Department of Chemistry, Fujita Health University School of Health Sciences, Toyoake, Aichi, Japan.
Pigment Cell Melanoma Res. 2012 Jul;25(4):434-45. doi: 10.1111/j.1755-148X.2012.01011.x.
Eumelanin is photoprotective while pheomelanin is phototoxic to pigmented tissues. Ultraviolet A (UVA)-induced tanning seems to result from the photooxidation of pre-existing melanin and contributes no photoprotection. However, data available for melanin biodegradation remain limited. In this study, we first examined photodegradation of eumelanin and pheomelanin in human black hairs and found that the ratio of Free (formed by peroxidation in situ) to Total (after hydrogen peroxide oxidation) pyrrole-2,3,5-tricarboxylic acid (PTCA) increases with hair aging, indicating fission of the dihydroxyindole moiety. In red hair, the ratio of thiazole-2,4,5-tricarboxylic acid (TTCA) to 4-amino-3-hydroxyphenylalanine (4-AHP) increases with aging, indicating the conversion from benzothiazine to benzothiazole moiety. These photodegradation of melanins were confirmed by UVA (not UVB) irradiation of melanins from mice and human hairs and synthetic eumelanin and pheomelanin. These results show that both eumelanin and pheomelanin degrade by UVA and that Free/Total PTCA and TTCA/4-AHP ratios serve as sensitive indicators of photodegradation.
真黑素具有光保护作用，而褐黑素对色素组织则具有光毒性。紫外线 A（UVA）诱导的晒黑似乎是由于预先存在的黑色素的光氧化所致，并没有提供光保护作用。然而，关于黑色素生物降解的数据仍然有限。在这项研究中，我们首先检查了人黑色毛发中真黑素和褐黑素的光降解，发现游离（原位过氧化物形成）与总（过氧化氢氧化后）吡咯-2,3,5-三羧酸（PTCA）的比值随头发老化而增加，表明二羟基吲哚部分的分裂。在红发中，噻唑-2,4,5-三羧酸（TTCA）与 4-氨基-3-羟基苯丙氨酸（4-AHP）的比值随老化而增加，表明苯并噻嗪部分向苯并噻唑部分的转化。这些黑色素的光降解通过小鼠和人毛发以及合成的真黑素和褐黑素的 UVA（而非 UVB）照射得到了证实。这些结果表明，真黑素和褐黑素均可通过 UVA 降解，游离/总 PTCA 和 TTCA/4-AHP 比值可作为光降解的敏感指标。
