Interactions of 5-hydroxytryptamine with oxidative enzymes.

Peroxidase (EC 1.11.1.7)/H2O2, ceruloplasmin (human type X)/O2, and tyrosinase (EC 1.14.18.1)/O2 all oxidized the indolic neurotransmitter 5-hydroxytryptamine (5-HT) in the physiological pH domain. Peroxidase/H2O2 oxidized 5-HT at pH values down to about 2.5. All oxidation reactions generated complex mixtures of products which included at least one known neurotoxin, tryptamine-4,5-dione. In general, the enzymatic oxidation pathways paralleled the in vitro electrochemical oxidation of 5-HT which has permitted suggestions to be made concerning the probable mechanisms of the enzyme-mediated reactions.