Tryptophan (Essential amino acid)
Tryptophan is an α-amino acid that is used in the biosynthesis of proteins. It contains an α-amino group, an α-carboxylic acid group, and a side chain indole, making it a non-polar aromatic amino acid. It is essential in humans, meaning the body cannot synthesize it: it must be obtained from the diet. Tryptophan is also a precursor to the neurotransmitter serotonin and the hormone melatonin.
L-tryptophan is a precursor of serotonin. Serotonin is a brain neurotransmitter needed for sleep.
Amino acids, including tryptophan, are used as building blocks in protein biosynthesis, and proteins are required to sustain life. Many animals (including humans) cannot synthesize tryptophan: they need to obtain it through their diet, making it an essential amino acid. Tryptophan is among the less common amino acids found in proteins, but it plays important structural or functional roles whenever it occurs. For instance, tryptophan and tyrosine residues play special roles in "anchoring" membrane proteins within the cell membrane. In addition, tryptophan functions as a biochemical precursor for the following compounds:
L-tryptophan is a precursor of serotonin (hence its use as an antidepressant and sleep aid). In this process, tryptophan is converted to 5-hydroxy-tryptophan (5-HTP), 5-HTP in turn is converted to serotonin.
Serotonin is a brain neurotransmitter, platelet clotting factor and neuro-hormone found in organs throughout the body. It is essential in regulating appetite, sleep, mood, and pain. It is also a natural sedative.
Metabolism of tryptophan to serotonin requires nutrients such as vitamin B6, niacin and glutathione. Serotonin is then in turn used to synthesize melatonin (a neurohormone), via N-acetyltransferase and 5-hydroxyindole-O-methyltransferase enzymes.
Niacin, also known as vitamin B3, is synthesized from tryptophan via kynurenine and quinolinic acids.
Auxins (a class of phytohormones) are synthesized from tryptophan.
L-tryptophan is an essential amino acid that is necessary for normal growth in infants and for NITROGEN balance in adults.
Poultry is a rich source of tryptophan.
Tryptophan is present in most protein-based foods or dietary proteins. It is particularly plentiful in chocolate, oats, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, almonds, sunflower seeds, pumpkin seeds, buckwheat, spirulina, and peanuts. Contrary to the popular belief that turkey contains an abundance of tryptophan, the tryptophan content in turkey is typical of poultry.
Tryptophan deficiency can cause depression, mental retardation, and dementia.
High corn or other tryptophan-deficient diets can cause pellagra, which is a niacin-tryptophan deficiency disease with symptoms of dermatitis, diarrhea and dementia.
Inborn errors of tryptophan metabolism exist where a tumor (carcinoid) makes excess serotonin. Hartnup's disease is a disease where tryptophan and other amino acids are not absorbed properly. Tryptophan supplements may be useful in each condition, in carcinoid replacing the over-metabolized nutrient and in Hartnup's supplementing a malabsorbed nutrient.
Assessment of tryptophan deficiency is done through studying excretion of tryptophan metabolites in the urine or blood. Blood may be the most sensitive test because the amino acid tryptophan is transported in a unique way. Increased urination of tryptophan fragments correlates with increased tryptophan degradation, which occurs with oral contraception, depression, mental retardation, hypertension and anxiety states.
Potential side effects of tryptophan supplementation include nausea, diarrhea, drowsiness, lightheadedness, headache, dry mouth, blurred vision, sedation, euphoria, and involuntary eye movements.
Tryptophan taken as a dietary supplement (such as in tablet form) has the potential to cause serotonin syndrome when combined with antidepressants of the MAOI or SSRI class or other strongly serotonergic drugs. Because tryptophan supplementation has not been thoroughly studied in a clinical setting, its interactions with other drugs are not well known.
Potential side effects of tryptophan supplementation include nausea, diarrhea, drowsiness, lightheadedness, headache, dry mouth, blurred vision, sedation, euphoria, and nystagmus (involuntary eye movements).
Some disorders of excess tryptophan in the blood may contribute to mental retardation.
The requirement for tryptophan and protein decreases with age. Adults' minimum daily requirement is 3 mg/kg/day or about 200 mg a day. This may be an underestimation, for there are 400 mg of tryptophan in just a cup of wheat germ. A cup of low fat cottage cheese contains 300 mg of tryptophan and chicken and turkey contain up to 600 mg per pound.
In 2002, the U.S. Institute of Medicine set a Recommended Dietary Allowance (RDA) of 5 mg/kg body weight/day of Tryptophan for adults 19 years and over.