Melanin is the main chromophore of the human iris. This pigment is considered to be the most important factor that determines the color of the irides. Previous studies based mainly on chemical degradation methods showed that brown irides contain more melanin than blue ones. In our study, we used electron spin resonance (ESR) spectroscopy to detect and characterize melanin free radical centers and associated iron in human irides. Based on this method, we determined the amount of melanin in the irides and the relative content of iron in iridial melanin as a function of their color, shade, and the age of their donors. Chemical degradation of iridial homogenates enabled us to characterize the structure of eumelanin and determine the content of pheomelanin present in human and bovine irides. The ESR amplitude, the normalized intensity obtained by double integration of the ESR signal of melanin, and the content of the pigment in the irides depended on color and shade of the eyes being 40% higher in the brown group of the irides compared with all other groups. On the other hand, the relative iron content normalized to the melanin content in light blue irides showed a small decrease with age of donors. Melanin in human and bovine irides was mostly composed of eumelanin, and pheomelanin content was of the order of a few percent. Although some differences in the structure of eumelanin present in the human and bovine irides are possible, the results obtained in this study suggest that human irides contain eumelanin with very similar chemical properties.
"Melanosomes in the human RPE are formed early in the fetal development, and although pigment deposition in these organelles may continue for some time after birth, there is very little metabolic turnover of the formed pigment granules. Pigmentation in human uveal melanocytes exhibits significant variation; these cells contain both eumelanin and pheomelanin, and their relative content and total amount depend on the race and iris color (Prota et al., 1998; Wakamatsu et al., 2008; Wielgus and Sarna, 2005). "
[Show abstract][Hide abstract] ABSTRACT: 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.
"Unlike the skin and hair in which melanin is continuously produced and secreted, in the iris the melanosomes are retained and congest the cytoplasm of the melanocytes within the iris stroma. The quality of the melanin between eye colours has also been studied chemically, blue irides have been reported to have minimal pigment content whereas eumelanic and pheomelanic forms have been detected in other eye colours (Prota et al., 1998; Wielgus and Sarna, 2005). Notably, the quantity and type of melanin have also been correlated with iris colour, with the eumelanin:pheomelanin ratio much greater for darker eyes, whereas lighter eyes demonstrated slightly greater amounts of pheomelanin (Wakamatsu et al., 2008). "
[Show abstract][Hide abstract] ABSTRACT: The presence of melanin pigment within the iris is responsible for the visual impression of human eye colouration with complex patterns also evident in this tissue, including Fuchs' crypts, nevi, Wolfflin nodules and contraction furrows. The genetic basis underlying the determination and inheritance of these traits has been the subject of debate and research from the very beginning of quantitative trait studies in humans. Although segregation of blue-brown eye colour has been described using a simple Mendelian dominant-recessive gene model this is too simplistic, and a new molecular genetic perspective is needed to fully understand the biological complexities of this process as a polygenic trait. Nevertheless, it has been estimated that 74% of the variance in human eye colour can be explained by one interval on chromosome 15 that contains the OCA2 gene. Fine mapping of this region has identified a single base change rs12913832 T/C within intron 86 of the upstream HERC2 locus that explains almost all of this association with blue-brown eye colour. A model is presented whereby this SNP, serving as a target site for the SWI/SNF family member HLTF, acts as part of a highly evolutionary conserved regulatory element required for OCA2 gene activation through chromatin remodelling. Major candidate genes possibly effecting iris patterns are also discussed, including MITF and PAX6.
"The precise mechanism underlying the organ specificity of aminoglycoside-induced toxicity has not been fully estab- lished. The melanin pigment is a high molecular mass polymer that occurs widely in living organisms and particularly in the skin, hair, eye, ear and brain (Ings 1984; Zucca et al. 2004; Wielgus and Sarna 2005, Liu et al. 2005). It is known that various drugs and other chemicals are bound (Ibrahim and Aubry 1995; Lowrey et al. 1997; Mårs and Larsson 1999) and retained for long periods in pigmented tissues due to melanin affinity. "
[Show abstract][Hide abstract] ABSTRACT: The aim of this study was to examine in vitro the interaction between aminoglycoside antibiotics displaying adverse ototoxic effects and melanin which is a constituent of the inner ear. The binding of neomycin, tobramycin and amikacin to model synthetic melanin was studied. It has been demonstrated that all the investigated aminoglycosides form stable complexes with melanin biopolymer. The obtained results show that the amount of drug bound to melanin increases with the increase of initial drug concentration and the incubation time. An analysis of drugs binding to melanin by the use of Scatchard plots has shown that at least two classes of independent binding sites must be implicated in the studied aminoglycoside antibiotic-melanin complexes formation: strong binding sites (n1) with the association constant K1 approximately 0.2-2.0 x 10(5) M(-1) and weak binding sites (n2) with K2 approximately 1.0-4.9 x 10(3) M(-1). Based on the values of association constants the following order of drugs affinity to DOPA-melanin was found: tobramycin > amikacin > neomycin. The ability of the analyzed aminoglycoside antibiotics to form complexes with melanin in vitro may be one of the reasons for their ototoxicity in vivo, as a result of their accumulation in melanin in the inner ear.
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