Amino acid of tryptophan

Tryptophan is a precursor for serotonin (a neurotransmitter), melatonin (a neurohormone), and niacin. 

Serotonin is a monoamine, and derived from tryptophan is found in the gastrointestinal tract, platelets and in the central nervous system. It is thought to be a contributor to feelings of well-being and happiness. 

Tryptophan is an amino acid from which a substance involved in the constriction of blood vessel is formed, and is present in components of blood involved in clotting.

Like tyrosine, tryptophan is a neutral amino acid that also gains entry into the brain by the large neutral amino acid transporter. Thus, plasma tryptophan will competed with other neutral main acids such as tyrosine and phenylalanine.

It means that the concentration of brain tryptophan will be determined not only by the concentration of tryptophan in plasma but also the plasma concentration of other neutral amino acids.

Tryptophan is the least abundant anion acid in most proteins, accounting on the average of 1 to 1.5% of the total amino acids in typical plant (1%) and animal (1.5%) proteins.

In human infants, the requirement for growth is roughly 12 to 40 mg/kg. In adult humans, the minimum daily requirement has been estimated to be 250 mg/d in males and 160 mg/d in females.
Amino acid of tryptophan