Abstract
Melanin is a pigment found in all biological kingdoms, and plays a key role in protection against ultraviolet radiation, oxidizing agents, and ionizing radiation damage. Melanin exerts an antimicrobial activity against bacteria, fungi, and parasites. We demonstrated an antifungal activity of synthetic and human melanin against Candida sp. The members of the Cryptococcus neoformans and C. gattii species complexes are capsulated yeasts, which cause cryptococcosis. For both species melanin is an important virulence factor. To evaluate if cryptococcal and human melanins have antifungal activity against Cryptococcus species they both were assayed for their antifungal properties and physico-chemical characters. Melanin extracts from human hair and different strains of C. neoformans (n = 4) and C. gattii (n = 4) were investigated. The following minimum inhibitory concentrations were found for different melanins against C. neoformans and C. gattii were (average/range): 13.7/(7.8–15.6) and 19.5/(15.6–31.2) μg/mL, respectively, for human melanin; 273.4/(125–>500) and 367.2/(125.5–>500) μg/mL for C. neoformans melanin and 125/(62.5–250) and 156.2/(62–250) μg/mL for C. gattii melanin. Using Scanning Electron Microscopy we observed that human melanin showed a compact conformation and cryptococcal melanins exposed an amorphous conformation. Infrared spectroscopy (FTIR) showed some differences in the signals related to C-C bonds of the aromatic ring of the melanin monomers. High Performance Liquid Chromatography established differences in the chromatograms of fungal melanins extracts in comparison with human and synthetic melanin, particularly in the retention time of the main compound of fungal melanin extracts and also in the presence of minor unknown compounds. On the other hand, MALDI-TOF-MS analysis showed slight differences in the spectra, specifically the presence of a minor intensity ion in synthetic and human melanin, as well as in some fungal melanin extracts. We conclude that human melanin is more active than the two fungal melanins against Cryptococcus. Although some physico-chemical differences were found, they do not explain the differences in the antifungal activity against Cryptococcus of human and cryptococcal melanins. More detailed studies on the structure should be considered to associate structure and antifungal activity.
Highlights
Melanin is a pigment that can be found in all biological kingdoms, largely recognized for play an important role in the protection against ultraviolet light (UV), oxidizing agents and ionizing radiation (Eisenman and Casadevall, 2012)
minimum inhibitory concentrations (MICs) values observed against C. gattii were: 19.5 (15.6–31.2) μg/mL for human melanin; 367.2 μg/mL (125 – >500) for de C. neoformans melanin; 156.2 (62–250) μg/mL for C. gattii melanin; and 17.4 (3.9–31.2) μg/mL for synthetic melanin (Figure 2B)
We tested the antimicrobial activity of the melanin extracts against eight clinical strains of C. neoformans and observed MIC values corresponding to 31.4 μg/mL (15.6–62.5) for synthetic melanin; 15.6 μg/mL (7.8– 31.2) for human melanin, and 187.5 μg/mL (125–250) for cryptococcal melanin VG III WM175
Summary
Melanin is a pigment that can be found in all biological kingdoms, largely recognized for play an important role in the protection against ultraviolet light (UV), oxidizing agents and ionizing radiation (Eisenman and Casadevall, 2012). It participates in other important functions, including thermoregulation in lower vertebrates, camouflage, and sexual attraction in some species (Ito and Wakamatsu, 2003). The human melanin synthesis uses distinct intermediate products and enzymes (Figures 1A,B), which suggests that melanin may possess a different structure; resulting in various activities to the organisms it originated
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