Melanin structure and the potential functions of uveal melanosomes

This is the authors' abstract. We don't add key points for this paper.

Dear Sir,Wielgus and Sarna (2005) recently studied the melanin in human irides. Interestingly, in spite of the varying color of the iris, the melanin composition is similar among the eyes of different donors. Specifically, for all the human iris samples examined, the content of eumelanin and pheomelanin is similar as are the yields of both pyrrole-2,3,5-tricarboxylic acid and pyrrole-2,3-dicarboxylic acid after alkaline peroxide degradation. It is reasonable to postulate that the function of melanosomes is reflected by the chemical structure of the embedded melanin pigment. And then the similarity of melanin composition in the irides found in the above study suggests that iris melanosomes share common structural features that are embodied by the specific functions performed. It is worthwhile to consider these results within the context of studies on uveal bovine melanosomes, expanding the discussion of potential structure/function linkages to take into account differences in the locations and functions of two uveal tissues: the iris and the choroids. Melanosomes isolated from bovine irides and choroids have different melanin contents and more importantly, different structural motifs (Hong and Simon, 2005; Liu et al., 2005). We consider the possibility that the structure of the melanin in iris and choroid melanosomes differs in functionally significant ways, responding to the different functions of these tissues. While both iris and choroid belong the uveal tract of the eye, the microenvironments of these tissues are different. They are exposed to different levels and wavelength ranges of light: the iris – being located at the front of the lens in the eye – is exposed to a broad band of UV, visible and near IR radiation (λ > 295 nm) (Ambach et al., 1994; Boettner and Wolter, 1962); the choroids – bring in the back of the eye – is exposed mainly to visible light (λ = 380–760 nm) (Ambach et al., 1994; Boettner and Wolter, 1962; Dayhaw-Barker, 2002). In addition to this differential light exposure, the oxidative stress varies within these tissues. The choroid is highly vascularized and experiences significant oxidative stress; the oxidative stress of RPE-choroid complex is the highest of all the pigmented tissues in the eye (Schraermeyer and Heimann, 1999). In addition, ocular melanosomes are believed to protect the eye by absorbing light and quenching reactive oxygen species (Boulton, 1991, 1998; Boulton et al., 2001; Sarna, 1992). Given the different wavelength ranges impinging on the iris and choroids, it is reasonable to hypothesize that iris melanosomes need to be more efficient in absorbing UV light than their choroid counterparts, while choroid melanosomes must have a greater capacity to quench reactive oxygen species. These functional differences between iris and choroids melanosomes should be reflected in the underlying pigment structure. While the molecular details of melanin structure still need to be elucidated, differences in the populations of functional specific function groups have been elucidated for these uveal melanosomes (Hong and Simon, 2005; Liu et al., 2005). Specifically, chemical degradation analyses showed that choroid melanosomes are almost exclusively eumelaninic, while iris melanosomes, although also predominantly eumelanosomes, contain a measurable pheomelanin component. Elemental analysis showed that choroid melanosomes contain more 5,6-dihydroxyindole-2-carboxylic acid (DHICA) subunits than iris melanosomes. Solid state 13C NMR and IR analyses reveal that iris melanosomes have a higher degree of crosslinking and conjugation than their choroid counterparts. Let us now consider these physiochemical differences in terms of the functional differences highlighted above in terms of oxidative stress and light exposure. Consider first oxidative stress. The greater the ratio of eumelanin to pheomelanin, the more antioxidant the pigment (Chedekel et al., 1980; Vincensi et al., 1998). In addition, the greater the DHICA to 5,6-dihydroxyindole (DHI) ratio, the greater the antioxidant ability – the increased solubility of DHICA relative to DHI enhances the ability of the melanin to quench reactive oxygen species (Blarzino et al., 1999; Sarangarajan and Apte, 2005). The lack of pheomelanin in choroid melanosomes, the higher DHICA content and the less cross-linked molecular structure of its melanin is consistent with the need of these melanosomes to be more efficient in quenching reactive oxygen species than those of the iris. While the corresponding studies on choroidal melanin has never been done, human iridial samples, regardless the age of the donors and color of their eyes, photogenerated similar amounts of superoxide anion and hydrogen peroxide (Wielgus and Tadeusz, 2002). Of interest is that physiological concentrations of ascorbate significantly accelerate these melanin-mediated photoprocesses. The aqueous humor contains up to 1 mM ascorbate (Ringvold and Kjonniksen, 2000; Spector and Wang, 1998) and the steady-state concentration of hydrogen peroxide in this watery fluid that fills the anterior and posterior chambers of the eye can reach in normal human eyes about 25 μM (Spector and Wang, 1998). It is possible that the ascorbate concentration together with the photoreactivity of the iris melanin may contribute to protection against bacterial contamination by keeping the hydrogen peroxide concentration in the aqueous humor at a relatively high level. Secondly, with regard to the absorption of light, increased crosslinking within the pigment extends the absorption to longer wavelength and increases its absorption cross section. Therefore iris melanosomes are more efficient in light absorption and absorb over a larger range than their choroids counterparts, reflective of their different light exposures – both in intensity and wavelength range. The observations made above suggest that the structure of melanin is tailored to meet the functional needs of the tissue in which the melanosomes are embedded. Sincerely, LH and JDS thank the AFOSR for support through the MFEL program. TS thanks Ministry of Science and Information Technology for financial support.