This study characterized the melanin complex from mycelial forms of Fonsecaea pedrosoi, a causative agent of chromoblastomycosis. Researchers isolated and purified the dark pigment using alkaline extraction and analyzed its chemical composition through multiple analytical techniques. Electron microscopy revealed that melanin accumulates primarily on large intracellular electron-dense bodies called melanosomes and, to a lesser extent, on the cell wall surface. The melanin complex contained both aromatic and aliphatic/glycosidic structures, with predominance of the latter. Chemical analysis identified carbohydrates (rhamnose, mannose, galactose, and glucose), proteins (with acidic and neutral amino acids like aspartic acid, leucine, and alanine), and lipids (saturated and unsaturated fatty acids, predominantly palmitic and oleic acids). The pigment showed infrared absorption bands indicating hydroxyl, carbonyl, and carboxyl groups. Notably, non-pigmented fungal cells lacked the accumulation of these acid-insoluble melanin precursors, indicating melanin production is a specific biosynthetic product of pigmented cells. The findings suggest melanin contributes to fungal virulence through protection against immune destruction and may facilitate host-pathogen interactions.

Key findings

• Melanin in F. pedrosoi is primarily compartmentalized in intracellular melanosomes rather than solely on the cell wall, similar to melanin organization in animal cells
• The melanin complex comprises carbohydrates (10% total, containing rhamnose, mannose, galactose, and glucose), proteins (major amino acids: aspartic acid, leucine, glycine, alanine), and fatty acids (C16-C18, predominantly saturated)
• Non-pigmented cells failed to accumulate acid-insoluble melanin precursors, confirming melanin is a specific biosynthetic product of pigmented cells
• Infrared spectroscopy revealed both aromatic and aliphatic/glycosidic structures with predominance of the latter, including carbonyl groups naturally present in the pigment