Chemical characterization of pheomelanogenesis starting from dihydroxyphenylalanine or tyrosine and cysteine. Effects of tyrosinase and cysteine concentrations and reaction time

Avian colouration in a polluted world: a meta-analysis

Brilliant, diverse colour ornaments of birds were one of the crucial cues that led Darwin to the idea of sexual selection. Although avian colouration plays many functions, including concealment, thermoregulation, or advertisement as a distasteful prey, a quality-signalling role in sexual selection has attracted most research attention. Sexually selected ornaments are thought to be more susceptible to external stressors than naturally selected traits, and as such, they might be used as a test for environmental quality. For this reason, the last two decades have seen numerous studies on the impact of anthropogenic pollution on the expression of various avian colour traits. Herein, we provide the first meta-analytical summary of these results and examine whether there is an interaction between the mechanism of colour production (carotenoid-based, melanin-based and structural) and the type of anthropogenic factor (categorised as heavy metals, persistent organic pollutants, urbanisation, or other). Following the assumption of heightened condition dependence of ornaments under sexual selection, we also expected the magnitude of effect sizes to be higher in males. The overall effect size was close to significance and negative, supporting a general detrimental impact of anthropogenic pollutants on avian colouration. In contrast to expectations, there was no interaction between pollution types and colour-producing mechanisms. Yet there were significant differences in sensitivity between colour-producing mechanisms, with carotenoid-based colouration being the most affected by anthropogenic environmental disturbances. Moreover, we observed no significant tendency towards heightened sensitivity in males. We identified a publication gap on structural colouration, which, compared to pigment-based colouration, remains markedly understudied and should thus be prioritised in future research. Finally, we call for the unification of methods used in colour quantification in ecological research to ensure comparability of results among studies.

The "Unconventional" Effect of Cysteine on the In Vitro Synthesis of Melanin

This report details some of our observations regarding the impact of cysteine on the air-mediated oxidation of catecholamines, particularly epinephrine. The intent was to synthesize light-colored, pheomelanin-like materials. Pheomelanin is commonly described as a material generated from a mixture of catecholamines and cysteine. However, we observed that (1) the presence of cysteine resulted in a concentration-dependent delay in the onset of color formation and (2) the presence of cysteine resulted in darker, more eumelanin-like materials. These effects were particularly impactful in the case of epinephrine. More elaborate studies involving other amino acids or scaled-up reactions were conducted with epinephrine as the precursor. These studies show that other amino acids, e.g., methionine or serine, could lead to darker materials, but none were as impactful as cysteine. Although our results are in contrast to typical descriptions regarding the impact of cysteine on the synthesis of melanin, they may reflect crucial differences between the in vitrovsin vivo synthesis of pheomelanin.

The Enigmatic World of Fungal Melanin: A Comprehensive Review

Synthetic dyes are generally not safe for human health or the environment, leading to the continuous search and growing demand for natural pigments that are considered safer, biodegrade more easily, and are environmentally beneficial. Among micro-organisms, fungi represent an emerging source of pigments due to their many benefits; therefore, they are readily viable on an industrial scale. Among all the bioactive pigments produced by fungi, melanin is an enigmatic, multifunctional pigment that has been studied for more than 150 years. This dark pigment, which is produced via the oxidative polymerization of phenolic compounds, has been investigated for its potential to protect life from all kingdoms, including fungi, from biotic and abiotic stresses. Over time, the research on fungal melanin has attracted a significant amount of scientific interest due to melanin's distinct biological activities and multifarious functionality, which is well-documented in the literature and could possibly be utilized. This review surveys the literature and summarizes the current discourse, presenting an up-to-date account of the research performed on fungal melanin that encompasses its types, the factors influencing its bioactivity, the optimization of fermentation conditions to enhance its sustainable production, its biosynthetic pathways, and its extraction, as well as biochemical characterization techniques and the potential uses of melanin in a wide range of applications in various industries. A massive scope of work remains to circumvent the obstacles to obtaining melanin from fungi and exploring its future prospects in a diverse range of applications.

The potential impact of melanosomal pH and metabolism on melanoma

Melanin is synthesized in melanocytes and is transferred into keratinocytes to block the effects of ultraviolet (UV) radiation and is important for preventing skin cancers including melanoma. However, it is known that after melanomagenesis and melanoma invasion or metastases, melanin synthesis still occurs. Since melanoma cells are no longer involved in the sun tanning process, it is unclear why melanocytes would maintain melanin synthesis after melanomagenesis has occurred. Aside from blocking UV-induced DNA mutation, melanin may provide other metabolic functions that could benefit melanoma. In addition, studies have suggested that there may be a selective advantage to melanin synthesis in melanoma; however, mechanisms regulating melanin synthesis outside the epidermis or hair follicle is unknown. We will discuss how melanosomal pH controls melanin synthesis in melanocytes and how melanosomal pH control of melanin synthesis might function in melanoma. We will also discuss potential reasons why melanin synthesis might be beneficial for melanoma cellular metabolism and provide a rationale for why melanin synthesis is not limited to benign melanocytes.

Melanin biopolymer synthesis using a new melanogenic strain of Flavobacterium kingsejongi and a recombinant strain of Escherichia coli expressing 4-hydroxyphenylpyruvate dioxygenase from F. kingsejongi

BACKGROUND

Melanins are a heterologous group of biopolymeric pigments synthesized by diverse prokaryotes and eukaryotes and are widely utilized as bioactive materials and functional polymers in the biotechnology industry. Here, we report the high-level melanin production using a new melanogenic Flavobacterium kingsejongi strain and a recombinant Escherichia coli overexpressing F. kingsejongi 4-hydroxyphenylpyruvate dioxygenase (HPPD).

RESULTS

Melanin synthesis of F. kingsejongi strain was confirmed via melanin synthesis inhibition test, melanin solubility test, genome analysis, and structural analysis of purified melanin from both wild-type F. kingsejongi and recombinant E. coli expressing F. kingsejongi HPPD. The activity of F. kingsejongi HPPD was demonstrated via in vitro assays with 6 × His-tagged and native forms of HPPD. The specific activity of F. kingsejongi HPPD was 1.2 ± 0.03 μmol homogentisate/min/mg-protein. Bioreactor fermentation of F. kingsejongi produced a large amount of melanin with a titer of 6.07 ± 0.32 g/L, a conversion yield of 60% (0.6 ± 0.03 g melanin per gram tyrosine), and a productivity of 0.03 g/L·h, indicating its potential for industrial melanin production. Additionally, bioreactor fermentation of recombinant E. coli expressing F. kingsejongi HPPD produced melanin at a titer of 3.76 ± 0.30 g/L, a conversion yield of 38% (0.38 ± 0.03 g melanin per gram tyrosine), and a productivity of 0.04 g/L·h.

CONCLUSIONS

Both strains showed sufficiently high fermentation capability to indicate their potential as platform strains for large-scale bacterial melanin production. Furthermore, F. kingsejongi strain could serve as a model to elucidate the regulation of melanin biosynthesis pathway and its networks with other cellular pathways, and to understand the cellular responses of melanin-producing bacteria to environmental changes, including nutrient starvation and other stresses.

Microbial melanin: Recent advances in biosynthesis, extraction, characterization, and applications

Melanin is a common name for a group of biopolymers with the dominance of potential applications in medical sciences, cosmeceutical, bioremediation, and bioelectronic applications. The broad distribution of these pigments suggests their role to combat abiotic and biotic stresses in diverse life forms. Biosynthesis of melanin in fungi and bacteria occurs by oxidative polymerization of phenolic compounds predominantly by two pathways, 1,8-dihydroxynaphthalene [DHN] or 3,4-dihydroxyphenylalanine [DOPA], resulting in different kinds of melanin, i.e., eumelanin, pheomelanin, allomelanin, pyomelanin, and neuromelanin. The enzymes responsible for melanin synthesis belong mainly to tyrosinase, laccase, and polyketide synthase families. Studies have shown that manipulating culture parameters, combined with recombinant technology, can increase melanin yield for large-scale production. Despite significant efforts, its low solubility has limited the development of extraction procedures, and heterogeneous structural complexity has impaired structural elucidation, restricting effective exploitation of their biotechnological potential. Innumerable studies have been performed on melanin pigments from different taxa of life in order to advance the knowledge about melanin pigments for their efficient utilization in diverse applications. These studies prompted an urgent need for a comprehensive review on melanin pigments isolated from microorganisms, so that such review encompassing biosynthesis, bioproduction, characterization, and potential applications would help researchers from diverse background to understand the importance of microbial melanins and to utilize the information from the review for planning studies on melanin. With this aim in mind, the present report compares conventional and modern ideas for environment-friendly extraction procedures for melanin. Furthermore, the characteristic parameters to differentiate between eumelanin and pheomelanin are also mentioned, followed by their biotechnological applications forming the basis of industrial utilization. There lies a massive scope of work to circumvent the bottlenecks in their isolation and structural elucidation methodologies.

Benzamide derivative radiotracers targeting melanin for melanoma imaging and therapy: Preclinical/clinical development and combination with other treatments

Cutaneous melanoma arises from proliferating melanocytes, cells specialized in the production of melanin. This property means melanin can be considered as a target for monitoring melanoma patients using nuclear imaging or targeted radionuclide therapy (TRT). Since the 1970s, many researchers have shown that specific molecules can interfere with melanin. This paper reviews some such molecules: benzamide structures improved to increase their pharmacokinetics for imaging or TRT. We first describe the characteristics and biosynthesis of melanin, and the main features of melanin tracers. The second part summarizes the preclinical and corresponding clinical studies on imaging. The last section presents TRT results from ongoing protocols and discusses combinations with other therapies as an opportunity for melanoma non-responders or patients resistant to treatments.

A source of exogenous oxidative stress improves oxidative status and favors pheomelanin synthesis in zebra finches

Some organisms can modulate gene expression to trigger physiological responses that help adapt to environmental stress. The synthesis of the pigment pheomelanin in melanocytes seems to be one of these responses, as it may contribute to cellular homeostasis. We experimentally induced environmental oxidative stress in male zebra finches Taeniopygia guttata by the administration of the herbicide diquat dibromide during feather growth to test if the expression of genes involved in pheomelanin synthesis shows epigenetic lability. As pheomelanin synthesis implies decreasing the availability of the main cellular antioxidant (glutathione), it is expected to cause oxidative stress unless a protective mechanism limits pheomelanin synthesis and thus favors the antioxidant capacity. However, diquat exposure did not only improve the antioxidant capacity of birds, but also upregulated the expression of a gene (AGRP) that promotes pheomelanin synthesis in feather melanocytes, leading to the development of darker plumage coloration. No changes in the expression of other genes involved in pheomelanin synthesis (Slc7a11, Slc45a2, MC1R, ASIP and CTNS) were detected. DNA methylation levels only changed in MC1R, suggesting that epigenetic modifications other than changes in methylation may regulate AGRP expression lability. Our results suggest that exogenous oxidative stress induced a hormetic response that enhanced the oxidative status of birds and, consequently, promoted pheomelanin-based pigmentation, supporting the idea that birds adjust pheomelanin synthesis to their oxidative stress conditions.

Exposure to a competitive social environment activates an epigenetic mechanism that limits pheomelanin synthesis in zebra finches

Competitive environments promote high testosterone levels, produce oxidative stress and, consequently, impair cellular homeostasis. The regulation of genes involved in the synthesis of the pigment pheomelanin in melanocytes seems to help to maintain homeostasis against environmental oxidative stress. Here, we experimentally increased social interactions in some zebra finch (Taeniopygia guttata) males by keeping them in groups of six birds during feather growth, while others were kept alone, to test if melanocytes show epigenetic lability under a competitive social environment. As these changes may depend on the oxidative status, we administrated buthionine sulfoximine (BSO) to decrease the antioxidant capacity of some birds. The competitive environment downregulated a gene involved in pheomelanin synthesis (Slc7a11) by changing the level of DNA methylation in feather melanocytes. In other genes involved in pheomelanin synthesis (Slc45a2, MC1R and AGRP), DNA methylation was also affected, but no changes in expression were detected. Exposure to the competitive environment did not affect systemic oxidative stress and damage, indicating that a protective epigenetic mechanism that changes the expression of Slc7a11 may have been activated. However, no changes to the pigmentation phenotype of birds were found, probably due to the short duration or low intensity of the competitive environment. BSO treatment did not affect the epigenetic mechanism, suggesting that the antioxidant capacity of birds was high enough to deal with the competitive environment. An epigenetic mechanism limiting pheomelanin synthesis therefore becomes activated under exposure to a competitive environment in male zebra finches, which may help to avoid damage caused by competitive interactions.

Determination of eumelanin and pheomelanin in melanomas using solid-phase extraction and high performance liquid chromatography-diode array detection (HPLC-DAD) analysis

Determination of eumelanin and pheomelanin in melanomas that exhibit different pigmentation was carried using a solid-phase extraction (SPE) preparation method based on weak anion exchange chemistry. This extraction significantly enhanced the chromatographic profile obtained by reverse phase high performance liquid chromatography-diode array detection (RP-HPLC-DAD). The SPE method was developed using aqueous standards of melanin markers: thiazole-2,4,5-tricarboxylic acid (TTCA), thiazole-4,5-dicarboxylic acid (TDCA), pyrrole-2,3-dicarboxylic acid (PDCA) and pyrrole-2,3,5-tricarboxylic acid (PTCA) and non-pigmented cell lines spiked with those markers. An excellent average recovery, above 90%, was obtained for the four markers with a relative standard deviation below 7%. We have also optimized the stationary phase and the mobile phase (phosphate concentration and pH) to improve sensitivity and to reduce the analysis time. Elution of the four markers is achieved in 5 min and total analysis of biological samples is completed in 15 min. The quantification limits for TDCA, TTCA, PDCA and PTCA are 60, 50, 47 and 48 ng/mL respectively. Furthermore, DAD detection improves the marker identification in complex matrices through the analysis of UV spectra. We have successfully applied this method to melanoma tumors and cells. Murine B16BL6 tumor are highly pigmented with mostly eumelanin (98.1% of eumelanin) while human SK-MEL-3 tumor contain about 30% pheomelanin. B16BL6 and B16F10 are eumelanic cells lines and NHEM melanocytes contain about 24% of pheomelanin.

Melanins and melanogenesis: from pigment cells to human health and technological applications

During the past decade, melanins and melanogenesis have attracted growing interest for a broad range of biomedical and technological applications. The burst of polydopamine-based multifunctional coatings in materials science is just one example, and the list may be expanded to include melanin thin films for organic electronics and bioelectronics, drug delivery systems, functional nanoparticles and biointerfaces, sunscreens, environmental remediation devices. Despite considerable advances, applied research on melanins and melanogenesis is still far from being mature. A closer intersectoral interaction between research centers is essential to raise the interests and increase the awareness of the biomedical, biomaterials science and hi-tech sectors of the manifold opportunities offered by pigment cells and related metabolic pathways. Starting from a survey of biological roles and functions, the present review aims at providing an interdisciplinary perspective of melanin pigments and related pathway with a view to showing how it is possible to translate current knowledge about physical and chemical properties and control mechanisms into new bioinspired solutions for biomedical, dermocosmetic, and technological applications.

Tyrosinase Depletion Prevents the Maturation of Melanosomes in the Mouse Hair Follicle

The mechanisms that lead to variation in human skin and hair color are not fully understood. To better understand the molecular control of skin and hair color variation, we modulated the expression of Tyrosinase (Tyr), which controls the rate-limiting step of melanogenesis, by expressing a single-copy, tetracycline-inducible shRNA against Tyr in mice. Moderate depletion of TYR was sufficient to alter the appearance of the mouse coat in black, agouti, and yellow coat color backgrounds, even though TYR depletion did not significantly inhibit accumulation of melanin within the mouse hair. Ultra-structural studies revealed that the reduction of Tyr inhibited the accumulation of terminal melanosomes, and inhibited the expression of genes that regulate melanogenesis. These results indicate that color in skin and hair is determined not only by the total amount of melanin within the hair, but also by the relative accumulation of mature melanosomes.

Brain size and the expression of pheomelanin-based colour in birds

Eumelanin and pheomelanin are the most common vertebrate pigments. They generate different colours and are synthesized under different physiological conditions. While pheomelanogenesis requires high levels of a key intracellular antioxidant (glutathione, GSH), eumelanogenesis is inhibited by GSH. This implies that species that present the molecular basis to produce large amounts of pheomelanin might be more limited to perform other costly processes that generate oxidative stress than species that produce eumelanin. Brain development requires large amounts of energy and antioxidants during ontogeny, so that large-brained species may be constrained in their simultaneous synthesis of large amounts of pheomelanin, but not in their synthesis of eumelanin. Here, we tested this hypothesis in a large dataset of 323 bird species. After controlling for the effects of phylogeny, latitude and sexual dichromatism, the proportion of pheomelanic plumage colour was strongly negatively related to the relative brain mass of species, whereas no relationship was found for the proportion of eumelanic colour. This indicates that the production of pheomelanin is a costly process that cannot evolve together with complex neural structures and thus with large cognitive capacity. This is the first time that the expression of melanic traits is found to correlate with another phenotypic character across species.

Bird population declines due to radiation exposure at Chernobyl are stronger in species with pheomelanin-based coloration

Eumelanin and pheomelanin are the most common pigments providing color to the integument of vertebrates. While pheomelanogenesis requires high levels of a key intracellular antioxidant (glutathione, GSH), eumelanogenesis is inhibited by GSH. This implies that species that possess the molecular basis to produce large amounts of pheomelanin might be more limited in coping with environmental conditions that generate oxidative stress than species that produce eumelanin. Exposure to ionizing radiation produces free radicals and depletes antioxidant resources. GSH is particularly susceptible to radiation, so that species with large proportions of pheomelanic integument may be limited by the availability of GSH to combat oxidative stress and may thus suffer more from radiation effects. We tested this hypothesis in 97 species of birds censused in areas with varying levels of radioactive contamination around Chernobyl. After controlling for the effects of carotenoid-based color, body mass and similarity among taxa due to common phylogenetic descent, the proportion of pheomelanic plumage was strongly negatively related to the slope estimates of the relationship between abundance and radiation levels, while no effect of eumelanic color proportion was found. This represents the first report of an effect of the expression of melanin-based coloration on the capacity to resist the effects of ionizing radiation. Population declines were also stronger in species that exhibit carotenoid-based coloration and have large body mass. The magnitude of population declines had a relatively high phylogenetic signal, indicating that certain groups of birds, especially non-corvid passeriforms, are particularly susceptible to suffer from the effects of radioactive contamination due to phylogenetic inertia.

The red and the black

"Pigmentation, which is primarily determined by the amount, the type, and the distribution of melanin, shows a remarkable diversity in human populations, and in this sense, it is an atypical trait."--E. J. Parra. Melanin is found throughout the human body, skin, eye, brain, hair, and inner ear, yet its molecular structure remains elusive. Researchers have characterized the molecular building blocks of melanin but have not been able to describe how those components fit together in the overall architecture of the pigment. Melanin is categorized into two distinct classes, pheomelanin (red) and eumelanin (black). Although these classes share a common biosynthetic origin, specific molecular reactions occurring early in pigment production differentiate these two types. Pure eumelanin is found throughout nature, which has allowed researchers to characterize and quantify its chemical properties. However, pure pheomelanin is not observed in nature and rarely makes up more than ~25% of the total melanin present. In this Account, we explore our current understanding of the structure and reactivity of the red and black pigments. Epidemiological studies of skin and ocular cancers suggest that increasing relative proportions of pheomelanin correlate with increased risk factors for these diseases. Therefore, understanding the factors that control the relative abundance of the two pigments has become increasingly important. Consequently, researchers have worked to elucidate the chemistry of pheomelanin to determine whether the pigment could cause these cancers and, if so, by what mechanisms. The photoactivation of oxygen by pheomelanin in the UV-A range could contribute to the development of UV-induced cancers: recent measurement of the surface photoionization threshold of intact melanosomes reveals a lower photoionization potential for pheomelanin than eumelanin. A complementary study of intact human melanosomes isolated from different colored irides reveals that the absorption coefficient of the melanosome decreases with increasing pheomelanin content. These results suggest that the epidemiological data may simply result from an increased exposure of the underlying tissues to UV light.

Antioxidant machinery differs between melanic and light nestlings of two polymorphic raptors

Colour polymorphism results from the expression of multiallelic genes generating phenotypes with very distinctive colourations. Most colour polymorphisms are due to differences in the type or amount of melanins present in each morph, which also differ in several behavioural, morphometric and physiological attributes. Melanin-based colour morphs could also differ in the levels of glutathione (GSH), a key intracellular antioxidant, because of the role of this molecule in melanogenesis. As GSH inhibits the synthesis of eumelanin (i.e. the darkest melanin form), individuals of darker morphs are expected to have lower GSH levels than those of lighter morphs. We tested this prediction in nestlings of two polymorphic raptors, the booted eagle Hieraaetus pennatus and the Eleonora's falcon Falco eleonorae, both of which occur in two morphs differing in the extent of eumelanic plumage. As expected, melanic booted eagle nestlings had lower blood GSH levels than light morph eagle nestlings. In the Eleonora's falcon, however, melanic nestlings only had lower GSH levels after controlling for the levels of other antioxidants. We also found that melanic female eagle nestlings had higher levels of antioxidants other than GSH and were in better body condition than light female eagle nestlings. These findings suggest an adaptive response of melanic nestlings to compensate for reduced GSH levels. Nevertheless, these associations were not found in falcons, indicating species-specific particularities in antioxidant machinery. Our results are consistent with previous work revealing the importance of GSH on the expression of melanic characters that show continuous variation, and suggest that this pathway also applies to discrete colour morphs. We suggest that the need to maintain low GSH levels for eumelanogenesis in dark morph individuals may represent a physiological constraint that helps regulate the evolution and maintenance of polymorphisms.

The first global screening of protein substrates bearing protein-bound 3,4-Dihydroxyphenylalanine in Escherichia coli and human mitochondria

Protein hydroxylation at proline and lysine residues is known to have important effects on cellular functions, such as the response to hypoxia. However, protein hydroxylation at tyrosine residues (called protein-bound 3,4-dihydroxy-phenylalanine (PB-DOPA)) has not been carefully examined. Here we report the first proteomics screening of the PB-DOPA protein substrates and their sites in Escherichia coli and human mitochondria by nano-liquid chromatography/tandem mass spectrometry (nano-LC/MS/MS) and protein sequence alignment using the PTMap algorithm. Our study identified 67 novel PB-DOPA sites in 43 E. coli proteins and 9 novel PB-DOPA sites in 7 proteins from HeLa mitochondria. Bioinformatics analysis indicates that the structured region is more favored than the unstructured regions of proteins for the PB-DOPA modification. The PB-DOPA substrates in E. coli were dominantly enriched in proteins associated with carbohydrate metabolism. Our study showed that PB-DOPA may be involved in regulation of the specific activity of certain evolutionarily conserved proteins such as superoxide dismutase and glyceraldehyde 3-phosphate dehydrogenase, suggesting the conserved nature of the modification among distant biological species. The substrate proteins identified in this study offer a rich source for determining their regulatory enzymes and for further characterization of the possible contributions of this modification to cellular physiology and human diseases.

Current challenges in understanding melanogenesis: bridging chemistry, biological control, morphology, and function

Melanin is a natural pigment produced within organelles, melanosomes, located in melanocytes. Biological functions of melanosomes are often attributed to the unique chemical properties of the melanins they contain; however, the molecular structure of melanins, the mechanism by which the pigment is produced, and how the pigment is organized within the melanosome remains to be fully understood. In this review, we examine the current understanding of the initial chemical steps in the melanogenesis. Most natural melanins are mixtures of eumelanin and pheomelanin, and so after presenting the current understanding of the individual pigments, we focus on the mixed melanin systems, with a critical eye towards understanding how studies on individual melanin do and do not provide insight in the molecular aspects of their structures. We conclude the review with a discussion of important issues that must be addressed in future research efforts to more fully understand the relationship between molecular and functional properties of this important class of natural pigments.

Chemical analysis of late stages of pheomelanogenesis: conversion of dihydrobenzothiazine to a benzothiazole structure

Pheomelanogenesis is a complex pathway that starts with the oxidation of tyrosine (or DOPA, 3,4-dihydroxyphenylalanine) by tyrosinase in the presence of cysteine, which results in the production of 5-S-cysteinyldopa and its isomers. Beyond that step, relatively little has been clarified except for a possible intermediate produced, dihydro-1,4-benzothiazine-3-carboxylic acid (DHBTCA). We therefore carried out a detailed study on the course of pheomelanogenesis using DOPA and cysteine and the physiological enzyme tyrosinase. To elucidate the later stages of pheomelanogenesis, chemical degradative methods of reductive hydrolysis with hydroiodic acid and alkaline peroxide oxidation were applied. The results show that: (1) DHBTCA accumulates after the disappearance of the cysteinyldopa isomers, (2) DHBTCA is then oxidized by a redox exchange with dopaquinone to form ortho-quinonimine, which leads to the production of pheomelanin with a benzothiazine moiety, and (3) the benzothiazine moiety gradually degrades to form a benzothiazole moiety. This latter process is consistent with the much higher ratio of benzothiazole-derived units in human red hair than in mouse yellow hair. These findings may be relevant to the (photo)toxic effects of pheomelanin.

The expression of melanin-based plumage is separately modulated by exogenous oxidative stress and a melanocortin

Melanin-based traits involved in animal communication have been traditionally viewed as occurring under strict genetic control. However, it is generally accepted that both genetic and environmental factors influence melanin production. Medical studies suggest that, among environmental factors influencing melanization, oxidative stress could play a relevant role. On the other hand, genetic control would be exerted by the melanocortin system, and particularly by the alpha-melanocyte-stimulating hormone (alpha-MSH), which triggers the production of eumelanins (black pigments). To determine how the melanocortin system and an exogenous source of oxidative stress interact in the expression of melanin-based plumage, developing red-legged partridges (Alectoris rufa) were manipulated. Some partridges were injected with alpha-MSH, while other birds received a pro-oxidant molecule (diquat) in drinking water. Controls and birds receiving both treatments were also studied. Both alpha-MSH- and diquat-treated individuals presented larger eumelanin-based traits than controls, but alpha-MSH+diquat-treated birds showed the largest traits, suggesting that oxidative stress and melanocortins promote additive but independent effects. Diquat also induced a decline in the level of a key intracellular antioxidant (glutathione), which is associated with high expression of eumelanin-based signals in other bird species. Some scenarios for the evolution of melanin-based traits in relation to oxidative stress are proposed.

Challenges in applying photoemission electron microscopy to biological systems

Photoemission electron microscopy (PEEM) is a unique surface-sensitive instrument capable of providing real-time images with high spatial resolution. While similar to the more common electron microscopies, scanning electron microscopy and transmission electron microscopy, the imaging technology relies on the photogeneration of electrons emitted from a sample through light excitation. This imaging technique has found prominence in surface and materials sciences, being well suited for imaging flat surfaces, and changes that occur to that surface as various parameters are changed (e.g. temperature, exposure to reactive gases). Biologically focused PEEM received significant attention in the 1970s, but was not aggressively advanced since that pioneering work. PEEM is capable of providing important insights into biological systems that extend beyond simple imaging. In this article, we identify and establish important issues that affect the acquisition and analysis of biological samples with PEEM. We will briefly review the biological impact and importance of PEEM with respect to our work. The article also concludes with a discussion of some of the current challenges that must be addressed to enable PEEM to achieve its maximum potential with biological samples.

Insights into melanosomes and melanin from some interesting spatial and temporal properties

Melanosomes are organelles found in a wide variety of tissues throughout the animal kingdom and exhibit a range of different shapes: spheres of up to approximately 1 mum diameters and ellipsoids with lengths of up to approximately 2 mum and varying aspect ratios. The functions of melanosomes include photoprotection, mitigation of the effects of reactive oxygen species, and metal chelation. The melanosome contains a variety of biological molecules, e.g., proteins and lipids, but the dominant constituent is the pigment melanin, and the functions ascribed to melanosomes are uniquely enabled by the chemical properties of the melanins they contain. In the past decade, there has been significant progress in understanding melanins and their impact on human health. While the molecular details of melanin production and how the pigment is organized within the melanosome determine its properties and biological functions, the physical and chemical properties of the surface of the melanosome are central to their range of ascribed functions. Surprisingly, few studies designed to probe this biological surface have been reported. In this article, we discuss recent work using surface-sensitive analytic, spectroscopic, and imaging techniques to examine the structural and chemical properties of many types of natural pigments: sepia melanin granules, human and bovine ocular melanosomes, human hair melanosomes, and neuromelanin. N 2 adsorption/desorption measurements and atomic force microscopy provide novel insights into surface morphology. The chemical properties of the melanins present on the surface are revealed by X-ray photoelectron spectroscopy and photoemission electron microscopy. These technologies are also applied to elucidate changes in surface properties that occur with aging. Specifically, studies of the surface properties of human retinal pigment epithelium melanosomes as a function of age are stimulating the development of models for their age-dependent behaviors. The article concludes with a brief discussion of important unanswered research questions in this field.

Chemistry of mixed melanogenesis--pivotal roles of dopaquinone

Melanins can be classified into two major groups-insoluble brown to black pigments termed eumelanin and alkali-soluble yellow to reddish-brown pigments termed pheomelanin. Both types of pigment derive from the common precursor dopaquinone (ortho-quinone of 3,4-dihydroxyphenylalanine) which is formed via the oxidation of l-tyrosine by the melanogenic enzyme tyrosinase. Dopaquinone is a highly reactive ortho-quinone that plays pivotal roles in the chemical control of melanogenesis. In the absence of sulfhydryl compounds, dopaquinone undergoes intramolecular cyclization to form cyclodopa, which is then rapidly oxidized by a redox reaction with dopaquinone to give dopachrome (and dopa). Dopachrome then gradually and spontaneously rearranges to form 5,6-dihydroxyindole and to a lesser extent 5,6-dihydroxyindole-2-carboxylic acid, the ratio of which is determined by a distinct melanogenic enzyme termed dopachrome tautomerase (tyrosinase-related protein-2). Oxidation and subsequent polymerization of these dihydroxyindoles leads to the production of eumelanin. However, when cysteine is present, this process gives rise preferentially to the production of cysteinyldopa isomers. Cysteinyldopas are subsequently oxidized through redox reaction with dopaquinone to form cysteinyldopaquinones that eventually lead to the production of pheomelanin. Pulse radiolysis studies of early stages of melanogenesis (involving dopaquinone and cysteine) indicate that mixed melanogenesis proceeds in three distinct stages-the initial production of cysteinyldopas, followed by their oxidation to produce pheomelanin, followed finally by the production of eumelanin. Based on these data, a casing model of mixed melanogenesis is proposed in which a preformed pheomelanic core is covered by a eumelanic surface.

The surface oxidation potential of human neuromelanin reveals a spherical architecture with a pheomelanin core and a eumelanin surface

Neuromelanin (NM) isolated from the substantia nigra region of the human brain was studied by scanning probe and photoelectron emission microscopies. Atomic force microscopy reveals that NM granules are comprised of spherical structures with a diameter of approximately 30 nm, similar to that observed for Sepia cuttlefish, bovine eye, and human eye and hair melanosomes. Photoelectron microscopy images were collected at specific wavelengths of UV light between 248 and 413 nm, using the spontaneous-emission output from the Duke OK-4 free electron laser. Analysis of the data establishes a threshold photoionization potential for NM of 4.5 +/- 0.2 eV, which corresponds to an oxidation potential of -0.1 +/- 0.2 V vs. the normal hydrogen electrode (NHE). The oxidation potential of NM is within experimental error of the oxidation potential measured for human eumelanosomes (-0.2 +/- 0.2 V vs. NHE), despite the presence of a significant fraction of the red pigment, pheomelanin, which is characterized by a higher oxidation potential (+0.5 +/- 0.2 V vs. NHE). Published kinetic studies on the early chemical steps of melanogenesis show that in the case of pigments containing a mixture of pheomelanin and eumelanin, of which NM is an example, pheomelanin formation occurs first with eumelanin formation predominantly occurring only after cysteine levels are depleted. Such a kinetic model would predict a structural motif with pheomelanin at the core and eumelanin at the surface, which is consistent with the measured surface oxidation potential of the approximately 30-nm constituents of NM granules.

Ocular Melanogenesis: The Role of Antioxidants

Given the propensity of a large number of melanogenic pathways that can be modulated by cellular redox status, a causal role of the deficiency of ocular pigments such as melanin in the pathogenesis of age-related macular degeneration and evidence that melanin production does occur in the adult eye, it seems not improbable that antioxidants (or agents that modify cellular redox status) may have melanin stimulatory (or inhibitory) effects that are superimposible on their effects as mere free radical scavengers. More empirical studies are needed to investigate this phenomenon so that antioxidant therapy may prove more beneficial to patients with ocular degenerative diseases.

Low incidence of iris pigmentation and eyelash changes in 2 randomized clinical trials with unoprostone isopropyl 0.15%

OBJECTIVE

To assess whether iris color and eyelash changes occur with the use of unoprostone for 2 years.

DESIGN

The 2 clinical trials described herein were prospective, randomized, double-masked, active-controlled, parallel group, multicenter studies.

PARTICIPANTS

A total of 1131 patients with primary open-angle glaucoma or ocular hypertension participated in 2 clinical trials and received either unoprostone isopropyl 0.15% (659), timolol maleate 0.5% (331), or betaxolol hydrochloride 0.5% (141), 1 drop per eye twice daily for up to 24 months.

METHODS

Color photographs (1:1 magnification) were taken of the iris and eyelid of each patient at baseline and at regular intervals thereafter through month 24 using a standardized camera system. Photography included 7 views of each eye plus a calibration photograph and a patient identification photograph, for a total of 16 photographs per patient per visit. Two independent (masked) readers subjectively compared baseline iris colors to subsequent visits. Side view photographs of the upper and lower eyelashes were used for the eyelash length analysis, with each having sufficient depth of field and a sufficient number of eyelashes in focus. Similarly, frontal eyelash views were used for the eyelash density analysis.

MAIN OUTCOME MEASURES

Changes from baseline in iris color and eyelash length and density within and between treatment groups.

RESULTS

Seven cases of iris color change (1.06%) were confirmed in patients treated with unoprostone for up to 24 months; no confirmed cases were reported in the timolol or betaxolol groups. In the unoprostone group, cases of iris color change were confirmed at months 12 (1 case), 18 (2 cases), and 24 (4 cases). No clinically relevant differences were observed among treatment groups for changes from baseline in eyelash length or density.

CONCLUSION

Although iris hyperpigmentation and abnormal eyelash changes may occur after treatment with unoprostone, the incidence of these events appears to be low in the 2-year clinical study.

Melanins and Melanosomes From Llama (Lama glama L.)

Analysis of melanins and melanosomes in eight hair and skin samples taken of adult pigmented Argentine llamas (Lama glama L.) has been carried out. In each sample, eumelanins, pheomelanins and alkali-soluble melanins were identified. The total amount of melanins and the amount of eumelanins both decreased from black to reddish brown colour, while pheomelanins were found to be present in small quantities in each sample. Eumelanosomes were round and oval-shaped, displaying transverse striations clearly visible at low magnification. Dark brown samples revealed all four melanosomes stages. Stages I and II melanosomes appeared as large, asymmetrical vacuoles containing numerous microvesicles randomly scattered within an amorphous proteinaceous material (vesiculo-globular bodies). Stage III melanosomes had microgranular melanin deposits in the microvesicles and in the matrix. The fully melanized melanosomes (stage IV) were primarily round-shaped, showing an irregular outline and the electron-dense pigment was arranged to form large clusters. In light brown melanocytes, numerous melanosomes at different maturation stages could be found. Premelanosomes appeared ovoid, containing amorphous proteinaceous material and spotty and microgranular deposits. Mature melanosomes were fully melanized, homogeneously electron-dense, ovoid granules.

The IFPCS presidential lecture: a chemist's view of melanogenesis

The significance of our understanding of the chemistry of melanin and melanogenesis is reviewed. Melanogenesis begins with the production of dopaquinone, a highly reactive o-quinone. Pulse radiolysis is a powerful tool to study the fates of such highly reactive melanin precursors. Based on pulse radiolysis data reported by Land et al. (J Photochem Photobiol B: Biol 2001;64:123) and our biochemical studies, a pathway for mixed melanogenesis is proposed. Melanogenesis proceeds in three distinctive steps. The initial step is the production of cysteinyldopas by the rapid addition of cysteine to dopaquinone, which continues as long as cysteine is present (1 microM). The second step is the oxidation of cysteinyldopas to give pheomelanin, which continues as long as cysteinyldopas are present (10 microM). The last step is the production of eumelanin, which begins only after most cysteinyldopas are depleted. It thus appears that eumelanin is deposited on the preformed pheomelanin and that the ratio of eu- to pheomelanin is determined by the tyrosinase activity and cysteine concentration. In eumelanogenesis, dopachrome is a rather stable molecule and spontaneously decomposes to give mostly 5,6-dihydroxyindole. Dopachrome tautomerase (Dct) catalyses the tautomerization of dopachrome to give mostly 5,6-dihydroxyindole-2-carboxylic acid (DHICA). Our study confirmed that the role of Dct is to increase the ratio of DHICA in eumelanin and to increase the production of eumelanin. In addition, the cytotoxicity of o-quinone melanin precursors was found to correlate with binding to proteins through the cysteine residues. Finally, it is still unknown how the availability of cysteine is controlled within the melanosome.

Biosynthesis of fungal melanins and their importance for human pathogenic fungi

For more than 40 years fungi have been known to produce pigments known as melanins. Predominantly these have been dihydroxyphenylalanine (DOPA)-melanin and dihydroxynaphthalene (DHN)-melanin. The biochemical and genetical analysis of the biosynthesis pathways have led to the identification of the genes and corresponding enzymes of the pathways. Only recently have both these types of melanin been linked to virulence in some human pathogenic and phytopathogenic fungi. The absence of melanin in human pathogenic and phytopathogenic fungi often leads to a decrease in virulence. In phytopathogenic fungi such as Magnaporthe grisea and Colletotrichum lagenarium, besides other possible functions in pathogenicity, DHN-melanin plays an essential role in generating turgor for plant appressoria to penetrate plant leaves. While the function of melanin in human pathogenic fungi such as Cryptococcus neoformans, Wangiella dermatitidis, Sporothrix schenckii, and Aspergillus fumigatus is less well defined, its role in protecting fungal cells has clearly been shown. Specifically, the ability of both DOPA- and DHN-melanins to quench free radicals is thought to be an important factor in virulence. In addition, in several fungi the production of fungal virulence factors, such as melanin, has been linked to a cAMP-dependent signaling pathway. Many of the components involved in the signaling pathway have been identified.

Regulation of yellow pigment formation in mice: a historical perspective

Pigment synthesis by hair follicle melanocytes is modulated by a large number of environmental and genetic factors, many of which are discussed in this review. Eumelanic (non-yellow) pigment is produced by hair follicle melanocytes following the binding of alpha-melanocyte stimulating hormone to melanocortin receptor 1. Binding of this hormone to the melanocyte membrane is blocked by agouti signaling protein (ASP) which is encoded by the agouti locus and results in the synthesis of yellow pigment, instead of non-yellow (black/brown) pigment. The cyclical release of ASP by hair follicle cells results in a black/brown hair with a subapical yellow band. This is the wild-type coat color pattern of many mammals and is called agouti. Several dominant mutations at the agouti locus in mice, induced by retrotransposon-like intracisternal A particles, result in ectopic over-expression of ASP and animals with much higher proportions of all-yellow hairs. This abnormal presence of ASP in essentially all body cells results in the 'yellow agouti obese mouse syndrome.' The obesity has been associated with binding of ASP to melanocortin receptor 4 inactivating the latter. The syndrome also includes hyperinsulinemia, increased somatic growth, and increased susceptibility to hyperplasia and carcinogenesis. The physiologic and molecular bases for these syndrome components have not yet been elucidated. This historically orientated review is subdivided, where applicable, into pre- and post-1992 subsections to emphasize the impact of the cloning of the agouti and extension loci and their protein products on the identification of the molecular and physiological pathways modulating the manifold aspects of pheomelanogenesis.

Advanced chemical methods in melanin determination

Among the biopolymers, melanins are unique in many respects. The other essential biopolymers - proteins, nucleic acids, and carbohydrates - are chemically well characterized; their precursors (monomer units) and modes of connection between the monomer units are known, and sequences of their connection can be determined with well-established methodologies. In contrast, we still do not have a method to determine accurately the ratio of various units present in melanins. This is largely because of the chemical properties of melanins, such as their insolubility over a broad range of pH, the heterogeneity in their structural features, and also because of the lack of methods that can split melanin polymers into their monomer units (all other biopolymers can be hydrolysed to the corresponding monomer units). To overcome this difficulty, we developed a rapid and sensitive method for quantitatively analysing eumelanin and pheomelanin in biological samples by chemical degradation methods followed by HPLC determination. This HPLC microanalytical method for characterizing eumelanin and pheomelanin has become a useful tool for the study of melanogenesis. This review will summarize the usefulness and limitations of the various chemical and spectrophotometric methods used to analyse melanins at the biochemical, cellular, and tissue levels. Emphasis is given on the usefulness of 4-amino-3-hydroxyphenylalanine as a specific marker of pheomelanin.

The usefulness of 4-amino-3-hydroxyphenylalanine as a specific marker of pheomelanin

Reductive hydrolysis of pheomelanin with hydriodic acid (HI) gives two aminohydroxyphenylalanine isomers, 4-amino-3-hydroxyphenylalanine ('specific AHP') and 3-amino-4-hydroxyphenylalanine (3-aminotyrosine, AT), which derive from the oxidative polymerization of 5-S-cysteinyldopa, and 2-S-cysteinyldopa, respectively. Since we first introduced this analytical method, the combined amount of AHP and AT ('total AHP') has been extensively used as a marker of pheomelanin. However, one problem with using total AHP as a marker is that background levels originate from precursors other than pheomelanin. Considerable and variable amounts of background AT are produced from other sources, most likely nitrotyrosine residues in proteins. In order to overcome this problem, we developed HPLC conditions which enable the direct injection of the HI reduction products into the HPLC system allowing good separation of AHP and AT. In this way we could study the importance of both degradation products separately and their specificity as markers for pheomelanin. The usefulness of the present method is validated using human hair samples of various colours which were divided into dark, fair or red colours. The combined amount of specific AHP and AT shows an excellent correlation with total AHP, and the amount of specific AHP also correlates with the amount of total AHP. We also examined total AHP and specific AHP values against pyrrole-2,3,5-tricarboxylic acid (PTCA) values in the human hair samples. These results show that specific AHP measurement gives a more prominent segregation for the ratio of specific AHP to PTCA among hairs of various colours than the ratio of total AHP to PTCA. Thus, we conclude that 'specific AHP' is a more specific marker of pheomelanin than is 'total AHP'.