Quinone methide as a new intermediate in eumelanin biosynthesis

Iron catalyzed C–C dehydrogenative coupling reaction: synthesis of arylquinones from quinones/hydroquinones

An atom-economical approach for the synthesis of arylquinones was achieved successfully via direct oxidative C–C dehydrogenative coupling reaction of quinones/hydroquinones with electron-rich arenes using an inexpensive Fe–I₂–(NH₄)₂S₂O₈ system. The efficiency of this catalytic approach was established with a broad scope of substrates involving quinones and hydroquinones to give high yields (60–89%) of several arylated quinones. The present protocol is simple, practical, and shows good functional group tolerance.

The synthesis of arylquinones was achieved via direct oxidative C–H/C–H cross-coupling of quinones/hydroquinones with electron-rich arenes using Fe–I₂–(NH₄)₂S₂O₈ system involving quinones/hydroquinones to give high yields (60–89%) of arylquinones.

Drosophila yellow-h encodes dopaminechrome tautomerase: A new enzyme in the eumelanin biosynthetic pathway

Melanin is a widely distributed phenolic pigment that is biosynthesized from tyrosine and its hydroxylated product, dopa, in all animals. However, recent studies reveal a significant deviation from this paradigm, as insects appear to use dopamine rather than dopa as the major precursor of melanin. This observation calls for a reconsideration of the insect melanogenic pathway. While phenoloxidases and laccases can oxidize dopamine for dopaminechrome production, the fate of dopaminechrome remains undetermined. Dopachrome decarboxylase/tautomerase, encoded by yellow-f/f2 of Drosophila melanogaster, can convert dopaminechrome into 5,6-dihydroxyindole, but the same enzyme from other organisms does not act on dopaminechrome, suggesting the existence of a specific dopaminechrome tautomerase (DPT). We now report the identification of this novel enzyme that biosynthesizes 5,6-dihydroxyindole from dopaminechrome in Drosophila. Dopaminechrome tautomerase acted on both dopaminechrome and N-methyl dopaminechrome but not on dopachrome or other aminochromes tested. Our biochemical and molecular studies reveal that this enzyme is encoded by the yellow-h gene, a member of the yellow gene family, and advance our understanding of the physiological functions of this gene family. Identification and characterization of DPT clarifies the precursor for melanin biosynthetic pathways and proves the existence of an independent melanogenic pathway in insects that utilizes dopamine as the primary precursor.

Multi-colored shades of betalains: recent advances in betacyanin chemistry

Covering: 2001 to 2021Betacyanins cover a class of remarkable natural red-violet plant pigments with prospective chemical and biological properties for wide-ranging applications in food, pharmaceuticals, and the cosmetic industry. Betacyanins, forming the betalain pigment group together with yellow betaxanthins, have gained much attention due to the increasing social awareness of the positive impact of natural products on human health. Betalains are commercially recognized as natural food colorants with preliminarily ascertained, but to be further investigated, health-promoting properties. In addition, they exhibit a remarkable structural diversity based on glycosylated and acylated varieties. The main research directions for natural plant pigments are focused on their structure elucidation, methods of their separation and analysis, biological activities, bioavailability, factors affecting their stability, industrial applications as a plant-based food, natural colorants, drugs, and cosmetics as well as methods for high-yield production and stabilization. This review covers period of the last two decades of betacyanin research. In the first part of the review, we present an updated classification of all known betacyanins and their derivatives identified by chemical means as well as by mass spectrometric and NMR techniques. In the second part, we review the current research reports focused on the chemical properties of the pigments (decarboxylation, oxidation, conjugation, and chlorination reactions as well as the acyl group migration phenomenon) and describe the semi-synthesis of natural and artificial fluorescent betalamic acid conjugates, showing various prospective research directions.

Nonenzymatic Spontaneous Oxidative Transformation of 5,6-Dihydroxyindole

Melanin is an important phenolic skin pigment found throughout the animal kingdom. Tyrosine and its hydroxylated product dopa provide the starting material for melanin biosynthesis in all animals. Through a set of well-established reactions, they are converted to 5,6-dihydroxyindole (DHI) and DHI-2-carboxylic acid (DHICA). Oxidative polymerization of these two indoles produces the brown to black eumelanin pigment. The steps associated with these transformations are complicated by the extreme instability of the starting materials and the transient and highly reactive nature of the intermediates. We have used mass spectral studies to explore the nonenzymatic mechanism of oxidative transformation of DHI in water. Our results indicate the facile production of not only dimeric and trimeric products but also higher oligomeric forms of DHI upon exposure to air in solution, even under nonenzymatic conditions. Such instantaneous polymerization of DHI avoids toxicity to self-matter and ensures the much-needed deposition of melanin at (a) the wound site and (b) the infection site in arthropods. The rapid deposition of DHI melanin is advantageous for arthropods given their open circulatory system; the process limits blood loss during wounding and prevents the spread of parasites by encapsulating them in melanin, limiting the damage.

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.

Iron-mediated site-selective oxidative C–H/C–H cross-coupling of aryl radicals with quinones: synthesis of β-secretase-1 inhibitor B and related arylated quinones

Phenoxy radicals were converted intoparasite selectiveC-aryl radicals and coupled with quinones, using an inexpensive FeCl₃–K₂S₂O₈system, to obtain several arylated quinones, in good to moderate yields.

Molecular basis of dimer formation during the biosynthesis of benzofluorene-containing atypical angucyclines

Lomaiviticin A and difluostatin A are benzofluorene-containing aromatic polyketides in the atypical angucycline family. Although these dimeric compounds are potent antitumor agents, how nature constructs their complex structures remains poorly understood. Herein, we report the discovery of a number of fluostatin type dimeric aromatic polyketides with varied C−C and C−N coupling patterns. We also demonstrate that these dimers are not true secondary metabolites, but are instead derived from non-enzymatic deacylation of biosynthetic acyl fluostatins. The non-enzymatic deacylation proceeds via a transient quinone methide like intermediate which facilitates the subsequent C–C/C−N coupled dimerization. Characterization of this unusual property of acyl fluostatins explains how dimerization takes place, and suggests a strategy for the assembly of C–C and C–N coupled aromatic polyketide dimers. Additionally, a deacylase FlsH was identified which may help to prevent accumulation of toxic quinone methides by catalyzing hydrolysis of the acyl group.

Benzofluorene-containing angucyclines, bacterial natural compounds with potential use as therapeutics/antibiotics, occur as dimers. Here, the authors elucidated the dimerization mechanism which turned out to work spontaneously, without enzymatic catalysis.

A Three-Ring Circus: Metabolism of the Three Proteogenic Aromatic Amino Acids and Their Role in the Health of Plants and Animals

Tyrosine, phenylalanine and tryptophan are the three aromatic amino acids (AAA) involved in protein synthesis. These amino acids and their metabolism are linked to the synthesis of a variety of secondary metabolites, a subset of which are involved in numerous anabolic pathways responsible for the synthesis of pigment compounds, plant hormones and biological polymers, to name a few. In addition, these metabolites derived from the AAA pathways mediate the transmission of nervous signals, quench reactive oxygen species in the brain, and are involved in the vast palette of animal coloration among others pathways. The AAA and metabolites derived from them also have integral roles in the health of both plants and animals. This review delineates the de novo biosynthesis of the AAA by microbes and plants, and the branching out of AAA metabolism into major secondary metabolic pathways in plants such as the phenylpropanoid pathway. Organisms that do not possess the enzymatic machinery for the de novo synthesis of AAA must obtain these primary metabolites from their diet. Therefore, the metabolism of AAA by the host animal and the resident microflora are important for the health of all animals. In addition, the AAA metabolite-mediated host-pathogen interactions in general, as well as potential beneficial and harmful AAA-derived compounds produced by gut bacteria are discussed. Apart from the AAA biosynthetic pathways in plants and microbes such as the shikimate pathway and the tryptophan pathway, this review also deals with AAA catabolism in plants, AAA degradation via the monoamine and kynurenine pathways in animals, and AAA catabolism via the 3-aryllactate and kynurenine pathways in animal-associated microbes. Emphasis will be placed on structural and functional aspects of several key AAA-related enzymes, such as shikimate synthase, chorismate mutase, anthranilate synthase, tryptophan synthase, tyrosine aminotransferase, dopachrome tautomerase, radical dehydratase, and type III CoA-transferase. The past development and current potential for interventions including the development of herbicides and antibiotics that target key enzymes in AAA-related pathways, as well as AAA-linked secondary metabolism leading to antimicrobials are also discussed.

Re-evaluation of insect melanogenesis research: Views from the dark side

Melanins (eumelanin and pheomelanin) are synthesized in insects for several purposes including cuticle sclerotization and color patterning, clot formation, organogenesis, and innate immunity. Traditional views of insect immunity detail the storage of pro-phenoloxidases inside specialized blood cells (hemocytes) and their release upon recognition of foreign bodies. Activated phenoloxidases convert monophenols into reactive quinones in a two-step enzymatic reaction, and until recently, the mechanism of tyrosine hydroxylation remained a mystery. Herein, we present our interpretations of these enzyme-substrate complexes. The resultant melanins are deposited onto the surface of microbes to immobilize, agglutinate, and suffocate them. Phenoloxidase activity and melanin production are not limited to the blood (hemolymph) or cuticle, as recent evidence points to more diverse, sophisticated interactions in the gut and with the resident symbionts. This review offers insight into the somewhat neglected areas of insect melanogenesis research, particularly in innate immunity, its role in beneficial insects such as pollinators, the functional versatility of phenoloxidases, and the limitations of common experimental approaches that may impede progress inadvertently.

Critical Analysis of the Melanogenic Pathway in Insects and Higher Animals

Animals synthesize melanin pigments for the coloration of their skin and use it for their protection from harmful solar radiation. Insects use melanins even more ingeniously than mammals and employ them for exoskeletal pigmentation, cuticular hardening, wound healing and innate immune responses. In this review, we discuss the biochemistry of melanogenesis process occurring in higher animals and insects. A special attention is given to number of aspects that are not previously brought to light: (1) the molecular mechanism of dopachrome conversion that leads to the production of two different dihydroxyindoles; (2) the role of catecholamine derivatives other than dopa in melanin production in animals; (3) the critical parts played by various biosynthetic enzymes associated with insect melanogenesis; and (4) the presence of a number of important gaps in both melanogenic and sclerotinogenic pathways. Additionally, importance of the melanogenic process in insect physiology especially in the sclerotization of their exoskeleton, wound healing reactions and innate immune responses is highlighted. The comparative biochemistry of melanization with sclerotization is also discussed.

Reactivities of Quinone Methides versus o-Quinones in Catecholamine Metabolism and Eumelanin Biosynthesis

Melanin is an important biopolymeric pigment produced in a vast majority of organisms. Tyrosine and its hydroxylated product, dopa, form the starting material for melanin biosynthesis. Earlier studies by Raper and Mason resulted in the identification of dopachrome and dihydroxyindoles as important intermediates and paved way for the establishment of well-known Raper-Mason pathway for the biogenesis of brown to black eumelanins. Tyrosinase catalyzes the oxidation of tyrosine as well as dopa to dopaquinone. Dopaquinone thus formed, undergoes intramolecular cyclization to form leucochrome, which is further oxidized to dopachrome. Dopachrome is either converted into 5,6-dihydroxyindole by decarboxylative aromatization or isomerized into 5,6-dihydroxyindole-2-carboxylic acid. Oxidative polymerization of these two dihydroxyindoles eventually produces eumelanin pigments via melanochrome. While the role of quinones in the biosynthetic pathway is very well acknowledged, that of isomeric quinone methides, however, remained marginalized. This review article summarizes the key role of quinone methides during the oxidative transformation of a vast array of catecholamine derivatives and brings out the importance of these transient reactive species during the melanogenic process. In addition, possible reactions of quinone methides at various stages of melanogenesis are discussed.

Mechanism of dopachrome tautomerization into 5,6-dihydroxyindole-2-carboxylic acid catalyzed by Cu(II) based on quantum chemical calculations

BACKGROUND

Tautomerization of dopachrome to 5,6-dihydroxyindole-2-carboxylic acid (DHICA) is a biologically crucial reaction relevant to melanin synthesis, cellular antioxidation, and cross-talk among epidermal cells. Since dopachrome spontaneously converts into 5,6-dihydroxyindole (DHI) via decarboxylation without any enzymes at physiologically usual pH, the mechanism of how tautomerization to DHICA occurs in physiological system is a subject of intense debate. A previous work has found that Cu(II) is an important factor to catalyze the tautomerization of dopachrome to DHICA. However, the effect of Cu(II) on the tautomerization has not been clarified at the atomic level.

METHODS

We propose the reaction mechanism of the tautomerization to DHICA by Cu(II) from density functional theory-based calculation.

RESULTS

We clarified that the activation barriers of α-deprotonation, β-deprotonation, and decarboxylation from dopachrome are significantly reduced by coordination of Cu(II) to quinonoid oxygens (5,6-oxygens) of dopachrome, with the lowest activation barrier of β-deprotonation among them. In contrast to our previous work, in which β-deprotonation and quinonoid protonation (O5/O6-protonation) were shown to be important to form DHI, our results show that the Cu(II) coordination to quinonoid oxygens inhibits the quinonoid protonation, leading to the preference of proton rearrangement from β-carbon to carboxylate group but not to the quinonoid oxygens.

CONCLUSION

Integrating these results, we conclude that dopachrome tautomerization first proceeds via proton rearrangement from β-carbon to carboxylate group and subsequently undergoes α-deprotonation to form DHICA.

GENERAL SIGNIFICANCE

This study would provide the biochemical basis of DHICA metabolism and the generalized view of dopachrome conversion which is important to understand melanogenesis.

Effect of pH on elementary steps of dopachrome conversion from first-principles calculation

Dopachrome conversion, in which dopachrome is converted into 5,6-dihydroxyindole (DHI) or 5,6-dihydroxyindole-2-carboxylic acid (DHICA) upstream of eumelanogenesis, is a key step in determining the DHI/DHICA monomer ratio in eumelanin, which affects the antioxidant activity. Although the ratio of DHI/DHICA formed and the conversion rate can be regulated depending on pH, the mechanism is still unclear. To clarify the mechanism, we carried out first-principles calculations. The results showed the kinetic preference of proton rearrangement to form quinone methide intermediate via β-deprotonation. We also identified possible pathways to DHI/DHICA from the quinone methide. The DHI formation can be achieved by spontaneous decarboxylation after proton rearrangement from carboxyl group to 6-oxygen. α-Deprotonation, which leads to DHICA formation, can also proceed with a significantly reduced activation barrier compared with that of the initial dopachrome. Considering the rate of the proton rearrangements in a given pH, we conclude that the conversion is suppressed at acidic pH.

Actinobacterial melanins: current status and perspective for the future

Melanins are enigmatic pigments that are produced by a wide variety of microorganisms including several species of bacteria and fungi. Melanins are biological macromolecules with multiple important functions, yet their structures are not well understood. Melanins are frequently used in medicine, pharmacology, and cosmetics preparations. Melanins also have great application potential in agriculture industry. They have several biological functions including photoprotection, thermoregulation, action as free radical sinks, cation chelators, and antibiotics. Plants and insects incorporate melanins as cell wall and cuticle strengtheners, respectively. Actinobacteria are the most economically as well as biotechnologically valuable prokaryotes. However, the melanin properties are, in general, poorly understood. In this review an evaluation is made on the present state of research on actinobacterial melanins and its perspectives. The highlights include the production and biotechnological applications of melanins in agriculture, food, cosmetic and medicinal fields. With increasing advancement in science and technology, there would be greater demands in the future for melanins produced by actinobacteria from various sources.

Structure, biosynthesis and possible function of tunichromes and related compounds

Several species of ascidians (phylum Chordata, subphylum Urochordata) contain a group of oligopeptides called "tunichromes" in their blood cells. These peptides have been implicated in (a) metal chelation and accumulation/sequestration of vanadium or iron; (b) crosslinking of structural fibers in tunic formation, (c) wound healing and (d) defense reactions. However, their biosynthesis, metabolism, and biological function remain largely un-elucidated due to their extreme instability and high reactivity. Tunichromes and related compounds uniquely possess dehydrodopamine moieties, all originating from post-translational modification of peptidyl tyrosine. It is conceivable that the presence of such novel post-translationally modified groups provide attributes that are crucial for their biological roles. Therefore, we examined the chemistry and reactivity of tunichromes in light of the available knowledge of the biochemistry of simple monomeric dehydro-N-acyldopamine units. Based on the reactivity of such simple compounds, the potential biological activities of tunichromes are predicted. Their possible biosynthetic route from peptidyl tyrosine is critically evaluated to provide a better basis for unraveling their biological functions. Prevalence of dehydro-N-acyldopamine units in different tunichromes, some marine antibiotic compounds, insect cuticular sclerotizing precursors and some bioadhesive marine proteins may aid in the de novo design of unique biomaterials with potential antibiotic/adhesive properties.

New pathways of betanidin and betanin enzymatic oxidation

Betanidin is a basic betacyanin with a 5,6-dihydroxyl moiety which causes its high antioxidant activity. For the purpose of structural study, the enzymatic oxidation of betanidin and betanin (5-O-glucosylated betanidin), followed by chromatographic separation of the oxidation products with spectrophotometric and mass spectrometric detection (LC-DAD-MS/MS) was performed. Within the pH 4-8 range, two main oxidation peaks of betanidin were observed, betanidin quinonoid (possibly betanidin o-quinone) and 2-decarboxy-2,3-dehydrobetanidin, whereas at pH 3 only dehydrogenated and decarboxylated derivatives were detected, suggesting different stabilities of the products at different pH values. The presence of two prominent oxidation products, 2-decarboxy-2,3-dehydrobetanidin and 2,17-bidecarboxy-2,3-dehydrobetanidin, at pH 3 indicates their generation via two possible reaction routes with two different quinonoid intermediates: dopachrome derivative and quinone methide. Both reaction paths lead to the decarboxylative dehydrogenation of betanidin. Subsequent oxidation and rearrangement of the conjugated chromophoric system results in the formation of 14,15-dehydrogenated derivatives. Betanin is oxidized with generation of a quinone methide intermediate, which rearranges to 2,3-dehydro- or neoderivatives. The products of enzymatic oxidation of betacyanins thus formed are derivatives of 5,6-dihydroxyindole and related structures known as the key intermediates in melanogenesis.

New insights into the active site structure and catalytic mechanism of tyrosinase and its related proteins

Tyrosinases are widely distributed in nature. They are copper-containing oxidases belonging to the type 3 copper protein family, together with catechol oxidases and haemocyanins. Tyrosinases are essential enzymes in melanin biosynthesis and therefore responsible for pigmentation of skin and hair in mammals, where two more enzymes, the tyrosinase-related proteins (Tyrps), participate in the pathway. The structure and catalytic mechanism of mammalian tyrosinases have been extensively studied but they are not completely understood because of the lack of information on the tertiary structure. The availability of crystallographic data of one plant catechol oxidase and one bacterial tyrosinase has improved the model of the three-dimensional structure of the active site of the enzyme. Furthermore, sequence comparison of tyrosinase and the Tyrps reveals that the three orthologue proteins share many key structural features, because of their common origin from an ancestral gene, although the specific residues responsible for their different catalytic capabilities have not been identified yet. This review summarizes our current knowledge of tyrosinase and Tyrps structure and function and describes the catalytic mechanism of tyrosinase and Dct/Tyrp2, which are better characterized.

Inhibition of Enzyme Activities and the Antiwrinkle Effect of Polyphenol Isolated from the Persimmon Leaf (Diospyros kaki folium) on Human Skin

BACKGROUND

Many studies have been conducted to find a natural material that has high biologic functions for human skin without any side effects. Persimmon leaf has a substantial amount of tannins in different forms; therefore, it was selected as a target material.

OBJECTIVE

The biosynthesis rate of the collagen was also investigated to clarify the beneficial functions for the human skin.

METHODS

Persimmon leaves were obtained, extracted with 80% ethanol, and isolated into PFs I, II, and III after column chromatography using a Sephadex LH-20 column followed by thin-layer chromatography.

RESULTS

The xanthine oxidase inhibition effect of both PFs II and III was over 40% at a 100 ppm concentration. PF II, containing higher flavonoids levels, had a significantly higher tyrosinase inhibition than that of PF III. Collagenase inhibition was 16.3 and 8.1% for PF III and PF II, respectively, at 100 ppm. On the other hand, elastase inhibition activity was significantly higher in PF II than PF III. Collagen biosynthesis rates of PF III were over 25% from a 1 to 10 ppm concentration. Consequently, PFs isolated from the persimmon leaf can be used as natural materials or additives for human skin owing to their beneficial biologic functions, including the antiwrinkle effect and the inhibition of skin problems, for food or cosmetic compositions.

Role of dopachrome conversion enzyme in the melanization of filarial worms in mosquitoes

Melanization is an effective defence reaction of mosquito hosts against invading parasites. In mosquitoes, the biosynthesis of melanin is initiated by the hydroxylation of tyrosine to DOPA by phenoloxidase (PO). DOPA is a branch point of the melanization reaction; it may be oxidized to dopaquinone by PO or be decarboxylated to dopamine by dopa decarboxylase. Further oxidation of dopaquinone by PO produces dopachrome. Dopachrome is then converted to 5, 6-dihydroxyindole by dopachrome conversion enzyme (DCE) to produce melanin. The conversion of dopachrome is a rate-limiting step of the melanization reaction, and the presence of PO and DCE significantly accelerates melanization reactions. In this study, a cDNA encoding DCE was cloned from the mosquito Armigeres subalbatus. Real-time PCR analysis revealed increased transcripts from haemocytes in microfilariae (mf)-inoculated mosquitoes. Gene silencing using double-stranded RNA was used to elucidate the role of DCE in the melanization reaction of parasites in Ar. subalbatus. The levels of both DCE transcripts and protein in gene knockdown mosquitoes were dramatically reduced. Compared with controls, the degree of melanization of mf in DCE-knockdown mosquitoes was significantly decreased. These results suggest that DCE is a critical enzyme that is required for effective melanization immune responses.

An oxygen transporter hemocyanin can act on the late pathway of melanin synthesis

5,6-Dihydroxyindole (DHI) and 5,6-dihydroxyindole-2-carboxylic acid (DHICA) are precursors of eumelanin. The effects of crustacean hemolymph proteins on these eumelanin-related metabolites were investigated. Zymogram analysis indicated that polymers of hemocyanin (Hc) subunits converted DHI into black pigment while no effects were observed using DHICA as a substrate. Spectrum changes for mixtures of purified Hc and DHI showed a profile similar to oxidized DHI by mushroom tyrosinase while Hc had only slight effects on DHICA. Typical inhibitors of tyrosinase and phenoloxidase severely hampered the production of oxidized DHI. Taken together with previous results, these data indicate that Hc plays a crucial role in the conversion of DHI in the hemolymph of crustaceans, which promotes late reactions in the melanin synthetic pathway as well as early reactions (oxidation of tyrosine and DOPA to dopaquinone).

Melanogenesis and associated cytotoxic reactions: applications to insect innate immunity

Insects transmit the causative agents for such debilitating diseases as malaria, lymphatic filariases, sleeping sickness, Chagas' disease, leishmaniasis, river blindness, Dengue, and yellow fever. The persistence of these diseases provides testimony to the genetic capacity of parasites to evolve strategies that ensure their successful development in two genetically diverse host species: insects and mammals. Current efforts to address the problems posed by insect-borne diseases benefit from a growing understanding of insect and mammalian immunity. Of considerable interest are recent genomic investigations that show several similarities in the innate immune effector responses and associated regulatory mechanisms manifested by insects and mammals. One notable exception, however, is the nearly universal presence of a brown-black pigment accompanying cellular innate immunity in insects. This response, which is unique to arthropods and certain other invertebrates, has focused attention on the elements involved in pigment synthesis as causing or contributing to the death of the parasite, and has even prompted speculation that the enzyme cascade mediating melanogenesis constitutes an ill-defined recognition mechanism. Experimental evidence defining the role of melanin and its precursors in insect innate immunity is severely lacking. A great deal of what is known about melanogenesis comes from studies of the process occurring in mammalian systems, where the pigment is synthesized by such diverse cells as those comprising portions of the skin, hair, inner ear, brain, and retinal epithelium. Fortunately, many of the components in the metabolic pathways leading to the formation of melanin have been found to be common to both insects and mammals. This review examines some of the factors that influence enzyme-mediated melanogenic responses, and how these responses likely contribute to blood cell-mediated, target-specific cytotoxicity in immune challenged insects.

Aluminum Facilitation of the Iron-Mediated Oxidation of DOPA to Melanin

Aluminum, a trivalent cation unable to undergo redox reactions, is shown to faciliate iron-initiated DOPA oxidation in the melanin pathway under acidic condition of pH 5.5, which is a favored medium for aluminum facilitation of iron-induced lipid peroxidation. In the process of oxidation of DOPA to melanin in the presence of the metal ions, Fe3+ and H2O2 oxidize DOPA to dopachrome (DC), then Al3+ catalyzes the conversion of DC to 5,6-dihydroxyindole (DHI) and finally Fe3+ oxidizes DHI to indole-5,6-quinone (IQ), which polymerizes immediately to melanochrome and melanin. The reactions involve the intermediate complexes of metal ions and DOPA or its derivative. The present results indicate that aluminum can enhance the oxidative stress on iron-mediated DOPA oxidation in melanin pathway under acidic condition through the cooperation of iron and aluminum ions.

Aluminum ions accelerated the oxidative stress of copper-mediated melanin formation

A comparison between the effects of aluminum and cupric ions on the dopachrome (DC) conversion and the cooperation effect of the both ions in the DOPA oxidation to melanin pathway has been studied by UV-Vis spectrophotometric method. Both aluminum and cupric ions catalyze the DC conversion reaction, which is an important step in the melanin synthesis pathway. However, cupric ions catalyze the conversion of DC to yield 5,6-dihydroxyindole-2-carboxylic acid (DHICA) but the product of DC conversion catalyzed by aluminum is 5,6-dihydroxyindole (DHI). DOPA oxidation catalyzed by aluminum and cupric ions is studied in the presence of hydrogen peroxide. The results from our experiments provide evidence that aluminum can markedly increase the oxidative stress of copper-mediated the melanin formation and influence the properties of the melanin by means of changing the ratio of DHICA/DHI in the acidic environment (pH 5.5).

Effect of aluminum (III) on the conversion of dopachrome in the melanin synthesis pathway

The effect of aluminum ions on the kinetics and mode of the conversion of dopachrome (DC) in acidic environment has been studied using UV-Vis spectrophotometric and cyclic voltammetric methods. The DC conversion step is an important reaction in melanogenesis. Aluminum ions catalyze greatly the decarboxylative transformation of DC to give 5,6-dihydroxyindole (DHI) rather than 5,6-dihydroxyindole-2-carboxylic acid (DHICA) at pH 5.5, which enhance the ratio of formation DHI/DHICA in melanin synthesis pathway. The kinetics of DC conversion catalyzed by aluminum ions is dependent on the concentration of DC and aluminum ions. These results provide evidence that aluminum ions could play a role in the synthesis of melanin pathway in acidic condition through catalyzing the DC decarboxylative transformation to yield DHI and influence the melanin structure and properties.

Comparative biochemistry of eumelanogenesis and the protective roles of phenoloxidase and melanin in insects

The phenolic biopolymer eumelanin is an important skin pigment found throughout the animal kingdom. The enzyme, tyrosinase, initiates melanogenesis in mammals. The biogenesis is assisted by a number of mammalian protein factors including dopachrome tautomerase and 5,6-dihydroxyindole-2-carboxylate oxidase. Invertebrates, such as insects, employ phenoloxidase and dopachrome (decarboxylating) isomerase for melanin biosynthesis. Recently generated molecular biological and biochemical data indicate that tyrosinase and phenoloxidase are distinctly different enzymes in spite of possessing both monophenol monooxygenase activity as well as o-diphenoloxidase activity. Similarly, insect dopachrome isomerase also differs significantly from its mammalian counterpart in several of its properties including the nature of the enzymatic reaction. In addition, there are considerable differences in the eumelanogenic pathways of these two animal groups that include the utility of substrates, use of dihydroxyindoles and the nature of eumelanin pigment. Thus, the biochemistry and molecular biology of melanogenesis in mammals and insects are significantly different. The advantages of generating different eumelanin pigments and intermediates by the insects are discussed.

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.

Insect melanogenesis. III. Metabolon formation in the melanogenic pathway-regulation of phenoloxidase activityy by endogenous dopachrome isomerase (decarboxylating) from Manduca sexta

Tyrosinase initiates melanogenesis in a variety of organisms. The nature of melanin formed is modified subsequently by dopachrome isomerase and other melanogenic proteins. Earlier, we reported the partial purification of dopachrome isomerase (decarboxylating) from the hemolymph of Manduca sexta and demonstrated the generation of a new quinone methide intermediate during melanogenesis (Sugumaran, M., and Semensi, V. (1991) J. Biol. Chem. 266, 6073-6078). In this paper, we report the purification of this enzyme to homogeneity and a novel inhibition mechanism for regulation of phenoloxidase activity. The activity of phenoloxidase isolated from M. sexta was markedly inhibited by purified dopachrome isomerase. In turn, phenoloxidase also reciprocated by inhibiting the isomerase activity. Preformed dopaminechrome did not serve as the substrate for the isomerase; but dopaminechrome that generated in situ by phenoloxidase was readily converted to melanin pigment by the phenoloxidase/isomerase mixture. Furthermore, the isomerase, which has a molecular weight of about 40,000 in native state, exhibited retardation during affinity electrophoresis on sodium dodeyl sulfate (SDS)-polyacrylamide gel electrophoresis gel copolymerized with tyrosinase and migrated with a molecular weight of 50,000, indicating complex formation with phenoloxidase. Electrophoresis of pupal cuticular extract on polyacrylamide gel, followed by activity staining revealed the presence of a protein band carrying both phenoloxidase and isomerase activity. Accordingly, a high-molecular-weight melanogenic complex was isolated from the pharate cuticle of M. sexta. The complex catalyzed the generation of melanochrome from dopa, while the free phenoloxidase produced only dopachrome from the same substrate. When the complex was treated with trace amounts of SDS, which inhibited the activity of dopachrome isomerase present in the complex, then only the conversion of dopa to dopachrome was observed. These studies confirm the formation of a melanogenic complex between phenoloxidase and dopachrome isomerase. By forming a complex and regulating each other's activity, these two enzymes seem to control the levels of endogenous quinones.

Characterization of a new phenoloxidase inhibitor from the cuticle of Manduca sexta

Melanin, the phenolic biopolymer that serves as a skin- and hair pigment-protecting agent against harmful solar radiation and a free radical trap, is biosynthesized in animals mainly by the action of tyrosinase also known as phenoloxidase. Regulation of tyrosinase and hence melanogenesis is vital for all animals. In this report, we present the isolation and characterization of a new, heat-labile glycoprotein inhibitor of phenoloxidase from the larvae of Manduca sexta. The inhibitor was isolated from the live larval cuticle by buffer extraction and purified to homogeneity employing ammonium sulfate precipitation, dialysis, and concanavalin A-Sepharose chromatography. It migrated with a molecular weight of 380,000 on SDS-PAGE gels and inhibited the activity of insect and plant as well as fungal phenoloxidases. Inhibitor formed a tight complex with phenoloxidases, which resisted dissociation even by 1% Triton X-100 or SDS. Selective inhibition of phenoloxidase, while acting on certain but not all different substrates, was observed. The physiological importance of this newly discovered high-molecular-weight phenoloxidase inhibitor is discussed.

A method for detecting dopaminechrome isomerase activity on gels

Melanogenesis involves oxidation of 3,4-dihydroxyphenylalanine (dopa) to dopachrome which then is converted into 5,6-dihydroxyindole by dopachrome isomerase. 5,6-Dihydroxyindole is oxidized to its quinone which in turn is metabolized nonenzymatically to melanin. In addition to dopachrome isomerase, a new dopminechrome isomerase activity involved in the conversion of dopaminechrome into 5,6-dihydroxyindole has been observed in the larva of Rhinoceros oryctes. This dopaminechrome isomerase differs from dopachrome isomerase in its electrophoretic mobility and substrate specificity. The present study reports a specific, sensitive and rapid staining method for detecting dopaminechrome isomerase activity after electrophoresis. Using this new method, the presence of the dopaminechrome isomerase activity, which is involved in melanogenesis, could easily be detected by staining tyrosinase embedded native gels in dopamine solution. Tyrosinase entrapped in the gels converts dopamine in dopaminechrome. The dopaminechrome isomerase separated in the gels catalyzes dopaminechrome to 5,6-dihydroxyindole which is oxidized further by tyrosinase to colored melanochrome. The dopaminechrome isomerase appears as a bluish purple band against a pink background.

Enzymatic grafting of a natural product onto chitosan to confer water solubility under basic conditions

Chitosan is a natural biopolymer whose rich amine functionality confers water solubility at low pH. At higher pH's (greater than 6. 5), the amines are deprotonated and chitosan is insoluble. To attain water solubility under basic conditions we enzymatically grafted the hydrophilic compound chlorogenic acid onto chitosan. Despite its name, chlorogenic acid is a nonchlorinated phenolic natural product that has carboxylic acid and hydroxyl functionality. The enzyme in this study was tyrosinase, which converts a wide range of phenolic substrates into electrophilic o-quinones. The o-quinones are freely diffusible and can undergo reaction with the nucleophilic amino groups of chitosan. Using slightly acidic conditions (pH = 6), it was possible to modify chitosan under homogeneous conditions. When the amount of chlorogenic acid used in the modification reaction exceeded 30% relative to chitosan's amino groups, the modified chitosan was observed to be soluble under both acidic and basic conditions, and to have a pH window of insolubility at near neutral pH. 1H NMR spectra confirmed that chitosan was chemically modified, although the degree of modification was low. Copyright 1999 John Wiley & Sons, Inc.

Insect melanogenesis. II. Inability of Manduca phenoloxidase to act on 5,6-dihydroxyindole-2-carboxylic acid

Eumelanins in animals are biosynthesized by the combined action of tyrosinase, 3,4-dihydroxyphenylalanine (DOPA)chrome isomerase, and other factors. Two kinds of eumelanins were characterized from mammalian systems; these are 5,6-dihydroxyindole (DHI)-melanin and 5,6-dihydroxyindole-2-carboxylic acid (DHICA)-melanin. In insects, melanin biosynthesis is initiated by phenoloxidase and supported by DOPAchrome isomerase (decarboxylating). Based on the facts that DOPA is a poor substrate for insect phenoloxidases and DHI is the sole product of insect DOPAchrome isomerase reaction, it is proposed that insects lack DHICA-melanin. Accordingly, the phenoloxidase isolated from the hemolymph of Manduca sexta failed to oxidize DHICA. Control experiments reveal that mushroom tyrosinase, as well as laccase, which is a contaminant in the commercial preparations of mushroom tyrosinase, are capable of oxidizing DHICA. Neither the whole hemolymph nor the cuticular extracts of M. sexta possessed any detectable oxidase activity towards this substrate. Thus, insects do not seem to produce DHICA-eumelanin. A useful staining procedure to localize DHICA oxidase activity on gels is also presented.

Cooperation of dopachrome conversion factor with phenoloxidase in the eumelanin pathway in haemolymph of Locusta migratoria (Insecta)

Dopachrome Conversion Factor (DCF) was found in the plasma of the locust Locusta migratoria. It has an apparent molecular mass of 85,000. Its K(m) was 0.2 mM at 22 degrees C and pH 7 with L-dopachrome as substrate. It had a high substrate specificity for L-dopachrome, methyl-L-dopachrome and L-dopachrome methyl ester but no activity on the corresponding D-isomers or on dopaminechrome. DCF was devoid of any phenoloxidase activity. Under action of DCF, L-dopachrome was converted into dihydroxyindole, which showed that a decarboxylation occured in the course of reaction. Locust DCF was inhibited by indole-3-propionic acid but not by indole-3- or indole-2-carboxylic acid. It was also inhibited by L-tryptophan in a competitive manner. Inhibition and substrate specificity suggest that a carboxyl group, either free or as a methyl ester, is necessary but not sufficient for enzyme recognition. When purified prophenoloxidase was activated and then added to dihydroxyindole either prepared by chemical synthesis or obtained by action of purified DCF on dopachrome, black pigments with a maximum absorption at 540 nm were generated. Therefore in the eumelanin pathway of locust plasma, phenoloxidase can catalyze the reaction that converts the product generated by DCF.

Facile oxidative decarboxylation of 3,4-dihydroxyphenylacetic acid catalyzed by copper and manganese ions

Under physiological conditions, we observed the rapid, pH- and temperature-dependent, oxidative decarboxylation and hydration of 3,4-dihydroxyphenylacetic acid (DOPAC) to form 3,4-dihydroxybenzyl alcohol (DBAlc). This product was oxidized and underwent tautomerization to form 3,4-dihydroxybenzaldehyde (DBAld). This reaction did not occur in the presence of EDTA, was catalyzed by copper (CuI, CuII) and manganese (MnII) and was oxygen dependent. A variety of mono- and dihydroxyphenyl carboxylic acids were tested and the reaction producing DBAlc as an intermediate was observed to be unique to DOPAC. 3.4-Dihydroxymandelic acid (DOMA) was rapidly oxidatively decarboxylated to form DBAld directly. The substrate and catalyst selectivity of this reaction suggest that this may have physiological relevance in the neurotoxic consequences of manganese and copper to the dopaminergic system in man.

Characterization of a defense complex consisting of interleukin 1 and phenol oxidase from the hemolymph of the tobacco hornworm, Manduca sexta

Hemolymph of fifth instar Manduca sexta larvae collected under non-sterile conditions exhibited the presence of a novel high molecular weight protein complex, which was absent from the hemolymph collected aseptically. The high molecular weight complex consisted of, at least prophenol oxidase, phenol oxidase, and an interleukin 1-like molecule, thereby demonstrating the generation of this complex as a consequence of a host defense response. While the native phenol oxidase and the interleukin 1-like molecule possessed molecular weights of about 80,000 and 17,000, respectively, the complex had a molecular weight of about 400,000. Apart from prophenol oxidase, phenol oxidase, and interleukin 1, dopachrome isomerase and other, as of yet unidentified, proteins may be part of the complex as judged by the presence of additional bands observed during SDS-polyacrylamide gel electrophoresis. The significance of the assembly of this defense complex for insect host defense strategies is discussed.

Oxidation of 3,4-dihydroxybenzylamine affords 3,4-dihydroxybenzaldehyde via the quinone methide intermediate

Dopamine and related compounds are known to be toxic to melanoma cells. Some of their toxicity may be related, in part, to the oxidation products generated from them upon their interaction with melanogenic enzymes. In this paper, we present our studies on the oxidation chemistry of 3,4-dihydroxybenzylamine, the lower homolog of dopamine. Mushroom tyrosinase catalyzed oxidation of 3,4-dihydroxybenzylamine rapidly generated the corresponding quinone. However, aminomethyl-o-benzoquinone thus formed did not accumulate in the reaction mixture, but readily transformed to another product that exhibited absorbance maxima at 280 and 310 nm. This compound was identified to be 3,4-dihydroxybenzaldehyde based on its HPLC elution profile, cochromatography with authentic sample and UV spectral properties. Possible mechanism for the formation 3,4-dihydroxybenzaldehyde from 3,4-dihydroxybenzylamine and the nature of cytotoxic quninonoid intermediates formed are discussed.

Fibroblasts co-expressing tyrosinase and the b-protein synthesize both eumelanin and phaeomelanin

Melanin synthesis in the mouse involves the interaction of many pigmentation loci. Tyrosinase, the product of the albino (c) locus, catalyses the first step of the pathway. The brown (b) locus protein has significant homology to tyrosinase and controls black/brown coat coloration, but its function is controversial. To investigate the function of the b-protein and its interaction with tyrosinase, we established cell lines expressing both tyrosinase and the b-protein by transfecting tyrosinase-expressing fibroblasts with a b-protein expression vector. The tyrosinase-expressing parent line does not have L-dopachrome tautomerase activity, but this enzyme is detectable in double transfectants as well as in fibroblasts expressing the b-protein alone. Cells expressing both proteins have a higher steady-state level of tyrosinase than fibroblasts expressing tyrosinase alone, and contain elevated levels of melanin intermediates. This is thought to result from interaction of tyrosinase with the b-protein. Only phaeomelanin is detectable in fibroblasts expressing tyrosinase alone, whereas double transfectants synthesise significantly more phaeomelanin and detectable eumelanin.

Complex formation between mushroom tyrosinase and Manduca dopachrome isomerase

Melanin biosynthesis in animals is initiated by the ubiquitously present tyrosinase and is aided by dopachrome isomerase. We have characterized a novel dopachrome isomerase (decarboxylating) from the hemolymph of Manduca sexta that generates a new quinone methide intermediate during melanogenesis (Sugumaran, M. and Semensi, V. (1991) J. Biol. Chem. 266, 6073-6078). This enzyme has the ability to form a complex with mushroom tyrosinase as judged by a number of physicochemical studies. The isomerase exhibited a marked inhibitory effect on tyrosinase and tyrosinase reciprocated by inhibiting the isomerase. While the isomerase showed no activity toward preformed dopaminechrome, it readily influenced the stability of dopaminechrome generated in situ by tyrosinase. Moreover, mushroom tyrosinase, which lacked specific binding to Concanavalin A Sepharose column, after complexing with the isomerase exhibited binding to this column. The complex formation also affected the pI value as well as mobility on a size exclusion column of these enzymes. Enzymes executing sequential metabolic transformation are known to form complexes called metabolons. Based on these above studies, it is concluded that both the enzymes involved in insect melanogenic pathway--phenoloxidase and dopachrome isomerase--are able to form a metabolon complex.

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.

The mouse brown (b) locus protein functions as a dopachrome tautomerase

The mouse b locus controls black/brown coat coloration. Its product, the b-protein or TRP-1, has significant homology to tyrosinase, and this has led to suggestions that the b-protein is itself a melanogenic enzyme. In order to investigate its function, we have used lines of mouse fibroblasts stably expressing the b-protein. We were unable to confirm previous reports that the b-protein has tyrosinase or catalase activity, but detected stereospecific dopachrome tautomerase activity in b-protein-expressing fibroblasts. This dopachrome tautomerase binds to Concanavalin A-Sepharose, and the major product of its action on L-dopachrome is 5,6-dihydroxyindole-2-carboxylic acid, as expected for the mammalian enzyme. Since this activity is not present in untransfected fibroblasts we conclude that the b-protein has dopachrome tautomerase activity. Further supporting evidence comes from the analysis of melanin metabolites produced by fibroblasts expressing tyrosinase alone, or in combination with the b-protein. Culture medium from the line expressing both proteins contains significant amounts of methylated carboxylated indoles, such as 6-hydroxy-5-methoxyindole-2-carboxylic acid, which would be expected in cells with an active dopachrome tautomerase. The levels of these compounds in medium from cells expressing tyrosinase alone are approximately 20-fold lower, and not significantly above background. Hence, it appears that the b-protein acts as a dopachrome tautomerase in vivo as well as in vitro.