title: Synthesis and structural characterization of soluble neuromelanin analogs provides important clues to its biosynthesis.
abstract: Elucidating the structure and biosynthesis of neuromelanin (NM) would be an important step towards understanding its putative role in the pathogenesis of Parkinson's disease. A useful complement to studies aimed at unraveling the origin and properties of this essentially insoluble natural substance is the preparation of synthetic derivatives that resemble NM. With this aim in mind, water-soluble conjugates between dopamine-derived melanin and bovine serum albumin (BSA) were synthesized. Melanin-BSA adducts were prepared with both eumelanic oligomers obtained through the oxidative polymerization of dopamine and pheomelanic oligomers obtained under the same conditions from dopamine and cysteine. Iron ions were added during the synthesis to understand the interaction between the pigment and this metal ion, as the NM in neurons in several human brain regions contains significant amounts of iron. The structures of the conjugates were analyzed by (1)H NMR spectroscopy and controlled proteolysis/MS experiments. The binding of iron(III) ions was evaluated by ICP analysis and EPR spectroscopy. The EPR signal from bound iron(III) indicated high-spin octahedral sites and, as also seen for NM, the signal is coupled to a signal from a radical associated with the melanic components of the conjugates. However, the intensity of the EPR signal from iron suggested a reduced fraction of the total iron, indicating that most of the iron is strongly coupled in clusters within the matrix. The amount of paramagnetic, mononuclear iron(III) was greater in the pheomelanin-BSA conjugates, suggesting that iron clustering is reduced in the sulfur-containing pigment. Thus, the melanin-BSA conjugates appear to be good models for the natural pigment.
authors.affiliation: Dipartimento di Chimica, Università degli Studi di Pavia, Via Taramelli 12, 27100 Pavia, Italy.
authors.forename: Emanuele
authors.initials: E
authors.lastname: Ferrari
authors.forename: Mireille
authors.initials: M
authors.lastname: Engelen
authors.forename: Enrico
authors.initials: E
authors.lastname: Monzani
authors.forename: Michela
authors.initials: M
authors.lastname: Sturini
authors.forename: Stefania
authors.initials: S
authors.lastname: Girotto
authors.forename: Luigi
authors.initials: L
authors.lastname: Bubacco
authors.forename: Luigi
authors.initials: L
authors.lastname: Zecca
authors.forename: Luigi
authors.initials: L
authors.lastname: Casella
mesh_terms: D000818:Animals; D002417:Cattle; D060326:Chemistry Techniques, Synthetic; D006801:Humans; D057927:Hydrophobic and Hydrophilic Interactions; D007501:Iron / Q000378:metabolism; D008543:Melanins / Q000096:biosynthesis / Q000138:chemical synthesis* / Q000737:chemistry* / Q000378:metabolism; D008958:Models, Molecular; D011487:Protein Conformation; D012710:Serum Albumin, Bovine / Q000737:chemistry / Q000378:metabolism; D012995:Solubility
chemical_list: D008543:Melanins; C014121:neuromelanin; D012710:Serum Albumin, Bovine; D007501:Iron