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