Isolation, Characterization, and Antioxidant Activity of Melanin from Auricularia auricula (Agaricomycetes)

The cell wall of Auricularia auricula fruit bodies is extremely tough, making it difficult to dissolve the melanin using the traditional preparation method. To investigate the efficient preparation of melanin and its resistance to oxidative stress, this paper first used ultrasound-assisted alkaline cellulase to optimize the optimal wall-breaking parameters through a Box-Behnken design based on a single-factor experiment. After optimization, the yield of melanin from A. auricula reached 3.201 +/- 0.018%. Then, different types and different proportions of deep eutectic solvents (DES) were used for further extraction. When choline chloride and urea were selected and the ratio was 1:2, the melanin yield was up to 25.99% +/- 2.36%. Scanning electron microscope (SEM) images showed that the melanin was amorphous mass with no crystal structure. X-ray photoelectron spectroscopy (XPS) analysis revealed that the melanin was mainly composed of C (5.38%), O (15.69%) and N (30.29%), as was the typical composition of eumelanin. The melanin had a concentration-dependent relationship with both ABTS(+) and hydroxyl radical scavenging ability; at the concentration of 0.5 mg/mL, it significantly prolonged Caenorhabditis elegans survival under hydrogen peroxide and methyl viologen stress and increased the glutathione level and enzyme (total superoxide dismutase and catalase) activities in vivo compared with the negative control (P < 0.05), indicating that the melanin enhances oxidative stress resistance in C. elegans.