Melanogenesis from 5-hydroxytryptamine, 5,6- and 5,7-dihydroxytryptamines. An in vitro study using MALDI-TOF.

The role of tyrosinase and peroxidase in melanogenesis of 5-hydroxytryptamine, 5,6- and 5,7-dihydroxytryptamines was investigated by matrix-assisted laser desorption/ionization mass spectrometry. Each enzyme was incubated with the tryptamine derivatives and samples were drawn at various times, ultrafiltered and immediately lyophilized. The results indicated that peroxidase promotes oligomerization of 5-HT with fast kinetics but with yields lower than those achieved by tyrosinase. 5,6- and 5,7-DHT formed low molecular mass oligomers in the presence of peroxidase alone. The addition of hydrogen peroxide evidences different reactivity of the two isomers: 5,6-DHT formed immediately a black precipitate while oligomers of the molecule itself and of its oxidation products were detectable for 5,7-DHT.

Chemical and enzyme-mediated oxidation of the serotonergic neurotoxin 5,7-dihydroxytryptamine: mechanistic insights.

The oxidation chemistry and biochemistry of the serotonergic neurotoxin 5,7-dihydroxytryptamine (1) has been studied under anaerobic and aerobic conditions in aqueous solution at physiological pH. Under anaerobic conditions, one-electron oxidants (ferricytochrome c, peroxidase/H2O2, ceruloplasmin, Cu2+) generate a radical intermediate. Dimerization of the C(6)-centered resonance form of this radical followed by secondary oxidations yields 3-(2-aminoethyl)-6-[3-(2-aminoethyl)-1,7-dihydro- 5-hydroxy-7-oxo-6H-indol-6-ylidene]-1-H-indole-5,7(4H,6H)-dione. Under aerobic conditions, molecular O2 attacks the C(4)-centered 1 radical to yield a hydroperoxy radical which decomposes to 5-hydroxytryptamine-4,7-dione (2). Autoxidation of 1 proceeds by primary attack by molecular O2 on a C(4)-centered carbanion to form a superoxide-radical complex. This rearranges to a C(4)-centered hydroperoxide which decomposes to 2. A C(6)-centered carbanion of 1 combines with 2 to give, ultimately, 6,6'-bi-5-hydroxytryptamine-4,7-dione (3). Trace concentrations of transition metal ions (Fe3+, Fe2+, Cu2+, Mn2+) catalyze the autoxidation of 1 by catalytic cycles in which a hydroperoxide intermediate plays key roles. A byproduct of the transition metal-catalyzed oxidation of 1 is superoxide, O2-. Because of its enormous basicity O2- facilitates deprotonation of 1. The C(4)-centered carbanion so produced is oxidized by molecular O2 or by the hydroperoxy radical (HO2) to give radical intermediates and thence 2 and 3. Mechanistic pathways leading to the various products of oxidation of 1 are proposed and the potential roles of oxidation reactions of the indolamine are related to its neurodegenerative properties.

Mechanism of autoxidation of 5,7-dihydroxytryptamine: effect of fluorine substitution at positions 4 and/or 6.

Analogs of 5,7-dihydroxytryptamine (5,7-DHT), namely, 4-fluoro-, 6-fluoro-, and 4,6-difluoro-5,7-DHT's (30a-c) were synthesized starting from 4-fluorophenol (7a), 4-fluorobenzyl alcohol (12) and 2,4-difluorophenol (7b), respectively. Regiospecific hydroxylation and formylation ortho to fluoro groups, both via aryllithium intermediates, were made possible by the blocking effect of tert-butyldimethylsilyloxy functions and allowed the conversion of the starting materials to the key intermediates, namely, 3,5-bis(tert-butyldimethylsilyloxy)-2-fluoro-, 4-fluoro- and 2,4-difluorobenzaldehydes (11a, b and 19, respectively). The latter were converted in one step to the corresponding benzyloxybenzaldehydes, from which indole-2-carboxylates 22a-c were synthesized via azidostyrenes 21a-c, respectively. Decarbonylation of the indole-2-carboxaldehydes (24a-c) produced from 22a-c in two steps gave 2,3-unsubstituted indoles 25a-c, respectively. Introduction of the aminoethyl side chains on C-3 of 25a-c via the corresponding indole-3-acetonitriles, and subsequent debenzylation generated the hydroxytryptamines, which were isolated as their creatinine sulfate salts 30a-c, respectively. Cyclic voltammetric studies indicated that like 5,7-DHT, 30a-c undergo electrochemical oxidation in 1 M H2SO4 via the corresponding p-quinoneimine derivatives 31a-c by an electrochemical-chemical-electrochemical (ECE) process. The voltammetrically detectable products of the ECE process appear to be the corresponding 5-hydroxytryptamine-4,7-dione (6) derivatives 33a-c. The nature of the interaction of dissolved O2 with 30a-c at pH 7.4 appears to be strikingly different from that of 5,7-DHT, which undergoes autoxidation at pH 7.4 via the 4-hydroperoxy derivative 4 to the quinone 6.(ABSTRACT TRUNCATED AT 250 WORDS)