Occurrence and analysis of melatonin in food plants

In vertebrates, the essential amino acid l-tryptophan is the precursor of 5-methoxyindoleamines, or indoleamines/tryptamines, including melatonin (N-acetyl-5-methoxytryptamine), through the intermediate serotonin (5-HT, 5-hydroxytryptamine) and the activity of hydroxyindoleamine-O-methyltransferase (HIOMT) (Figure 29.1) (Axelrod and Weissbach 1961; Hardeland 2008). In mammals, melatonin is synthesized in the pineal gland, predominantly during the nighttime, though it can be also produced in other organs, such as retina, gastrointestinal tract, lymphocytes, and bone marrow cells. Conversely, light at night has an inhibitory effect on pineal melatonin biosynthesis which is initiated by the uptake of tryptophan from the circulation into pinealocytes (Reiter 1991; Hardeland 2008). Once synthesized, melatonin is not stored in the pineal cells, but it is released into the bloodstream with a circadian rhythm, from which it reaches other body fluids, including urine, saliva, cerebrospinal fluid, bile, semen, and amniotic fluid (Tan et al. 1999; Tamura et al. 2009). The circadian rhythm of melatonin secretion is generated by the biological clock, situated in the suprachiasmatic nucleus of the hypothalamus, via a neuronal pathway that begins in the retina and involves the retinohypothalamic tract (Reuss 2003). In mammals, melatonin acts in part via membrane receptors MT1 and MT2 (Dubocovich and Markowska 2005). Physiological processes regulated by melatonin presumably via a receptor-mediated mechanism include the control of the sleep/wake cycle, modulation of reproductive function, and bone metabolism (Reiter et al. 2007a). Apart from these receptor-mediated processes, melatonin and its metabolites exert a series of receptor-independent functions, mainly due to their powerful antioxidant activity (Tan et al. 1993; Peyrot and Ducrocq 2008; Hardeland et al. 2009). Melatonin can directly scavenge free radical species (both reactive oxygen and nitrogen species) and stimulate the activity of antioxidant enzymes (Reiter et al. 2009).