Influence of Nitrative Stress on the Synthesis of Neuromelanin Model Systems

Neuromelanin is a dark pigment present in the human brain and involved in the onset of Parkinson's disease. Since natural pigment can only be extracted from the human brain in very small quantities, synthetic models of neuromelanin have been developed in recent years for research purposes, consisting of melanin conjugates and proteins made up of dopamine and β-lactoglobulin. Here, we studied the influence of nitrative stress on the synthesis of neuromelanin models, as it is known that reactive nitrogen species are present in vivo under pathophysiological conditions. HPLC-MS/MS analysis and ¹H NMR and UV-vis absorption spectra show that nitration on the protein component does not affect the conjugate synthesis, while increasing nitration on the melanic component, by replacing dopamine with its nitrate derivative, 6-nitrodopamine, gradually inhibits the melanization. Moreover, although 6-nitrodopamine is not able to trigger melanization alone, in the presence of dopamine, it is incorporated into the conjugate. This study represents a step forward toward the synthesis of models that are increasingly similar to human neuromelanin, considering the influence of nitrative stress on neuromelanin pigment properties and biosynthesis.