Tryptophan hydroxylase catalyzes the first rate-limiting step of serotonin biosynthesis, converting l-tryptophan to 5-hydroxytryptophan (5-HTP). 5-HTP is then converted to serotonin.
Vitamin B6 is a cofactor for the endogenous conversion of 5-HTP to serotonin via the enzyme 5-HTP decarboxylase. Serotonin controls a variety of functions in the human body and brain.
Inadequate serotonin is believed to play a role in depression and mood disorders. Serotonin is also converted to melatonin in the pineal gland, where it plays an important role in the human sleep cycle (circadian rhythm).