Melanin, particularly eumelanin, is considered an efficient photoprotective pigment, even though its residual photochemistry is responsible for the generation of reactive oxygen species (ROS). However, the flux of ROS, photogenerated by melanin under typical conditions, is too low to cause significant disruption of pigmented cells. The phototoxic potential of melanin could substantially increase with an enhanced photosensitizing ability of the melanin. We have tested this postulate by analyzing the photochemical and photobiological properties of human hair melanosomes obtained from individuals of different skin phototypes subjected to different degrees of experimental photobleaching [Mokrzynski et al., 2023]. Melanosomes of skin phototypes II and III were most susceptible to bleaching induced by intense violet light. Upon excitation with UVA or blue light, melanin from control melanosomes of phototype II and III exhibited the highest yield to photogenerate singlet oxygen. Experimental bleaching of hair melanosomes modified their photochemical properties. Thus bleached melanosomes of all tested skin phototypes revealed higher efficiency to photogenerate singlet oxygen. On the other hand, the ability of bleached melanosomes, except melanosomes from skin phototype V, to photogenerate superoxide anion, was lower than that of control unbleached melanosomes. The antioxidant capacity of hair melanosomes was consistently reduced by photobleaching. Photobleached melanosomes of all skin phototypes, except phototype V, were phototoxic to human epidermal keratinocytes (HaCaT cell line). Irradiation of such cells, containing photobleached melanosomes, with light from a solar simulator, decreased the cell's mitochondrial membrane potential and increased the cellular level of lipid hydroperoxides. The data suggest that photoaging of melanin could substantially elevate its photosensitizing ability.
Reference:
K. Mokrzynski et al., Journal of Photochemistry & Photobiology, B: Biology 2023, 243: 1-16, 112704.
Supported by Poland National Science Centre (project OPUS-2021/43/B/ST4/03255).