title: Melatonin: a hormone, a tissue factor, an autocoid, a paracoid, and an antioxidant vitamin.
abstract: Melatonin, a derivative of an essential amino acid, tryptophan, was first identified in bovine pineal tissue and subsequently it has been portrayed exclusively as a hormone. Recently accumulated evidence has challenged this concept. Melatonin is present in the earliest life forms and is found in all organisms including bacteria, algae, fungi, plants, insects, and vertebrates including humans. Several characteristics of melatonin distinguish it from a classic hormone such as its direct, non-receptor-mediated free radical scavenging activity. As melatonin is also ingested in foodstuffs such as vegetables, fruits, rice, wheat and herbal medicines, from the nutritional point of view, melatonin can also be classified as a vitamin. It seems likely that melatonin initially evolved as an antioxidant, becoming a vitamin in the food chain, and in multicellular organisms, where it is produced, it has acquired autocoid, paracoid and hormonal properties.
authors.affiliation: Department of Cellular and Structural Biology, The University of Texas Health Science Center at San Antonio, San Antonio, TX 78229-3900, USA.
authors.forename: Dun-Xian
authors.initials: DX
authors.lastname: Tan
authors.forename: Lucien C
authors.initials: LC
authors.lastname: Manchester
authors.forename: Rüdiger
authors.initials: R
authors.lastname: Hardeland
authors.forename: Silvia
authors.initials: S
authors.lastname: Lopez-Burillo
authors.forename: Juan C
authors.initials: JC
authors.lastname: Mayo
authors.forename: Rosa M
authors.initials: RM
authors.lastname: Sainz
authors.forename: Russel J
authors.initials: RJ
authors.lastname: Reiter
mesh_terms: D000818:Animals; D006801:Humans; D008550:Melatonin / Q000502:physiology*; D010870:Pineal Gland / Q000378:metabolism; D011956:Receptors, Cell Surface / Q000378:metabolism; D018160:Receptors, Cytoplasmic and Nuclear / Q000378:metabolism; D044094:Receptors, Melatonin
chemical_list: D011956:Receptors, Cell Surface; D018160:Receptors, Cytoplasmic and Nuclear; D044094:Receptors, Melatonin; D008550:Melatonin