Extraction and Purification of a Melanin from Aureobasidium melanogenum with Evaluation of Its Stability and In Vitro Antitumor Activity

Microbial melanins are natural pigments with broad application potential, and Aureobasidium melanogenum is a promising strain for industrial production of melanin. However, the separation, purification, and biological activities of melanin derived from this strain remain largely unexplored. In this study, an efficient extraction and purification process for melanin from the fermentation broth of A. melanogenum GXZ-6 was established through single-factor optimization, and the final purity of the melanin reached 94.4%. Spectroscopic stability tests showed that the melanin was highly stable under dark, natural light, a wide pH range (2–12), and temperatures ≤ 37 °C. However, UV light, high temperature (>50 °C), ultrasonication, and certain metal ions (especially Cr3+) markedly altered its absorbance at 225 nm. More importantly, the purified melanin exhibited concentration-dependent antiproliferative activity against human cancer cell lines of A549, HCT116, MCF-7, and HepG2, with IC50 values ranging from 25.50 to 72.33 μg/mL. HepG2 cells were the most sensitive (IC50 = 25.50 μg/mL), and the melanin showed moderate selectivity toward normal hepatocytes (LO2). Mechanistic studies revealed that melanin effectively inhibited HepG2 cell migration, induced apoptosis, depolarized the mitochondrial membrane potential, and activated the caspase-3/PARP pathway. In summary, this study provides a simple, scalable, and environmentally friendly process for producing high-purity melanin with potential antitumor activity.