Application of matrix-assisted laser desorption/ionization mass spectrometry to the detection of melanins formed from Dopa and dopamine.

Abstract

Matrix-assisted laser desorption/ionization (MALDI) mass spectrometry was used to study melanogenesis starting from Dopa and dopamine, the latter considered one of the precursors of neuromelanins. These substrates were left to react with the peroxidase - H(2)O(2) system, which is postulated to play an important role in melanin biosynthesis. Samples were prepared by ultrafiltering the substrate - enzyme solution after 30, 60, 120, 240 and 360 min of reaction and aliquots were immediately lyophilized. The reaction of dopamine with peroxidase - H(2)O(2) favoured the formation of dopamine oligomers up to octamers. In contrast, the action of either peroxidase or H(2)O(2) alone, studied for comparison, did not lead to melanin production and only dimeric and trimeric species were observed. Also for Dopa, analogous results were obtained in the presence of either peroxidase or H(2)O(2) alone, without melanin formation. Conversely, Dopa with the peroxidase - H(2)O(2) system led to the formation of a black precipitate after 120 min of reaction, and oligomers of 5,6-dihydroxyindole (DHI), an intermediate of melanogenesis, were detected, together with products of further oxidation. Faster kinetics were observed when Dopa was treated with tyrosinase, the enzyme catalysing the oligomerization of tyrosine to melanins, leading to the formation mainly of DHI oligomers.