Photobleaching of retinal pigment epithelium melanosomes reduces their ability to inhibit iron-induced peroxidation of lipids Andrzej Zadlo 1 , Malgorzata B. Rozanowska 1,2 , Janice M. Burke 3 and Tadeusz J. Sarna 1 1 Department of Biophysics, Jagiellonian University, Krakow, Poland 2 School of Optometry and Vision Sciences, Cardiff University, Cardiff, UK 3 Department of Ophthalmology, Medical College of Wisconsin, Milwaukee, WI, USA *Address correspondence to T. Sarna, e-mail: tsarna@mol.uj.edu.pl Summary Melanin in the human retinal pigment epithelium (RPE) is believed to play an important photoprotec- tive role. However, unlike in skin, melanosomes in the RPE are rather long-lived organelles, which increases their risk of modifications resulting from significant fluxes of light and high oxygen tension. In this work, we subjected purified bovine RPE melano- somes to prolonged aerobic exposure with intense visible and near ultraviolet radiation and studied the effects of irradiation on the melanosome’s capacity to inhibit peroxidation of lipids induced by iron/ ascorbate. We found that control, untreated melano- somes show a concentration-dependent inhibition of the accumulation of lipid hydroperoxides and the accompanying consumption of oxygen, but photo- lysed melanosomes lose their antioxidant efficiency and even became prooxidant. The prooxidant action of partially photobleached melanosomes was observed for pigment granules with a melanin con- tent reduced by about 50% compared with untreated melanosomes, as determined by electron spin reson- ance spectroscopy. We have previously shown that a similar loss in the content of the RPE melanin occurs during human lifetime, which may suggest that the normal antioxidant properties of human RPE melanin become compromised with aging. Key words: RPE melanosomes/melanin photobleaching/ lipid peroxidation/antioxidant properties/prooxidant action Received 22 May 2006, revised and accepted for publi- cation 22 September 2006 Introduction Melanin in the human skin and eye is believed to act as a natural sunscreen and photoprotective pigment by absorbing and scattering solar radiation, particularly the energetic UV and short wavelength visible photons. There is a clear correlation between skin pigmentation and the skin’s resistance to UV-induced erythema and sunburn and the incidence of solar radiation-related skin cancer is higher in individuals with low skin pigmenta- tion and poor tanning ability (Rees, 2004; Armstrong and Kricker, 2001). Furthermore, an inverse correlation appears to exist between skin pigmentation and the susceptibility to so-called photo-ageing (Wlaschek et al., 2001), and to the extent of UV-induced DNA damage (Tadokoro et al., 2003). In cultured skin melanocytes, melanin has been shown to protect against major UVB- induced DNA lesions (Barker et al., 1995; Smit et al., 2001) and against UVA-induced membrane damage (Kvam and Dahle, 2003). It is not clear if ocular pigmen- tation similarly protects the eye, specifically the retina, from chronic exposure to solar radiation, perhaps redu- cing the incidence of diseases like age-related macular degeneration (AMD) (Young, 1988). AMD is more often found in individuals with a lower content of the uveal melanin (Nicolas et al., 2003), but similar comparative data do not exist for melanin in the retinal pigment epi- thelium (RPE), a tissue located in close association with retinal photoreceptors. The molecular and cellular mechanism of photopro- tection offered by melanin is not fully understood. Of course, the ability of melanin pigments to absorb light is intrinsically protective, providing that the energy of the absorbed photons is rapidly utilized in non-photo- chemical processes. Indeed a very efficient non-radia- tive de-excitation of melanin following the absorption of ultraviolet and visible photons has been reported in several studies (Crippa et al., 1991; Gallas et al., 1988; Forrest et al., 2000). These studies clearly show that melanin is a system in which a very efficient thermal relaxation occurs; that is, the energy absorbed by mel- anin photons is rapidly converted to heat via internal conversion. As a result, the risk of potentially dam- aging photochemical reactions mediated by melanin is significantly reduced. Copyright ª 2006 The Authors Journal compilation ª 2006 Blackwell Munksgaard doi: 10.1111/j.1600-0749.2006.00350.x 52 Pigment Cell Res. 20; 52–60