Explanation for the Disparity among Absorption and Action Spectra of Eumelanin

Nofsinger, J. Brian; Forest, Susan E.; Simon, John D.

doi:10.1021/jp992640y

Cited by 98 publications

(142 citation statements)

References 24 publications

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“…Based on the results with the acidic synthetic melanin samples where aggregation and TREPR-observed activity is high, the melanin in these less-susceptible RPE cells is expected to be more aggregated than in RPE cells exhibiting high susceptibility to apoptosis. Accordingly, the physical state of the melanin biopolymer is relevant to the production of free radicals in agreement with photoacoustical studies (57). Our observations indicate that aggregation of melanin could serve a photoprotective role by providing highly diffusive melanin radicals that can quickly scavenge dangerous free radicals including ROS.…”

Section: Discussionsupporting

confidence: 86%

Melanin photoprotection in the human retinal pigment epithelium and its correlation with light-induced cell apoptosis

Seagle

Rezai

Kobori

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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Time-resolved electron paramagnetic resonance (TREPR) spectroscopy was used to study melanin free radicals in human retinal pigment epithelium (RPE) cells and tyrosine-derived synthetic melanin. TREPR signal traces from RPE cells reveal in vivo light-induced melanin free radical photochemistry in more detail than previously known. Electron spin polarization reflecting a non-Boltzmann population within the energy levels of the spin system is observed in RPE cells as the result of the triplet state photoproduction and subsequent disappearance of free radicals in the melanin polymer. In a set of RPE cells cultured from individual sources, differences in optical absorption, continuous wave EPR spectra, and TREPR signals were correlated with apoptosis assays performed by flow cytometry. Continuous wave EPR spectra of RPE cells and TREPR of acidified synthetic melanin suggest that increased melanin aggregation provides an increase in photoprotection in the RPE cells that are relatively less susceptible to blue light-induced apoptosis.continuous wave EPR ͉ electron spin polarization ͉ time-resolved EPR I n the eye, retinal pigment epithelium (RPE) is a monolayer of cuboidal cells between the photoreceptors and choriocapillaris of the eye and is specialized to uptake, phagocytize, and recycle the outer segment of photoreceptors and retinaldehyde, the chromophore of rhodopsin (1). RPE cell death plays a major role in the pathogenesis of age-related macular degeneration, the leading cause of blindness in the population Ͼ60 years of age in the developed world (2). Possible reasons given for the degeneration of RPE cells are their exposure to high oxidative stress and damaging irradiation (2-6).The pigment melanin, found in RPE cells, is a heterogeneous polymer consisting of various monomers believed to be oxidation products of dopa (dihydroxyphenylalinine) derived from tyrosine (7). Melanin contains intrinsic, indolesemiquinone-like doublet-state radicals (8, 9). Extrinsic indolesemiquinone-like radicals are reversibly photogenerated under visible or UV irradiation (9). RPE melanin serves a photoprotective role by absorbing radiation and scavenging free radicals and reactive oxygen species (ROS) (8-11). Evidence also exists for a phototoxic role for melanin in RPE cells, especially in aged cells, including measurable ROS photoproduction (8,(12)(13)(14)(15)(16)(17)(18). Electron paramagnetic resonance (EPR) spectroscopy enables a sensitive and nondestructive analysis and differentiation of natural melanin (9,19,20). The photophysical, redox, and aggregation properties of melanin as well as its ability to bind metals, proteins, and drugs have stimulated numerous studies in chemical, physical, and medical research (8, 9). The diversity and interrelations of its properties and the lack of chemical structures have hampered a clear understanding of the roles of melanin in RPE cells (8,9). Nanosecond time-resolved (TR) EPR studies have not been reported despite the relevance of TREPR to the study of melanin photochemistry (9). El...

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Section: Discussionsupporting

confidence: 86%

Melanin photoprotection in the human retinal pigment epithelium and its correlation with light-induced cell apoptosis

Seagle

Rezai

Kobori

et al. 2005

Proc. Natl. Acad. Sci. U.S.A.

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“…23,27 In 1999, we put forth an explanation for this disparity in the case of eumelanin by demonstrating that the action spectrum matches the absorption spectra of oligomeric constituents of the natural melanins. 28 Furthermore, aggregation of these constituents affects their aerobic reactivity. Miles Chedekel and co-workers were the first to report detailed studies on the photodynamic properties of pheomelanin, finding that excitation through the UV region resulted in the formation of superoxide radical anions.…”

Section: Modes Of Behaviormentioning

confidence: 99%

The Red and the Black

2010

Self Cite

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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."sE. 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.

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“…11, shows that the absorption rapidly decreases with wavelength. 22 In the analysis of FWHM angles, absorption by melanin is dominant in the short wavelength range and is, therefore, not accounted for since the laser produces light in the longer wavelength region of 633 nm. As for the effect of absorption, a small amount of absorption merely reduces the amount of scattering, and larger amounts of absorption will reduce the amount of light, which, while reducing amplitude of the scattering profiles from the nuclei, will not change their shapes.…”

Section: Discussionmentioning

confidence: 99%

Refractometry of melanocyte cell nuclei using optical scatter images recorded by digital Fourier microscopy

Seet

Nieminen

Zvyagin³

2009

J. Biomed. Opt.

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Abstract. The cell nucleus is the dominant optical scatterer in the cell. Neoplastic cells are characterized by cell nucleus polymorphism and polychromism-i.e., the nuclei exhibits an increase in the distribution of both size and refractive index. The relative size parameter, and its distribution, is proportional to the product of the nucleus size and its relative refractive index and is a useful discriminant between normal and abnormal ͑cancerous͒ cells. We demonstrate a recently introduced holographic technique, digital Fourier microscopy ͑DFM͒, to provide a sensitive measure of this relative size parameter. Fourier holograms were recorded and optical scatter of individual scatterers were extracted and modeled with Mie theory to determine the relative size parameter. The relative size parameter of individual melanocyte cell nuclei were found to be 16.5± 0.2, which gives a cell nucleus refractive index of 1.38± 0.01 and is in good agreement with previously reported data. The relative size parameters of individual malignant melanocyte cell nuclei are expected to be greater than 16.5.

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Explanation for the Disparity among Absorption and Action Spectra of Eumelanin

Cited by 98 publications

References 24 publications

Melanin photoprotection in the human retinal pigment epithelium and its correlation with light-induced cell apoptosis

Melanin photoprotection in the human retinal pigment epithelium and its correlation with light-induced cell apoptosis

The Red and the Black

Refractometry of melanocyte cell nuclei using optical scatter images recorded by digital Fourier microscopy

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