Scheme 1.

The hypothesized biosynthetic pathway of NM is schematically shown to proceed through four main phases. In the initial Phase 1, the oxidation products of DA, or the oxidized DA metabolites [22], generated by catalytic oxidation mediated by Fe³⁺, or Cu²⁺, react with fibrillary seeds of aggregated peptides/proteins (blue) at the exposed side chains of Cys, His and Lys forming melanin (orange)-protein conjugates maintaining the fibrillary core as the only structural element. In Phase 2, the undegradable conjugates are engulfed by the phagophore and isolated from the cytosol through a double lipid membrane (yellow). The autophagic vacuoles fuse with lysosomes (blue circle) in Phase 3, causing interaction between the melanin-protein conjugates with other proteins, enzymes and lipids carried by lysosomes, and are transformed into autolysosomes. The resulting autolysosomes fuses with other vesicles containing lipids that attach to melanin-protein conjugates to form NM. Finally, in Phase 4 these organelles fuse with other autophagic vacuoles containing NM precursors or with mature NM organelles. A more detailed description of the entire process can be found in Zucca et al., 2018 [14].