidant content and a considerable decrease in browning.

Oxidation of 4-tert-butylcatechol and dopamine by hydrogen peroxide catalysed by horseradish peroxidase

The catalytic cycle of horseradish peroxidase (HRP; donor:hydrogen peroxide oxidoreductase; EC 1.11.1.7) is initiated by a rapid oxidation of it by hydrogen peroxide to give an enzyme intermediate, compound I, which reverts to the resting state via two successive single electron transfer reactions from reducing substrate molecules, the first yielding a second enzyme intermediate, compound II. To investigate the mechanism of action of horseradish peroxidase on catechol substrates we have studied the oxidation of both 4-tert-butylcatechol and dopamine catalysed by this enzyme. The different polarity of the side chains of both o-diphenol substrates could help in the understanding of the nature of the rate-limiting step in the oxidation of these substrates by the enzyme. The procedure used is based on the experimental data to the corresponding steady-state equations and permitted evaluation of the more significant individual rate constants involved in the corresponding reaction mechanism. The values obtained for the rate constants for each of the two substrates allow us to conclude that the reaction of horseradish peroxidase compound II with o-diphenols can be visualised as a two-step mechanism in which the first step corresponds to the formation of an enzyme-substrate complex, and the second to the electron transfer from the substrate to the iron atom. The size and hydrophobicity of the substrates control their access to the hydrophobic binding site of horseradish peroxidase, but electron density in the hydroxyl group of C-4 is the most important feature for the electron transfer step.

4-Hydroxyanisole: the most suitable monophenolic substrate for determining spectrophotometrically the monophenolase activity of polyphenol oxidase from fruits and vegetables

A continuous spectrophotometric method for determining the monophenolase activity of polyphenol oxidase from several plant sources is described. This assay method is based on the coupling reaction between 3-methyl-2-benzothiazolinone hydrazone and the quinone product of the oxidation of 4-hydroxyanisole in the presence of polyphenol oxidase. 4-Hydroxyanisole proved to be the best monophenol assayed to measure the monophenolase activity of polyphenol oxidase from apple, artichoke, avocado, medlar, pear, and strawberry. Kinetic constants of 4-hydroxyanisole were compared to those of p-hydroxyphenyl propionic acid, a very sensitive monophenol previously reported to assay the monophenolase activity of polyphenol oxidase from apple, pear, and mushroom. The high values of the maximum steady state rate obtained for 4-hydroxyanisole suggest the existence of high catalytic constant toward this monophenol. These kinetic values were supported by nuclear magnetic resonance assays which predicted the highest reactivity of 4-hydroxyanisole. Therefore nuclear magnetic resonance assays proved to be a valuable and useful tool to predict the best monophenolic substrate for plant polyphenol oxidases. The 3-methyl-2-benzothiazlolinone-adduct for 4-hydroxyanisole was stable, with high molar absorptivity at the optimum pHs of the polyphenol oxidases assayed. All this together makes the use of 4-hydroxyanisol as monophenolic substrate and 3-methyl-2-benzothiazolinone as coupling reagent the most sensitive and precise assay method up to date reported in the literature to determine the monophenolas activity of polyphenol oxidase from fruits and vegetables.

Polyphenol oxidase activity staining in polyacrylamide electrophoresis gels

An analytical method allowing the detection of polyphenol oxidase activity on polyacrylamide gel electrophoresis (PAGE) is described. The method is rapid, sensitive and specific and is based on a coupling reaction between 4-tert-butyl-o-benzoquinone and the aromatic amine, 4-amino-N,N-diethylaniline sulphate. Catecholase activity of polyphenol oxidase appears as blue stained bands on a colourless background.

A stopped spectrophotometric assay for the dopa oxidase activity of tyrosinase

A stopped spectrophotometric assay for the dopa oxidase activity of tyrosinase has been developed to enable large numbers of samples to be screened very rapidly. The assay measures the pink pigment formed by the reaction of Besthorn's hydrazone (3-methyl-2-benzothiazoninone hydrazone, or MBTH) with dopaquinone, the product of oxidation of L-dopa by tyrosinase. Addition of perchloric acid stops the reaction and precipitates protein, enabling turbid as well as non-turbid samples to be assayed. Stability of the pink product is enhanced in acid solution and the pigment has a sharp absorbance maximum at 505 nm such that it is easily measured spectrophotometrically. Using the stopped assay, tyrosinase is detectable only in mammalian cell lines expected to express the enzyme, and the specificity of the assay has also been confirmed using tyrosinase inhibitors. The stopped MBTH assay is approx. 15-times more sensitive than the widely used dopachrome assay and can reliably detect the formation of as little as 350 pmol of product.

Enzymatic browning reactions in apple and apple products

This review examines the parameters of enzymatic browning in apple and apple products that is, phenolic compounds, polyphenoloxidases, and other factors (ascorbic acid and peroxidases), both qualitatively and quantitatively. Then the relationships between intensity of browning and the browning parameters are discussed, including a paragraph on the methods used for browning evaluation. Finally, the different methods for the control of browning are presented.

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.