Decolourization of natural and synthetic non-fungal melanins by white rot fungi

Skin lightening products have become increasingly popular in the past few years. Their applications include lightening or whitening of the skin as well as treatment of pigmentary disorders such as chloasma, freckles, pregnancy marks and age spots. In addition, decolourization of hair as preliminary step to dyeing is a most common practice in hair cosmetics. The major determinants of the colour of skin, hairs and eyes in man and other mammals are melanins [1]. These are heterogeneous polymers made up of 5,6-dihydroxyindoles units possibly at different oxidation states [1]. This structural feature is similar to lignin or coal, in which polymers are composed of indolic or phenolic subunits. White-rot fungi produce various extracellular oxidases and peroxidases, capable of oxidizing a broad spectrum of structurally different substrates, including lignin or coal as well as highly toxic compounds and azo-dye [2]. Such capabilities of lignin-degrading enzymes suggest their use in decolourisation of melanins. Previous studies were carried out using fungal melanins [3], and only a report has appeared on the bleaching of a synthetic melanin, but the preparation procedure and the substrate were not provided [4]. Very recently, Mohorcic and co-workers [5] published cosmetic applications of a bleaching enzyme of fungal origin. We report here the results of a study in which the two white-rot fungi Phanerochaete chrysosporium [6] and Pleurotus ostreatus and their lignin-degrading enzymes [7] were tested for efficiency in decolourization of melanin from Sepia officinalis, a widely accepted standard of natural melanins. The former fungus proved more effective than the latter in melanin decolourisation. In liquid cultures, the added melanin was initially adsorbed onto the fungal mycelium, and then the colour was smoothly discharged over ten to fifteen days from either the medium or the mycelium. Decolourization was due to extensive melanin degradation, as shown by HPLC identification in the culture broth of PTCA (pyrrole-2,3,5-tricarboxylic acid), the typical product of chemical degradation of eumelanin [8]. Yields of PTCA, obtained by the action of the fungus after 10 days of incubation, was at least 80-90% of those obtained after chemical degradation of the same melanin under oxidative alkaline conditions. The effects of melanin concentration on fungal efficiency of decolourization was also evaluated. Synthetic melanins prepared by oxidation of the main biosynthetic precursors 5,6-dihydroxyindole and its 2-carboxy derivative were also efficiently degraded by the fungus. Work is in progress to identify the enzyme(s) involved in melanin degradation and its cellular localization.