Melatonin (N-acetyl-5-methoxytryptamine) is a hormone of the pineal gland, a part of the diencephalon. It is produced by the pinealocytes in the pineal gland. Melatonin promotes sleep and controls the day-night rhythm.
Synthesis
Melatonin is produced from the essential amino acid tryptophan via the intermediate serotonin. Synthesis proceeds as follows:
- L-tryptophan is converted to 5-hydroxytryptophan and finally to 5-hydroxytryptamine (serotonin) with the help of tryptophan hydroxylase. Important cofactors here are the vitamins B6 and B3 and magnesium.
- Serotonin is N-acetylated with acetyl coenzyme A and N-acetylserotonin is formed (catalyst is the enzyme serotonin N-acetyltransferase (AANAT)).
- N-acetylserotonin is methylated with S-adenosylmethionine by acetylserotonin O-methyltransferase to form melatonin.
Melatonin is synthesized only at night with the onset of darkness. The maximum of formation is reached between 2:00 and 4:00, after which it falls again. Daylight reaching the eye inhibits melatonin secretion. This is especially true of morning light, which has the highest blue light content. During the course of the day, the blue light content continuously decreases and the melatonin level slowly builds up towards the evening. Melatonin induces deep sleep and is a stimulus for the release of the growth hormone somatotropic hormone (STH). Melatonin production is controlled by the circadian clock and especially by ambient light (sunshine, indoor lighting). Furthermore, there is evidence that melatonin levels are reduced by caffeine, alcohol, tobacco consumption as well as stress and obesity.Nocturnal production ranges from 10 µg to 80 µg of melatonin. The concentration of melatonin is age-dependent. Infants have the highest concentration. After that, melatonin production decreases continuously. Therefore, the average sleep duration decreases with age and sleep problems occur more frequently. In addition to the pineal gland, melatonin is produced in small amounts in the gastrointestinal tract (gastrointestinal tract) and in the retina (retina of the eye).
Absorption
Melatonin taken orally can be 100% absorbed by adults. With increasing age, the absorption rate decreases to 50%. If melatonin is taken in combination with, for example, an evening meal, the absorption rate is delayed. The bioavailability of melatonin is 15%.
Transport and distribution
Synthesized melatonin is immediately released and circulates in the bloodstream. Plasma melatonin levels correlate with the amount produced in the pineal gland. In saliva, 40% of the melatonin present can be measured. The sleep-promoting effect results from the binding of melatonin to the MT1 and MT2 receptors. Melatonin is rapidly metabolized in the liver. The half-life is only 10 to 60 minutes. Excretion occurs through the urine. The metabolite 6-sulphatoxymelatonin (6-SMT) measured here corresponds to the serum melatonin level and is used to determine melatonin levels.
