Roles of reactive oxygen species in <scp>UVA</scp>‐induced oxidation of 5,6‐dihydroxyindole‐2‐carboxylic acid‐melanin as studied by differential spectrophotometric method

<jats:title>Summary</jats:title><jats:p>Eumelanin photoprotects pigmented tissues from ultraviolet (<jats:styled-content style="fixed-case">UV</jats:styled-content>) damage. However, <jats:styled-content style="fixed-case">UVA</jats:styled-content>‐induced tanning seems to result from the photooxidation of preexisting melanin and does not contribute to photoprotection. We investigated the mechanism of <jats:styled-content style="fixed-case">UVA</jats:styled-content>‐induced degradation of 5,6‐dihydroxyindole‐2‐carboxylic acid (<jats:styled-content style="fixed-case">DHICA</jats:styled-content>)‐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 (<jats:styled-content style="fixed-case">ROS</jats:styled-content>) to determine how <jats:styled-content style="fixed-case">ROS</jats:styled-content> is involved in the <jats:styled-content style="fixed-case">UVA</jats:styled-content>‐induced oxidative modifications. The results show that <jats:styled-content style="fixed-case">UVA</jats:styled-content> radiation induces the oxidation of <jats:styled-content style="fixed-case">DHICA</jats:styled-content> 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 <jats:styled-content style="fixed-case">DHICA</jats:styled-content>‐melanin was confirmed by direct measurements of singlet oxygen phosphorescence.</jats:p>