Natural Melanin Research Articles

The effect of ocular pigmentation on transscleral delivery of triamcinolone acetonide.

To determine the capacity and kinetics of the binding between triamcinolone acetonide (TA) and the ocular pigment for a better understanding of the transscleral delivery. In the in vitro study, natural melanin (sepia officinalis, Sigma-Aldrich) was incubated at 37°C with different concentrations of TA and the binding capacity/binding affinity was measured. The TA releasing profile from the melanin was also studied through repeated incubation of TA-melanin in fresh phosphate-buffed saline. In the ex vivo study, the effect of the choroidal pigment on the trans sclera/choroid permeability of TA was investigated through Franz-type vertical diffusion cells using both a TA suspension and a saturated TA solution. The amount of TA bound to melanin increases with the increase of the TA concentration and with an increase in the incubation time. A Scatchard analysis revealed that the maximum number of moles of TA bound to melanin is predicted to be 22.43 nmol/mg, with a binding affinity of K=2.4×10(-5) nM(-1). TA released from a pigment showed a fast phase within the first 24 h and a slow phase thereafter. About 40% of the bound TA released in the first day and 73.94% of accumulative release was observed after 5 days. The TA suspension showed more TA penetration through the scleral-choroid complex than the saturated solution (P=0.0104). The apparent permeability coefficients for the suspension across the sclera-choroid of pigmented and albino rabbits are 7.48±1.53×10(-6) cm/s and 10.78±2.49×10(-6) cm/s, respectively. TA can bind to and release from the ocular pigment, which may extend the TA ocular half-life and therapeutic duration when TA is delivered through a subtenon injection. A further in vivo study is warranted to validate the findings and to quantitate the magnitude of the difference between pigmented and albino animals.

The Synthetic Melanin Nanoparticles Having An Excellent Binding Capacity of Heavy Metal Ions

Spherical-shape melanin nanoparticles with good water-dispersibility were successfully synthesized by a simple oxidation polymerization of 3,4-dihydroxy-phenylalanin (DOPA) with . Similar features to those known from natural and synthetic melanin polymers were observed from prepared melanin nanoparticles by FT-IR, UV-Vis., and ESR spectroscopic methods. Their binding ability with several heavy metal ions from aqueous solution was quantitatively investigated, and the maximum binding capacities with melanin nanoparticles to lead, copper, and cadmium ions were obtained as 2.45, 2.17 and 1.88 mmol/g, respectively, which are much larger values than those reported from natural and synthetic melanin polymers. The large binding capacity and fast binding rate of melanin nanoparticles to metal ions can make them an excellent candidate for the remediation of contaminated water.

Compton Scattering by Internal Shields Based on Melanin-Containing Mushrooms Provides Protection of Gastrointestinal Tract from Ionizing Radiation

There is a need for radioprotectors that protect normal tissues from ionizing radiation in patients receiving high doses of radiation and during nuclear emergencies. We investigated the possibility of creating an efficient oral radioprotector based on the natural pigment melanin that would act as an internal shield and protect the tissues via Compton scattering followed by free radical scavenging. CD-1 mice were fed melanin-containing black edible mushrooms Auricularia auricila-judae before 9 Gy total body irradiation. The location of the mushrooms in the body before irradiation was determined by in vivo fluorescent imaging. Black mushrooms protected 80% of mice from the lethal dose, while control mice or those given melanin-devoid mushrooms died from gastrointestinal syndrome. The crypts of mice given black mushrooms showed less apoptosis and more cell division than those in control mice, and their white blood cell and platelet counts were restored at 45 days to preradiation levels. The role of melanin in radioprotection was proven by the fact that mice given white mushrooms supplemented with melanin survived at the same rate as mice given black mushrooms. The ability of melanin-containing mushrooms to provide remarkable protection against radiation suggests that they could be developed into oral radioprotectors.

Eumelanin fibrils

We describe the auto-oxidation of 3, 4-dihydroxy-L-phenylalanine (L-DOPA) in the synthesis of eumelanin to spontaneously produce fibrils upon drying. The self-assembled fibrils are of characteristic diameter ∼1 to 2 μm, composed of filaments, and are unidirectional, apart from branches that are formed at typically an angle of 20 to 22 deg. The fibrils are characterized using fluorescence spectroscopy, fluorescence decay times, scanning electron microscopy, atomic force microscopy, and fluorescence lifetime imaging microscopy. The fibrils mimic natural melanin in consisting of core eumelanin with efficient nonradiative properties, but they also display pockets of electronically isolated species with higher radiative rates on the nanosecond timescale. Eumelanin fibrils formed occasionally in solution are tentatively attributed to a scaffold of bacteria or fungus. Fabricating and characterizing novel synthetic eumelanin structures such as fibrils are of interest in helping to reveal a functional structure for eumelanin, in understanding its photophysics, in learning more about L-DOPA as it is used in the treatment of Parkinson's disease, and in producing novel materials which might embody some of the diverse properties of eumelanin.

Hydrophilic prodrug approach for reduced pigment binding and enhanced transscleral retinal delivery of celecoxib.

Transscleral retinal delivery of celecoxib, an anti-inflammatory and anti-VEGF agent, is restricted by its poor solubility and binding to the melanin pigment in choroid-RPE. The purpose of this study was to develop soluble prodrugs of celecoxib with reduced pigment binding and enhanced retinal delivery. Three hydrophilic amide prodrugs of celecoxib, celecoxib succinamidic acid (CSA), celecoxib maleamidic acid (CMA), and celecoxib acetamide (CAA) were synthesized and characterized for solubility and lipophilicity. In vitro melanin binding to natural melanin (Sepia officinalis) was estimated for all three prodrugs. In vitro transport studies across isolated bovine sclera and sclera-choroid-RPE (SCRPE) were performed. Prodrug with the highest permeability across SCRPE was characterized for metabolism and cytotoxicity and its in vivo transscleral delivery in pigmented rats. Aqueous solubilities of CSA, CMA, and CAA were 300-, 182-, and 76-fold higher, respectively, than celecoxib. Melanin binding affinity and capacity were significantly lower than for celecoxib for all three prodrugs. Rank order for the % in vitro transport across bovine sclera and SCRPE was CSA > CMA ~ CAA ~ celecoxib, with the transport being 8-fold higher for CSA than celecoxib. CSA was further assessed for its metabolic stability and in vivo delivery. CSA showed optimum metabolic stability in all eye tissues with only 10-20% conversion to parent celecoxib in 30 min. Metabolic enzymes responsible for bioconversion included amidases, esterase, and cytochrome P-450. In vivo delivery in pigmented BN rats showed that CSA had 4.7-, 1.4-, 3.3-, 6.0-, and 4.5-fold higher delivery to sclera, choroid-RPE, retina, vitreous, and lens than celecoxib. CSA has no cytotoxicity in ARPE-19 cells in the concentration range of 0.1 to 1000 μM. Celecoxib succinamidic acid, a soluble prodrug of celecoxib with reduced melanin binding, enhances transscleral retinal delivery of celecoxib.

Detection and quantitation of a pheomelanin component in melanin pigments using pyrolysis–gas chromatography/tandem mass spectrometry system with multiple reaction monitoring mode

Here, we describe the reliable method for the detection and quantitation of a pheomelanin component in melanin pigments. Synthetic melanins with various contents of pheomelanin-type structural units were thermally degraded, and the multiple reaction monitoring mode was applied to detect the pheomelanin markers in the pyrolysates by GC/MS/MS. The method allowed the specific detection and quantitation of a pheomelanin component in melanin with the incorporation of pheomelanin-type units as low as 0.05%. Considering highly universal character of the pheomelanin markers, the method could be applied for structural studies of natural melanin pigments being mixtures of eumelanin and pheomelanin.

Studies on Synthetic and Natural Melanin and Its Affinity for Fe(III) Ion

In this work, we measured the metal-binding sites of natural and synthetic dihydroxyindole (DHI) melanins and their respective interactions with Fe(III) ions. Besides the two acid groups detected for the DHI system: catechol (Cat) and quinone-imine (QI), acetate groups were detected in the natural oligomer by potentiometric titrations. At acidic pH values, Fe(III) complexation with synthetic melanin was detected in an Fe(OH)(CatH2Cat) interaction. With an increase of pH, three new interactions occurred: dihydroxide diprotonated catechol, Fe(OH)2(CatH2Cat)−, dihydroxide monoprotonated catechol, [Fe(OH)2(CatHCat)]2−, and an interaction resulting from the association of one quinone-imine and a catechol group, [Fe(OH)2(Qi−)(CatHCat)]3−. In the natural melanin system, we detected the same interactions involving catechol and quinone-imine groups but also the metal interacts with acetate group at pH values lower than 4.0. Furthermore, interactions in the synthetic system were also characterized by infrared spectroscopy by using the characteristic vibrations of catechol and quinone-imine groups. Finally, scanning electronic microscopy (SEM) and energy-dispersive X-ray (EDS) analysis were used to examine the differences in morphology of these two systems in the absence and presence of Fe(III) ions. The mole ratio of metal and donor atoms was obtained by the EDS analysis.

Evaluation of Melanogenesis in A-375 Cells in the Presence of DMSO and Analysis of Pyrolytic Profile of Isolated Melanin

The increase of a skin malignant melanoma (melanoma malignum) incidence in the world has been observed in recent years. The tumour, especially in advanced stadium with metastases, is highly resistant to conventional treatment. One of the strategies is to modulate melanogenesis using chemical compounds. In this study, the processes of differentiation and melanogenesis induced by dimethylsulfoxide (DMSO) in human melanoma cells (A-375) were investigated. Natural melanin isolated from A-375 melanoma cell line treated with 0.3% DMSO was analyzed by pyrolysis-gas chromatography-mass spectrometry (Py-GC/MS) method. The products derived from pheomelanin have not been stated in the pyrolytic profile of analyzed melanin. Within all products derived from eumelanins, 1,2-benzenediol has been predominated. It has been shown that in the melanoma cells stimulated with 0.3% and 1% DMSO, the increase of transcriptional activity of the tyrosinase gene took place. It was accompanied by the rise of tyrosinase activity and an accumulation of melanin in the cells. The better knowledge about the structure of melanins can contribute to establish the uniform criteria of malignant melanoma morbidity risk.

Probing the Molecular Architecture and Assembly of Synthetic and Fungal Melanins with Solid-State NMR

Melanins are ubiquitous natural pigments that are associated with biological roles in protection against sunlight, camouflage, free radical scavenging, and microbial virulence. However, elucidating structural characteristics of these enigmatic pigments remains a challenging task because of their insolubility and heterogeneous properties. High-resolution solid-state NMR spectroscopy provides a unique tool to unravel the mysteries of melanin structure, and melanogenesis in Cryptococcus neoformans (CN) requiring exogenous obligatory catecholamine precursors such as L-dopa offers unprecedented opportunities to probe melanin biosynthesis and assembly in fungal cells. 1D solid-state 13C NMR has been used to assess the structural differences among CN melanins produced by systematic variation of catecholamine precursors and in cell-free media. The precursor preferences of CN melanogenesis have been demonstrated by competition studies in which the cells were provided with equimolar mixtures of catecholamines, and a mechanistic scheme adapted from the Mason-Raper pathway has been proposed to rationalize the possible intermediates for biosynthesis of indole-based melanin polymers. 13C and 15N solid-state NMR, including 2D through-space and through-bond correlation spectra, have been implemented to elucidate the ultimate molecular framework of the pigments and melanization in CN with exogenously 13C-enriched L-dopa and/or [U-13C6]-glucose supplied to the growth media. Findings include: (1) the main aromatic components are located spatially within 5-7 A of the long-chain aliphatics; (2) glucose scrambles metabolically to appear in oxygenated- and long-chain aliphatic structures; (3) some aromatic, oxygenated- and long-chain aliphatic moieties are attached covalently to carboxyl groups of the pigment; (4) the pigment is likely to be covalently bound to the polysaccharides of fungal cell wall. The molecular architecture of natural melanin revealed by the solid-state NMR study can guide the design of new drugs against microbial virulence and novel therapeutics for melanoma tumors.

Melanin-based coatings as lead-binding agents.

Interactions between metal ions and different forms of melanin play significant roles in melanin biochemistry. The binding properties of natural melanin and related synthetic materials can be exploited for nonbiological applications, potentially including water purification. A method for investigating metal ion-melanin interactions on solid support is described, with lead as the initial target. 2.5 cm discs of the hydrophobic polymer PVDF were coated with synthetic eumelanin from the tyrosinase-catalyzed polymerization of L-dopa, and with melanin extracted from human hair. Lead (Pb2+) binding was quantified by atomic absorption spectroscopy (flame mode), and the data was well fit by the Langmuir model. Langmuir affinities ranged from 3.4 · 103 to 2.2 · 104 M−1. At the maximum capacity observed, the synthetic eumelanin coating bound ~9% of its mass in lead. Binding of copper (Cu2+), zinc (Zn2+), and cadmium (Cd2+) to the synthetic-eumelanin-coated discs was also investigated. Under the conditions tested, the Langmuir affinities for Zn2+, Cd2+, and Cu2+ were 35%, 53%, and 77%, respectively, of the Langmuir affinity for Pb2+. The synthetic-eumelanin-coated discs have a slightly higher capacity for Cu2+ on a per mole basis than for Pb2+, and lower capacities for Cd2+ and Zn2+. The system described can be used to address biological questions and potentially be applied toward melanin-based water purification.

Obtaining of water soluble microbial melanin and study of its some properties

The effective sorption method for melanin isolation and purification from fermentation solutions of Bacillus thuringiensis serovar galleriae K1 has been elaborated, the principle process flowsheet is presented. The identification of obtained pigment with the samples of natural and synthetic melanin was done by IR-spectroscopy, and the intensity ratio of optical absorption at 650 and 500 nm allows to refer the isolated melanin to eumelanin class. By thermal treatment it was determined, that melanin's amorphous sediment is steady at up to 120 degrees C temperatures, at that the concentration of paramagnetic centers is changed from 0.053 x 10(18) spin/g (48 degrees C) to 0.25 x 10(18) spin/g (120 degrees C). The rising of the temperature of treatment up to 210 degrees C brings to substantial increase of the concentration of unpaired electrons, and at 280 degrees C its sharp growth is observed. At 350 degrees C growth stops, then sharp decrease is observed. The obtained results were confirmed by methods of IR spectroscopy and derivatographic analysis.

The Research Progress of Melanin

The sources of melanin and the sorts of melanin are introduced. New techniques for extraction and analysis of natural melanin were introduced. New analytical techniques are high performance liquid chromatography, high-speed countercurrent chromatography, high performance capillary electrophoresis and chromatography-mass spectrometry. The relationship of polyketide melanogenesis molecular biology to that of nonmelanin-producing pathways in a wide range of fungi and other organisms is discussed. The applications of melanin are introduced.

A comparative evaluation of photo-toxic effect of fractionated melanin and chlorpromazine hydrochloride on human (dermal fibroblasts and epidermal keratinocytes) and mouse cell line/s (fibroblast Balb/c 3T3)

Fractionated melanin (Mel-HEV), a bleached version of natural melanin, offers protection against the high energy visible (HEV/UVA) and ultraviolet (specifically UVA) irradiation making it a potential compound to be added to skin care and sunscreen formulations and other cosmetic and personal care products. Chlorpromazine (CPZ) has been shown to exhibit photosensitivity and phototoxicity reaction in vitro and in vivo. Comparative evaluation of chemotoxicity and phototoxicity using Mel-HEV and CPZ (as positive control) was performed on mouse fibroblast cell line ‘Balb/c 3T3’. This is the recommended method for evaluating the phototoxic potential of compounds under the European Center of Validation of Alternative Methods (ECVAM) guidelines (OECD, 2004). This study was expanded from a mouse cell line – Balb 3T3/c to two human cell lines – HDF and HEKn for two reasons: to compare the difference between the sensitivity and behavior of two fibroblast cell lines (Balb/c 3T3 vs. HDF) and to compare the differences between two fibroblast cell lines with the keratinocyte cell line (HDF & Balb/c 3T3 vs. HEKn). It was found that Balb/c 3T3 and HEKn were both sensitive to the phototoxic potential of CPZ. However, HDF showed insensitivity to phototoxic evaluation. The test compound, Mel-HEV, was found to be non-phototoxic. The mean toxic concentration (MTC) for CPZ during HEV and UVA exposure conditions was found to be similar using Balb/c 3T3 (36.25μg/ml) and HEKn (39.99μg/ml) showing that cells exhibit similar responses at HEV/UVA− conditions. However, Balb/c 3T3 showed more sensitivity to CPZ at HEV/UVA+ condition (MTC=0.87μg/ml; mean PIF=55.33; MPE=0.395) than HEKn (MTC=5.35μg/ml; PIF=7.61; MPE=0.276) making it the preferred cell line for phototoxicity evaluations.

Optimization of ultrasound-assisted extraction of melanin from Auricularia auricula fruit bodies

Abstract In this paper, ultrasound-assisted extraction (UAE) conditions of natural melanin from dried fruit bodies of Auricularia auricula (A. auricula) were investigated. By single-factor experiments, ultrasound power was determined at 250 W and the other factors (temperature, liquid–solid ratio and duration) were chosen to further optimize extraction conditions using response surface methodology (RSM). The Box–Behnken experimental results showed the optimum extraction conditions as follows: a temperature of 63 °C, a liquid–solid ratio of 43 mL/g and a duration of 36 min. Under these conditions, two extractions sufficiently reached the maximal melanin yield (120.05 mg/100 g). Light absorbance, solubility and redox properties of the melanin extracted from A. auricula were similar to those of the typical melanin. Industrial relevance Auricularia auricula (A. auricula) has been used as food and drug in China for a long time. The annual production of A. auricula in China was 1.44 million tons in 2007 ( http://www.mushroommarket.net/news/show.asp?id=19769 ). However, most of this precious macro-fungus product was only used as cuisine materials, and many of its functional components, such as melanin, a healthful natural food colorant, were not fully developed and employed. In this paper, ultrasound-assisted extraction (UAE) was applied to melanin extraction from A. auricula fruit bodies to improve the extraction efficiency. This research helps to develop a new and economical method of extracting natural melanin and to fully use this edible mushroom.

Infrared and Water Sorption Studies of the Hydration Structure and Mechanism in Natural and Synthetic Melanin

Although water is a structural and functional determinant in melanins, a direct study of the interaction between water and melanin is still lacking and is the subject of the present work. Melanin forms in cells and organisms' colloidal particles deriving from the hierarchical aggregation of smaller subunits such as protomolecules, stacking units, and small aggregates: its functions must be interpreted in terms of solid-state and surface properties. They are strictly connected to the porosity of the particles when they interact with water and other chemicals. The structural characteristics of water sorbed in the pores are investigated by means of FTIR in the OH stretching vibration region (4000-3000 cm(-1)) on Sepia and synthetic l-Dopa melanins at different and controlled hydration degrees (a(w) = 0.06 to 0.92). Three distinct component bands are recognized in the main OH stretching band, corresponding to three distinct water populations, showing differing behavior for the two kinds of melanin and thus accounting for different molecular structures. On this basis, the historical, albeit current, model of hydration structure of melanin granules, a "strongly" and a "weakly" bound fraction, is reassessed and rediscussed.

Natural Melanin Produced in Fungi Protects Cells from High Dose Ionizing Radiation: An EPR Study

Natural Melanin Produced in Fungi Protects Cells from High Dose Ionizing Radiation: An EPR Study

Transscleral visible/near-infrared spectroscopy for quantitative assessment of melanin in a uveal melanoma phantom of ex vivo porcine eyes

Optical spectroscopy has been used as a supplement to conventional techniques for analyzing and diagnosing cancer in many human organs. Because ocular tumors may be characterized by their different melanin content, we investigated the feasibility of using transscleral visible/near-infrared spectroscopy (Vis/NIRS) to estimate the quantity of melanin in a novel uveal melanoma phantom of ex vivo porcine eyes. The phantoms were made by injecting a freshly prepared suspension of 15% (wt/vol) gelatin, 10 mg/ml titanium dioxide (TiO 2), and natural melanin, isolated from the ink sac of cuttlefish ( Sepia officinalis), into the suprachoroidal space of 30 enucleated porcine eyes. The melanin concentrations used were 1 mg/ml, 2 mg/ml, and 3 mg/ml, with 10 eyes in each group. After gelation, the size and location of the phantoms were documented by B-scan ultrasonography and transillumination. Vis/NIRS recordings, covering the wavelength region from 550 to 1000 nm, were performed with two optical fibers separated by 6 mm to deliver and collect the light through the sclera. During all measurements, the exact pressure exerted by the fiber probe on the scleral surface was monitored by placing the eye on an electronic scale. Transscleral Vis/NIRS was performed across the phantom inclusion, as well as on the opposite (normal) side of each eye. A total of three consecutive measurements were carried out alternately on each side of the globe. The spectral data were analyzed using partial least squares regression. In the melanin concentration groups of 1 mg/ml ( n = 10), 2 mg/ml ( n = 10), and 3 mg/ml ( n = 10), the largest basal phantom diameters (mean ± SD) were 14.9 ± 1.6 mm, 14.6 ± 1.5 mm, and 14.3 ± 1.0 mm, respectively ( p > 0.05). The largest phantom thicknesses (mean ± SD) were 4.0 ± 0.5 mm, 4.4 ± 0.7 mm, and 4.5 ± 0.5 mm, respectively ( p > 0.05). Statistical regression modeling of the Vis/NIRS data revealed that it was possible to correctly classify the phantoms according to their melanin concentrations in 84.4% of cases. The correct classification rate for phantoms with the lowest (1 mg/ml) and highest (3 mg/ml) melanin concentrations was 99.2%. The study demonstrates that transscleral Vis/NIRS is a feasible and accurate method for predicting the content of melanin in choroidal lesions.

Influence of lipophilicity on drug partitioning into sclera, choroid-retinal pigment epithelium, retina, trabecular meshwork, and optic nerve.

In vitro bovine eye tissue/phosphate-buffered saline, pH 7.4, partition coefficients (Kt:b), in vitro binding to natural melanin, and in vivo delivery at 1 h after posterior subconjunctival injection in Brown Norway rats were determined for eight beta-blockers. The Kt:b was in the order intact tissue, dry weight method >or= intact tissue, wet weight method corrected for tissue water and drug in tissue water >> intact tissue, wet weight method > homogenized tissue. In intact tissue methods, Kt:b followed the order choroid-retinal pigment epithelium (RPE) > trabecular meshwork > retina > sclera approximately optic nerve; propranolol > betaxolol > pindolol approximately timolol approximately metoprolol > sotalol approximately atenolol approximately nadolol. Intact tissue, wet weight log (Kt:b) correlated positively with log D for all tissues (R(2) of 0.7-0.9). Log (melanin binding capacity) correlated positively with choroid-RPE log (Kt:b) (R(2) of 0.5). With an increase in concentration, Kt:b decreased in trabecular meshwork for all beta-blockers and for some lipophilic beta-blockers in choroid-RPE and sclera. With an increase in drug lipophilicity, in vivo tissue distribution increased in choroid-RPE, iris-ciliary body, sclera, and cornea but exhibited a declining trend in retina, vitreous, and lens. In vitro bovine intact tissue, wet weight Kt:b correlated positively with rat in vivo tissue/vitreous humor distribution for sclera, choroid-RPE, and retina (R(2) of 0.985-0.993). In vitro tissue partition coefficients might be useful in predicting in vivo drug distribution after trans-scleral delivery. Less lipophilic solutes exhibiting limited nonproductive binding in choroid-RPE might exhibit greater trans-scleral delivery to the retina and vitreous.

Electrical and optical properties of natural and synthetic melanin biopolymer

Abstract Natural and synthetic melanin have been investigated by means of optical, electrical and photoelectronic measurements. Optical measurements evidence absorption curves which allow to estimate optical gap for synthetic melanin, by using Tauc’s method. Dark conductivity and photoconductivity measurements were performed as a function of temperature and for different duration of thermal treatments. It has been evidenced that both quantities are thermally activated and thermal treatments play a very important role as far as gap states are concerned.

Effect of untreated and photobleached bovine RPE melanosomes on the photoinduced peroxidation of lipids

Melanin is usually considered a photoprotective pigment and antioxidant agent, but it is unclear how melanosomes protect pigmented cells against oxidative stress induced by light and whether aging modulates its photoprotective function, particularly in long-lived post-mitotic cells such as the retinal pigment epithelium (RPE). To address these issues, we analyzed the effects of untreated and experimentally photobleached melanosomes from cow RPE on the peroxidation of liposomal lipids induced by irradiation with intense visible light or by a rose Bengal photosensitized reaction. Photobleached melanosomes were used as a model of photo-aged pigment granules, and the progress of lipid peroxidation was monitored by electron spin resonance (EPR) oximetry and the iodometric determination of lipid hydroperoxides. We observed that while untreated melanosomes inhibited the rose Bengal induced peroxidation of lipids only moderately, partially photobleached melanosomes had very little effect on this process. Untreated melanosomes also inhibited peroxidation of liposomal lipids induced by intense visible light; however, the inhibitory effect markedly changed with the irradiation time. On the other hand, partially photobleached pigment granules accelerated the photoinduced peroxidation of lipids. The observed effects illustrate the limited efficiency of melanin within granules to scavenge and quench reactive oxygen species randomly generated by photosensitized reactions. The photosensitizing ability of photobleached melanosomes may arise from their capacity to photogenerate hydrogen peroxide. Collectively, our data indicate that natural melanin is only a moderately efficient photoprotective pigment, which upon photoaging may lose its antioxidant efficiency and even become a photosensitizer.