Melanosome degradation: fact or fiction.

Our mini review summarizes what is known about the (bio)degradation of melanosomes. Unlike melanosome biogenesis where our knowledge enables us to explain it in molecular terms posing many interesting questions on the relation between lysosomes and melanosomes, melanosome degradation has remained 'terra incognita'. Observations at optical and ultrastructural levels describe the disintegration of melanosomes in the lysosomal compartment (in auto- and heterophagosomes). Histochemical studies suggest the participation of acid hydrolases in the process of melanosome degradation. Biochemical data confirm the ability of lysosomal hydrolases to degrade melanosome constituents except the melanin moiety. The similarity of melanin structure to that of polycyclic aromatic hydrocarbons suggests that melanin should be sensitive mainly, if not exclusively, to oxidative breakdown. In vitro melanin can indeed be decomposed by an oxidative attack and the degradation is accompanied by fluorescence and decreasing absorbance. From enzymes engaged in the biotransformation of polycyclic hydrocarbons only phagosomal NADPH oxidase meets the criteria (particularly as for compartmental and catalytic properties) to be involved in melanin biodegradation. The in vivo biodegradation of melanin has so far been clearly demonstrated in Aspergillus and fungi melanins.