Roles of reactive oxygen species in UVA-induced oxidation of 5,6-dihydroxyindole-2-carboxylic acid-melanin as studied by differential spectrophotometric method

• Published in: Pigment cell and melanoma research Vol. 29; no. 3; pp. 340 - 351
• Main Authors: Ito, Shosuke, Kikuta, Marina, Koike, Shota, Szewczyk, Grzegorz, Sarna, Michal, Zadlo, Andrzej, Sarna, Tadeusz, Wakamatsu, Kazumasa
• Format: Journal Article
• Language: English
• Published: England Blackwell Publishing Ltd 01-05-2016; Wiley Subscription Services, Inc
• Subjects: 5,6‐dihydroxyindole‐2‐carboxylic acid; 6-dihydroxyindole-2-carboxylic acid; Acids; Animals; Ascorbic Acid - pharmacology; Borohydrides - pharmacology; Cattle; eumelanin; Hydrogen Peroxide - pharmacology; Indoles - chemistry; Indoles - radiation effects; Luminescence; Melanins - chemistry; Melanins - radiation effects; Oxidation; Oxidation-Reduction - drug effects; Oxidation-Reduction - radiation effects; Oxygen; reactive oxygen species; Reactive Oxygen Species - metabolism; singlet oxygen; Singlet Oxygen - metabolism; Spectrophotometry - methods; Spectroscopy, Near-Infrared; Time Factors; Ultraviolet Rays; UVA; UVA; singlet oxygen; 5,6-dihydroxyindole-2-carboxylic acid; reactive oxygen species; eumelanin
• ISSN: 1755-1471; 1755-148X
• DOI: 10.1111/pcmr.12469

Summary
Eumelanin photoprotects pigmented tissues from ultraviolet (UV) damage. However, UVA‐induced tanning seems to result from the photooxidation of preexisting melanin and does not contribute to photoprotection. We investigated the mechanism of UVA‐induced degradation of 5,6‐dihydroxyindole‐2‐carboxylic acid (DHICA)‐melanin taking advantage of its solubility in a neutral buffer and using a differential spectrophotometric method to detect subtle changes in its structure. Our methodology is suitable for examining the effects of various agents that interact with reactive oxygen species (ROS) to determine how ROS is involved in the UVA‐induced oxidative modifications. The results show that UVA radiation induces the oxidation of DHICA to indole‐5,6‐quinone‐2‐carboxylic acid in eumelanin, which is then cleaved to form a photodegraded, pyrrolic moiety and finally to form free pyrrole‐2,3,5‐tricarboxylic acid. The possible involvement of superoxide radical and singlet oxygen in the oxidation was suggested. The generation and quenching of singlet oxygen by DHICA‐melanin was confirmed by direct measurements of singlet oxygen phosphorescence.