Melatonin Derived from Serotonin

Melatonin, the sleep-regulating hormone, is biosynthesized directly from serotonin in the pineal gland. The correct answer to the multiple-choice question is option (3) Serotonin.

Correct Answer

Melatonin is derived from Serotonin (Option 3). This occurs through a two-step enzymatic process where serotonin is first acetylated by arylalkylamine N-acetyltransferase (AANAT) to form N-acetylserotonin, then methylated by hydroxyindole-O-methyltransferase (HIOMT) to produce melatonin.

Option Explanations

Epinephrine (Option 1)

Epinephrine (adrenaline) is a catecholamine hormone derived from tyrosine via dopamine and norepinephrine, synthesized in the adrenal medulla. It plays no role in melatonin production and instead functions in the fight-or-flight response.​

Nor-epinephrine (Option 2)

Nor-epinephrine (norepinephrine) is also a catecholamine from tyrosine, formed by hydroxylation of dopamine using dopamine β-hydroxylase. While it regulates pineal activity by stimulating AANAT indirectly, it is not a precursor to melatonin.

Serotonin (Option 3)

Serotonin (5-HT), derived from tryptophan, is the direct precursor to melatonin in pinealocytes. The pathway—tryptophan → 5-HT → N-acetylserotonin → melatonin—is well-established and rate-limited by AANAT activity during darkness.​

Histamine (Option 4)

Histamine derives from histidine via decarboxylation and acts as a neurotransmitter in allergic responses and gastric acid secretion. It has no biosynthetic connection to melatonin.​

Biosynthesis Pathway

The melatonin synthesis pathway starts with tryptophan uptake, converted to serotonin by tryptophan hydroxylase, followed by the two key steps mentioned. This process is circadian-regulated, peaking at night to align sleep-wake cycles.​