Influence of peroxidase on the oligomerization of 5,6- and 5,7-dihydroxytryptamine investigated by matrix-assisted laser desorption/ionization mass spectrometry

The influence of peroxidase in the melanogenesis of 5,6- and 5,7-dihydroxytryptamines (DHT) was investigated by matrix-assisted laser desorption/ionization mass spectrometry. Incubation was carried out with peroxidase, in the presence or absence of hydrogen peroxide. Samples were withdrawn at various times, ultrafiltered to remove the enzyme and immediately lyophilized. Reaction of the two isomers with peroxidase alone revealed processes similar to those observed in the case of reaction with tyrosinase, leading to low molecular mass oligomers. Reaction with peroxidase and hydrogen peroxide showed very different reactivity of the two isomers. 5,6-DHT immediately led to the formation of a black precipitate, probably due to the reaction of its oxidation products with the enzyme. In contrast, 5,7-DHT formed oligomers, up to nonamers, originating from the molecule itself and from its oxidation products. Both isomers showed the same chemical reactivity with respect to hydrogen peroxide alone, with the formation of up to nonamer species from both original molecules and their oxidation products. (C) 1998 John Wiley & Sons, Ltd.