We recently described that horseradish peroxidase (HRP) and myeloperoxidase (MPO) catalyze the oxidation of melatonin, forming the respective indole ring-opening product N-1-acetyI-N-2-formyl-5-methoxy-kynuramine (AFMK) (Biochem Biophys. Res. Commun. 279, 657-662, 2001). Although the classic peroxidatic enzyme cycle is expected to participate in the oxidation of melatonin, the requirement of a low HRP:H2O2 ratio suggested that other enzyme paths might also be operative. Here we followed the formation of AFMK under two experimental conditions: predominance of HRP compounds I and II or presence of compound III. Although the consumption of substrate is comparable under both conditions, AFMK is formed in significant amounts only when compound III predominates during the reaction. Using tryptophan as substrate, N-formyl-kynurenine is formed in the presence of compound III. Both, melatonin and tryptophan efficiently prevents the formation of p-670, the inactive form of HRP. Since superoxide dismutase (SOD) inhibits the production of AFMK, we proposed that compound III acts as a source of O-2(-.) or participates directly in the reaction, as in the case of enzyme indoleamine 2,3-dioxygenase.