Effect of pH on the oxidation pathway of dopamine and dopa

How to Design Catechol-Containing Hydrogels for Cell Encapsulation Despite Catechol Toxicity

Catechol (cat) is a highly adhesive diphenol that can be chemically grafted to polymers such as chitosan (CH) to make them adhesive as well. However, catechol-containing materials experimentally show a large variability of toxicity, especially in vitro. While it is unclear how this toxicity emerges, most concerns are directed toward the oxidation of catechol into quinone that releases reactive oxygen species (ROS) which can, in turn, cause cell apoptosis through oxidative stress. To better understand the mechanisms at play, we examined the leaching profiles, hydrogen peroxide (H₂O₂) production, and in vitro cytotoxicity of several cat-chitosan (cat-CH) hydrogels that were prepared with different oxidation levels and cross-linking methods. To create cat-CH with different propensities toward oxidation, we grafted either hydrocaffeic acid (HCA, more prone to oxidation) or dihydrobenzoic acid (DHBA, less prone to oxidation) to the backbone of CH. Hydrogels were cross-linked either covalently, using sodium periodate (NaIO₄) to trigger oxidative cross-linking, or physically, using sodium bicarbonate (SHC). While using NaIO₄ as a cross-linker increased the oxidation levels of the hydrogels, it also significantly reduced in vitro cytotoxicity, H₂O₂ production, and catechol and quinone leaching in the media. For all gels tested, cytotoxicity could be directly related to the release of quinones rather than H₂O₂ production or catechol release, showing that oxidative stress may not be the main reason for catechol cytotoxicity, as other pathways of quinone toxicity come into play. Results also suggest that the indirect cytotoxicity of cat-CH hydrogels fabricated through carbodiimide chemistry can be reduced if (i) catechol groups are chemically bound to the polymer backbone to prevent leaching or (ii) the chosen cat-bearing molecule has a high resistance to oxidation. Coupled with the use of other cross-linking chemistries or more efficient purification methods, these strategies can be adopted to synthesize various types of cytocompatible cat-containing scaffolds.

Polymeric Tissue Adhesives

Polymeric tissue adhesives provide versatile materials for wound management and are widely used in a variety of medical settings ranging from minor to life-threatening tissue injuries. Compared to the traditional methods of wound closure (i.e., suturing and stapling), they are relatively easy to use, enable rapid application, and introduce minimal tissue damage. Furthermore, they can act as hemostats to control bleeding and provide a tissue-healing environment at the wound site. Despite their numerous current applications, tissue adhesives still face several limitations and unresolved challenges (e.g., weak adhesion strength and poor mechanical properties) that limit their use, leaving ample room for future improvements. Successful development of next-generation adhesives will likely require a holistic understanding of the chemical and physical properties of the tissue-adhesive interface, fundamental mechanisms of tissue adhesion, and requirements for specific clinical applications. In this review, we discuss a set of rational guidelines for design of adhesives, recent progress in the field along with examples of commercially available adhesives and those under development, tissue-specific considerations, and finally potential functions for future adhesives. Advances in tissue adhesives will open new avenues for wound care and potentially provide potent therapeutics for various medical applications.

Metal ion removal using waste byssus from aquaculture

Byssus is a thread-like seafood waste that has a natural high efficiency in anchoring many metal ions thanks to its richness of diverse functional groups. It also has structural stability in extreme chemical, physical and mechanical conditions. The combination of these properties, absent in other waste materials, has novelty suggested its use as matrix for water remediation. Thus, pristine byssus, upon de-metalation, was studied to remove metal ions from ideal solutions at pH 4 and 7, as model chemical systems of industrial and environmental polluted waters, respectively. The byssus matrix's uptake of metal ions was determined by ICP-OES and its surface microstructure investigated by SEM. The results showed that the byssus matrix excellently uptakes metal ions slightly reorganizing its surface micro-structure. As example of its efficiency: 50 mg of byssus absorbed 21.7 mg·g-1 of Cd2+ from a 10 mM solution at pH 7. The adsorption isotherm models of Freundlich and Langmuir were mainly used to describe the system at pH 7 and pH 4, respectively. In conclusion, we showed that the byssus, a waste material that is an environmental issue, has the potential to purify polluted industrial and environmental waters from metal ions.

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.

Synthesis, Follow-Up, and Characterization of Polydopamine-like Coatings Departing from Micromolar Dopamine-o-Quinone Precursor Concentrations

The understanding of oxidized species derived from the neurotransmitter dopamine (DA) is a relevant topic for both the medical field (Parkinson's disease) as well as for the field of materials science where the formation process of polydopamine (PDA) films is an active area of research. Polymers that interact strongly with almost all surfaces but have a low electrical conductivity have been obtained by the chemical oxidation of DA. Since electrical conductivity is a desired property for several applications, deposition alternatives such as electrochemical PDA synthesis have been proposed, but the results are still insufficient. In this context, we propose a new PDA chemical-electrochemical deposition process on glassy carbon electrodes. The chemical oxidation step that converts dopamine into dopamine-o-quinone previous to the electrochemical deposition was crucial to decrease the precursor concentration to the micromolar range. The PDA-like films synthesized by this method had high adhesion and low charge-transfer resistance, which was evidenced by impedance measurements and the successful electrodeposition of a polypyrrole coating on top of a PDA-like film. In addition, we observed that anodization of GC surfaces increases sensitivity toward six electroactive couples derived from DA oxidation in the pH regimes studied. These results show the complexity of the intermediates formed during the electrochemical polymerization of PDA.

Catechol-thiol-based dental adhesive inspired by underwater mussel adhesion

The critical problem associated with the underwater mussel adhesive catechol-based 3,4-dihydroxy-L-phenylalanine (DOPA) is its sensitivity to oxidation. To overcome this problem, mussels underwent etching in the presence of acidic pH conditions (<3.0), and thiol chemistry was used to control the propensity of DOPA for oxidation. Similar strategies deployed by mussels are also actively utilized in dental adhesives which undergo etching in the presence of phosphoric acid derivatives to maximize the bonding strength and adapt thiol chemistries to minimize shrinkage stress. In view of the similarities between dental and underwater mussel adhesives, we employ in this study the strategy of mussel adhesion-the combination of DOPA and thiol chemistry with acid etching-to one of the most critical issues in dental adhesives, namely, the dentin bonding with zirconia. As a result, the adhesion bonding between zirconia and dentin, one of the most elusive problems in dentistry, has improved compared to the commercially available adhesive resin formulation. In addition, in view of the similar human oral and mussel adhesive environments, our findings will considerably contribute to the translation of the adhesive system inspired by mussels. STATEMENT OF SIGNIFICANCE: Mussels are effectively operated by creating an acidic environment when adhering with 3,4-dihydroxy-l-phenylalanine (DOPA)-thiol redox chemistry for underwater bonding. Similarly, in dental adhesives, phosphoric acid-based etching is used for dentin-bonding materials. In view of the similarity between dental adhesives and underwater mussel adhesives, the combination of DOPA and thiol chemistry with acid etching can be used to overcome one of the most critical issues in dentin medical adhesives. The proposed adhesion method produces high adhesion strengths compared to those currently used in dentin and zirconia adhesives. Here, we extend and evaluate dentin and zirconia dental adhesives by mixing with mussel (DOPA)-thiol redox chemistry and acid etching.

Effect of release of dopamine on iron transformations and reactive oxygen species (ROS) generation under conditions typical of coastal waters

Seasonally persistent blooms of Ulvaria obscura var. blyttii, the prominent species present in green tides in the northern Pacific and Atlantic, have been well documented in recent decades. The synthesis and release of dopamine (DA) by Ulvaria obscura var. blyttii has been proposed to be associated with the suppression and inhibition of the growth of other organisms competing for limited resources. To better understand the potential benefits obtained from the release of DA, the transformation of DA as well its concomitant impact on the local seawater environment are investigated in this study. The results show that, despite several toxic quinones being produced during the oxidation of DA, aminochrome (DAC) is likely to be the only quinone playing an allelopathic role in view of its expected accumulation in the surrounding environment. As a consequence of the direct oxidation of DA and DA induced generation of 5,6-dihydroxyindole (DHI), high concentrations of H₂O₂ accumulate over time, especially in the presence of elements including iron, calcium and magnesium. The oxidative stress to other organisms induced by the release of DA may be particularly detrimental as a result of H₂O₂ induced reduction in photosynthesis, inactivation of antioxidant systems or even the generation of ˙OH. DA induced iron mobilization may benefit the continuously persistent blooms of Ulvaria obscura var. blyttii or even the whole community via alleviation in iron deficiency within the bloom region.

Electroanalytical study of dopamine oxidation on carbon electrodes: from the macro- to the micro-scale

The oxidation of dopamine in strongly acidic (pH = 0) solution is investigated using microdisc, microcylinder and macro-electrodes together with a range of voltage scan rates.

Recent approaches in designing bioadhesive materials inspired by mussel adhesive protein

Marine mussels secret protein-based adhesives, which enable them to anchor to various surfaces in a saline, intertidal zone. Mussel foot proteins (Mfps) contain a large abundance of a unique, catecholic amino acid, Dopa, in their protein sequences. Catechol offers robust and durable adhesion to various substrate surfaces and contributes to the curing of the adhesive plaques. In this article, we review the unique features and the key functionalities of Mfps, catechol chemistry, and strategies for preparing catechol-functionalized polymers. Specifically, we reviewed recent findings on the contributions of various features of Mfps on interfacial binding, which include coacervate formation, surface drying properties, control of the oxidation state of catechol, among other features. We also summarized recent developments in designing advanced biomimetic materials including coacervate-forming adhesives, mechanically improved nano- and micro-composite adhesive hydrogels, as well as smart and self-healing materials. Finally, we review the applications of catechol-functionalized materials for the use as biomedical adhesives, therapeutic applications, and antifouling coatings. © 2016 The Authors. Journal of Polymer Science Part A: Polymer Chemistry Published by Wiley Periodicals, Inc. J. Polym. Sci., Part A: Polym. Chem. 2017, 55, 9-33.

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.

Mollusk glue inspired mucoadhesives for biomedical applications

Chitosan (CH), partially N-deacetylated chitin, is a biodegradable and biocompatible polymer that has shown great potential in drug delivery and tissue engineering applications. Although bioadhesive, CH has limited mucoadhesion in wet conditions due to weak interactions with biological surfaces. DOPA (3,4-dihydroxy-L-phenylalanine), a catechol-containing molecule naturally present in marine mussel foot proteins, has been shown to increase the mucoadhesion of several polymers. We report here a simple and bioinspired approach to enhance CH mucoadhesion in wet conditions by preparing mixed hydrogels including CH and different catechol-containing compounds, namely DOPA, hydrocaffeic acid (HCA), and dopamine (DA). We characterized the hydrogels for their swelling, release kinetics of the catechol compounds, and mucoadhesive strength to rabbit small intestine. The swelling of the hydrogels was pH dependent with maximum swelling at pH 1. The hydrogel swelling was higher in the presence of the DOPA and DA but lower in the presence of HCA. HCA/CH hydrogel also showed the slowest catechol release, most likely due to electrostatic interactions between CH and HCA. Lower hydrogel swelling and slower HCA release resulted in increased mucoadhesion: HCA/CH showed more than 2-fold enhancement of mucoadhesion to rabbit small intestine compared to CH alone. Since it is known that catechol compounds can be oxidized, we analyzed the oxidation of DOPA, HCA, and DA at different pH values and its effect on mucoadhesion. We found that oxidation occurring before contact with the intestinal mucosa did not improve mucoadhesion, while oxidation occurring during the contact further increased the mucoadhesion of HCA/CH hydrogels. These results show that mucoadhesion of CH hydrogels can be increased with a simple bioinspired approach, which has the potential to be applied to other polymers since it does not require any chemical modification.

Melanization in living organisms: a perspective of species evolution

Eumelanin is a heteropolymer that is generally composed of hydroxylated indole residues and plays diverse protective functions in various species. Melanin is derived from the amino acid tyrosine and production of melanin is a highly complex oxidative process with a number of steps that can either proceed enzymatically or non-enzymatically. Although melanin plays important protective roles in many species, during melanization, particularly in steps that can proceed non-enzymatically, many toxic intermediates are produced, including semiquinones, dopaquinone, indole-quinones and moreover, the production of many reactive oxygen species. To mitigate the production of reactive species, a number of proteins that regulate the biochemical process of melanization have evolved in various living species, which is closely related to adaptation and physiological requirements. In this communication, we discuss differences between non-enzymatic and enzymatic processes of melanization and the enzymatic regulation of melanization in difference species with an emphasis on differences between mammals and insects. Comparison between melanization and insect sclerotization is also emphasized which raises some interesting questions about the current models of these pathways.

Molecular cloning, sequence characteristics, and polymorphism analyses of the tyrosinase-related protein 2 / DOPAchrome tautomerase gene of black-boned sheep (Ovis aries)

Tyrosinase-related protein 2 (TYRP2) plays a pivotal role in the biosynthesis of eumelanin. Black-boned sheep have excessive melanin and eumelanin, resulting in dark (black) muscles and organs. This study was designed to investigate the effects of variants of the TYRP2 gene on black traits and coat colour of black-boned sheep. Melanin traits were measured in three populations of sheep (Nanping black-boned, Nanping normal, and Romney Marsh) and compared in this study. From the TYRP2 cDNA, all 8 exons and their flanking regions were amplified and characterized. Fifteen single nucleotide polymorphisms (SNPs) were identified in the exons and their flanking regions. Five exonic polymorphic sites, including two synonymous (c.93T>G and c.1140C>T) and three non-synonymous mutations (c.163C>T (p.R55W), c.605G>A (p.R202H), and c.1141A>G (p.T381A)), were retrieved. PCR-RFLP analysis of c.605G>A showed that the frequencies of allele G in the Nanping black-boned, Nanping normal, and Romney Marsh sheep were 0.632, 0.603, and 0.886, respectively. Sheep with the GG genotype had significantly (P < 0.05) lower tyrosinase activity, alkali-soluble melanin content, and ratio of eumelanin : total melanin than sheep with GA and AA genotypes when measured across all investigated samples but not when samples within each population of sheep were compared. However, there was no association of TYRP2 genotype at a single SNP position with coat colour across populations. Nonetheless, the two breeds with higher overall tyrosinase activity did produce darker and more varied coat colours than the breed with lower tyrosinase activity.

Pigmentation in Black-boned sheep (Ovis aries): association with polymorphism of the MC1R gene

Variations in vertebrate skin and hair color are due to varied amounts of eumelanin (brown/black) and phaeomelanin (red/yellow) produced by the melanocytes. The melanocortin 1 receptor (MC1R) is a regulator of eumelanin and phaeomelanin production in the melanocytes, and MC1R mutations causing coat color changes are known in many vertebrates. We have sequenced the entire coding region of the MC1R gene in Black-boned, Nanping indigenous and Romney Marsh sheep populations and found two silent mutation sites of A12G and G144C, respectively. PCR-RFLP of G144C showed that frequency of allele G in Black-boned, Nanping indigenous and Romney Marsh sheep was 0.818, 0.894 and 0, respectively. Sheep with GG genotype had significantly higher (P < 0.05) tyrosinase activity than sheep with CC genotype in the all investigated samples. Moreover, there was significant effect of MC1R genotype on coat color, suggesting that MC1R gene could affect coat color but not black traits. There would be merit in further studies using molecular techniques to elucidate the cause of black traits in these Black-boned sheep.

Effect of oxidation rate on cross-linking of mussel adhesive proteins

The cross-linking behavior of mussel adhesive protein Mefp-1 was studied by measuring the rate of aggregation of the protein by photon correlation spectroscopy. To be able to calculate the aggregation numbers, the hydrodynamic radius of monomer Mefp-1 (10 nm) was determined under reducing conditions. The aggregation is controlled by the redox potential of the solution, and the aggregation number varied, independent of pH, over a factor 2 within the experimentally accessible redox potential window. A kinetic model for cross-linking, based on the intricate interplay of the oxidation and auto-oxidation of the hydroquinones of Mefp-1, is proposed. The oxidation rate strongly depends on redox potential. The cross-linking rate is taken to be proportional to the rate of auto-oxidation. The model correctly predicts the experimentally observed phenomena. When the oxidation rate is slower than the auto-oxidation rate, cross-linking is efficient and controlled by the oxidation rate. When the rate of auto-oxidation rate is slower than the oxidation rate, the cross-linking is inefficient due to the quick exhaustion of the hydroquinones. The experimentally determined rate constant for cross-linking is found to be much smaller than those found for auto-oxidation of hydroquinones because of the excluded volume interactions imposed by the protein backbone. Tuning the interplay between oxidation and auto-oxidation presents the potential of controlling cross-linking density independent of the density of reactive groups.

Application of dopamine as an electroactive ligand for the determination of aluminum in biological fluids

Dopamine (3,4-dihydroxyphenylethylamine, DA) is applied as an electroactive chelant for indirect determination of aluminum (Al) in biological fluids. It is observed that the decrease of the differential pulse voltammetric (DPV) anodic peak current of DA is linear with the increase of Al concentration. Under optimum experimental conditions (pH 8.6, 2.0 x 10(-4) M DA, and 0.03 M NH4Ac-NH3 x H2O buffer solution), two linear ranges, 5.0 x 10(-8) - 4.0 x 10(-7) M and 4.0 x 10(-7) - 7.2 x 10(-6) M Al(III), are obtained. The detection limit of Al is 1.9 x 10(-8) M and the relative standard deviation for 4 x 10(-6) M Al(III) is 3.1% (N = 8). Many biologically active foreign species have been selected for interference. Excellent recoveries and accuracy have been obtained in the measurements of Al in biological samples such as synthetic renal dialysate, Ringer's solution, human whole blood, cerebrospinal fluid of demented patient, and urine of diabetic patient. The methodological principle that Al complexes with DA on the electroactive position result in the depression of electrochemical activities of DA has been verified by comparing both the electrochemical behaviors and the spectroscopic responses like UV-vis and Raman of DA in the presence and in the absence of Al.

Neurotransmitter dopamine applied in electrochemical determination of aluminum in drinking waters and biological samples

It was demonstrated that the decrease of the differential pulse voltammetric (DPV) anodic peak current of dopamine (3,4-dihydroxyphenylethylamine, DA) was linear with the increase of aluminum (Al) concentration. Under optimum experimental conditions (pH 4.6, 1.2 x 10(-3) M DA, and 0.04 M NaAc-HAc buffer solution), the linear range is 4.0 x 10(-7)-8.0 x 10(-5) M, the detection limit is 1.4 x 10(-7) M, and the relative standard deviation for 4 x 10(-5) M Al(III) is 3.5% (n=8). Many foreign species, especially some low-molecule-weight biological molecules, were chosen for interference testing. The proposed method was applied to the determination of Al in biological samples such as synthetic renal dialysate, Ringer's solution, human blood, cerebrospinal fluid of a patient, and urine of a diabetic patient. The corresponding recoveries were generally between 95 and 105%. The basic principle of the method was determined by examining Al complexed with DA. This results in the blockage of the electroactive sites on DA, followed eventually by the reduction of the electrochemical response of DA. This result was verified by examining the behavior of DA, both in the presence and absence of Al, using electrochemical, UV-Vis, Raman, and (13)C NMR spectroscopic methods.

Cloning and characterization of a dopachrome conversion enzyme from the yellow fever mosquito, Aedes aegypti

In this study we describe the purification and molecular cloning of a dopachrome conversion enzyme (DCE) from the yellow fever mosquito, Aedes aegypti. DCE catalyzes the conversion of L-dopachrome to 5,6-dihydroxyindole in the melanization pathway. Melanin biosynthesis is involved with crucial protective phenomena in mosquitoes, including egg chorion and cuticular tanning, wound healing, and the melanotic encapsulation immune response. The enzyme was purified to homogeneity by various chromatographic techniques from A. aegypti larvae and has a relative molecular mass of 51 kDa as-revealed by SDS-PAGE analysis. Physiochemical analysis of DCE revealed a pH optimum of 7.5-8.0 and substrate activity for L-dopachrome and aminochromes generated from dopa methyl ester, alpha-methyl dopa and dopamine. Trypsin digestion of the isolated DCE and subsequent reverse-phase separation resulted in the isolation of several polypeptide fragments, from which two partial internal amino acid sequences were obtained by Edman degradation. PCR amplification, using a degenerate primer based on one internal amino acid sequence and an oligo-dT primer, produced a 650 bp DNA fragment. Subsequent screening of an A. aegypti pupal cDNA library resulted in the isolation of a 1.6 kb clone containing coding sequence for both internal DCE amino acid sequences, thereby confirming the identity of the isolated gene product (pAaDce1) as DCE. Northern analysis revealed the constitutive expression of DCE message in developmental stages and adults, with the majority of transcript localized in the fat body and ovaries of adult females. AaDce1 mRNA increased in abundance above constitutive levels in adult females when a melanotic encapsulation immune response was initiated by the intrathoracic inoculation of Dirofilaria immitis microfilariae.

Some aspects of tyrosine secondary metabolism

The origin and fate of some tyrosine secondary metabolites within specialized eukaryotic cells are discussed in the light of our knowledge of the plasma environment to which they are exposed throughout their lifetime. Attention is focused on ar-dihydroxy and -trihydroxy derivatives and the corresponding quinoidal counterparts, as well as on the enzymic activities involved in the formation and degradation of these potentially toxic molecules. Some physiopathological and pharmacological implications of the above-mentioned topics are considered, taking into account the well known toxicity of reactive intermediates in molecular oxygen reduction, as well as the reactivity of both semiquinonic and quinonic products of catecholamine oxidation.

Chemical degradation of melanins: application to identification of dopamine-melanin

Melanocytes produce two chemically distinct types of melanin pigments, eumelanins and pheomelanins. These pigments can be quantitatively analyzed by acidic KMnO4 oxidation or reductive hydrolysis with hydriodic acid (HI) to form pyrrole-2,3,5-tricarboxylic acid (PTCA) or aminohydroxyphenylalanine (AHP), respectively. Dark brown melanin-like pigments are also widespread in nature, for example, in the substantia nigra of humans and primates (neuromelanin), in butterfly wings and in the fungus Cryptococcus neoformans. To characterize such diverse types of melanins, we have improved the alkaline H2O2 oxidation method of Napolitano et al. (Tetrahedron, 51:5913-5920, 1995) and re-examined the HI hydrolysis method of Wakamatsu et al. (Neurosci. Lett., 131:57-60, 1991). The results obtained with H2O2 oxidation show that 1) pyrrole-2,3-dicarboxylic acid (PDCA), a specific marker of 5,6-dihydroxyindole units in melanins, is produced in yields ten times higher than by acidic KMnO4 oxidation, and 2) PTCA is artificially produced from pheomelanins. The results with HI hydrolysis show that dopamine-melanin produces a 1:1 mixture of 3-amino and 4-amino isomers of aminohydroxyphenylethylamine, while the isomer ratio is about 0.2 in melanins prepared from dopamine and cysteine. These results indicate that alkaline H2O2 oxidation is useful in characterizing synthetic and natural eumelanins and that reductive hydrolysis with HI can be applied to analyzing oxidation products of dopamine such as neuromelanin.

Melanin biosynthesis in Cryptococcus neoformans

Pigment production by Cryptococcus neoformans is virulence associated. Dopamine- and 3,4-dihydroxyphenylalanine-melanin products were identified after acidic permanganate oxidation, alkaline hydrogen peroxide oxidation, or hydrolysis with hydriodic acid. These data provide direct chemical evidence for the formation of eumelanin polymers by catecholamine oxidation by laccase alone followed by oxidative coupling of dihydroxyindole.

Oxidation pathways of hydroxydopamines and their chloride and bromide salts: an electrochemical study in acidic solutions

The anodic oxidation of 6-hydroxydopamine and 5-hydroxydopamine and their chloride and bromide salts was studied in aqueous acidic solutions. The electrodes used were Pt and Pt/M (UPD) (M = Bi, Tl, Pb) modified surfaces. The electrochemical studies revealed that the first two-electron oxidation leads to formation of reactive quinoid intermediates. Reactions of these quinoid intermediates with nucleophiles yield substituted quinones that are not stable in solution. The reactions are initiated by the oxidation of the third hydroxyl group at more positive potentials. Cyclic voltammetry and rotating ring-disc electrode voltammetry were used to elucidate the reaction mechanisms. These techniques provide powerful tools to generate and detect transient intermediates and products and give valuable information regarding the pathways and mechanisms of the redox reactions. Key words: 6-hydroxydopamine, 5-hydroxydopamine, voltammetry, electrolysis, oxidation mechanism, nucleophilic attack.

The effects of hydroxyl radical attack on dopa, dopamine, 6-hydroxydopa, and 6-hydroxydopamine

High pressure liquid chromatography with electrochemical detection (HPLC-ED) was employed in conjugation with a sensitive and specific salicylate hydroxylation assay to evaluate the immediate effects of hydroxyl radical (.OH) attack on four catechol intermediates of eumelanin, dopamine (3,4-dihydroxyphenylethylamine), its precursor dopa (3,4-dihydroxyphenylalanine), and their respective neurotoxic trihydroxyphenyl derivatives, 6-hydroxydopamine (2,4,5-trihydroxyphenylethylamine,6-OHDA) and 6-hydroxydopa(2,4,5-trihydroxyphenylalanine, TOPA). Semiquinone and quinone species were identified as the initial products of the oxidation of these four catechol substrates. The enhanced oxidations of the catechols when exposed to .OH attack was accompanied by marked decreases in the level of each semiquinone species. Quinone levels were elevated in reactions involving .OH attack on dopamine and 6-OHDA, but absent in reactions involving radical attack on dopa or TOPA, suggesting that dopaquinone (DOQ) and TOPA p-quinone (TOPA p-Q) are oxidized more rapidly by .OH than are the quinones of dopamine and 6-OHDA. The formation of 6-OHDA p-quinone (6-OHDA p-Q) in incubations involving DA and .OH suggest that the .OH-mediated hydroxylation of DA may be a mechanism for generating this potentially cytotoxic trihydroxyphenyl. The results of this study demonstrate for the first time that semiquinone and quinone intermediates of eumelanin are the initial products derived from the .OH-mediated oxidations of dopa, DA, TOPA, and 6-OHDA. These observations suggest that if .OH is generated beyond the capabilities of cytoprotective mechanisms, the radical can rapidly oxidize catechol precursors, augment melanogenesis, and generate additional cytotoxic quinoid intermediates of eumelanin.

Egg chorion tanning in Aedes aegypti mosquito

The biochemical pathway of egg chorion tanning in the mosquito, Aedes aegypti, is described and compared with chorion protein crosslinking in Drosophila and silkmoths and the biochemical pathways of cuticular tanning in insects. Phenol oxidase, dopa decarboxylase and tyrosine are critical components involved in egg chorion tanning in A. aegypti. Tanning of the mosquito egg chorion is initiated following activation of phenol oxidase, which then catalyzes the hydroxylation of tyrosine to dopa and further oxidizes dopa and dopamine to their respective o-quinones. Because intramolecular cyclization is much slower in dopaminequinone than dopaquinone, the chance to react with external nucleophiles to participate in protein crosslinking reactions also is much greater in dopaminequinone than dopaquinone. This might partly explain the necessity for the involvement of dopa decarboxylase in mosquito chorion tanning. Intramolecular cyclization of dopaquinone and dopaminequinone to form dopachrome and dopaminechrome, respectively, the structural rearrangement of these aminochromes to produce 5,6-dihydroxyindole, and the subsequent oxidation of 5,6-dihydroxyindole by phenol oxidase also lead to melanin formation during egg chorion tanning.