The catabolism of melatonin, whether naturally occurring or ingested, takes place via two pathways: ∼ 70 % can be accounted for by conjugation (sulpho-and glucurono-conjugation), and ∼ 30 % by oxidation. It is commonly thought that the interferoninduced enzyme indoleamine 2,3-dioxygenase (EC 1.13.11.42), which oxidizes tryptophan, is also responsible for the oxidation of 5-hydroxytryptamine (serotonin) and its derivative, melatonin. Using the recombinant enzyme expressed in Escherichia coli, we show in the present work that indoleamine 2,3-dioxygenase indeed cleaves tryptophan; however, under the same conditions, it is incapable of cleaving the two other indoleamines. By contrast, myeloperoxidase (EC 1.11.1.7) is capable of cleaving the indole moiety of melatonin. However, when using the peroxidase conditions of assay -with H 2 O 2 as co-substrate -indoleamine 2,3-dioxygenase is able to cleave melatonin into its main metabolite, a kynurenine derivative. The present work establishes that the oxidative metabolism of melatonin is due, in the presence of H 2 O 2 , to the activities of both myeloperoxidase and indoleamine 2,3-dioxygenase (with lower potency), since both enzymes have K m values for melatonin in the micromolar range. Under these conditions, several indolic compounds can be cleaved by both enzymes, such as tryptamine and 5-hydroxytryptamine. Furthermore, melatonin metabolism results in a kynurenine derivative, the pharmacological action of which remains to be studied, and could amplify the mechanisms of action of melatonin.

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This is the first study showing that chronic treatment with an iNOS inhibitor, L-NIL, limits progression of preexisting atherosclerosis in hypercholesterolemic rabbits. Increased intimal collagen accumulation may participate in iNOS-induced atherosclerosis progression.