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What is Tryptophan ?
Tryptophan is one of the 20 amino
acid in the genetic code(codon UGG) that makes up a protein
structure. Only the L- stereoisomer is found in mammalian
proteins.D-isomer is occasionally found in natural materials(eg
in marine venom peptide contryphan). This amino acid was
isolated from milk protein casein in 1901. Tryptophan is an
aromatic amino acid with a chemical formula C11H12N2O2 , with
molecular weight of 204.23 g mol−1. and is abbreviated as trp.
Tryptophan is an essential amino acid.This type of amino acid
cannot be synthesized by the organism and therefore must be part
of its diet. Young adults require about 7 mg of this amino acid
per day per kg of body weight. Tryptophan has an indole ring
comprised of two fused rings and an NH group joined to a
methylene (-CH2-) group. The aromatic ring of tryptophan contain
delocalized p electrons that strongly absorb ultraviolet light(
280nm wavelength), a property exploited by researchers in the
characterization of proteins. The functional part of tryptophan
is the indole ring. Tryptophan is incorporated into proteins and
enzymes at the molar rate of 1.1% compared to other amino acids
making it the rarest amino acid found in proteins.
Dietary Sources of Tryptophan
Tryptophan, found as a component of dietary protein, is
particularly plentiful in chocolates, oats, bananas, dried
dates, yoghurt, milk, cottage cheese, meat, fish, turkey,
chicken, sesame, chickpeas, peanuts.
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Function
of Tryptophan
Tryptophan is a precursor for serotonin( a neurotransmitter),
melatonin(a neurohormone), and niacin(vitaminB3).
Precursor of Niacin
In people with low to moderate intake of vitaminB3, tryptophan
may be used by the liver to make the ratio of 60mg tryptophan to
one mg B3. Thus tryptophan is an amino acid which is a
provitamin of niacin. Pellagra which is characterized by high
sensitivity to sunlight, dermatitis, red skin lesions, insomnia,
dementia, results from a combined deficiency of niacin and
tryptophan. Long term deficiency leads to central nervous system
dysfunction manifested as confusion, apathy, disorientation, and
eventually coma and death. Protein, energy, riboflavin(vitamin
B2), vitamin B6, nutritional status and hormones affect
conversion of tryptophan |
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to niacin. Hartnup disease is an
inborn, autosomal recessive, metabolic disorder involving amino
acid tryptophan and is characterized by skin lesions,
coordination impairment, mild mental retardation,
gastrointestinal problems and central nervous system
abnormalities. In Hartnup disease it is believed that a
transport system in the kidney tubule that normally transports
tryptophan into the body is defective. As a result the
concentration of tryptophan increases in the urine and decreases
in the blood. Body is thus left with inadequate tryptophan
leading to deficiency in proteins and vitamin B3. Most of the
symptoms are caused by deficiency of vitaminB3.
Precursor of Serotonin
Tryptophan serves as a precursor of serotonin, a
neurotransmitter. Serotonergic neurons are known to modulate
mood, emotion, appetite, promote feelings of well being, calm,
relaxation, confidence. Serotonin is synthesized in 2 steps
involving a tetrahydrobiopterin dependent hydroxylation reaction
catalyzed by tryptophan hydroxylase and then a decarboxylation
catalyzed by L-amino acid decarboxylase. Increased uptake of
tryptophan in the diet will lead to an increased brain serotonin
content. . The initial step in serotonin synthesis is the
facilitated transport of the amino acid L-tryptophan from blood
into brain. . Certain other neutral amino acids, such as
phenylalanine, leucine and methionine, are transported into
brain by the same carrier. The entry of tryptophan into brain is
related not only to its concentration in blood but is also a
function of its concentration in relation to the concentrations
of other neutral amino acids. Consequently, lowering the dietary
intake of tryptophan while raising intake of the amino acids
that tryptophan competes with for transport into brain lowers
the content of 5-HTP in brain and changes certain behaviors
associated with 5-HTP function.
Direct administration of 5-HTP bypasses the conversion of L-tryptophan
into 5-HTP, does not require the presence of transport
molecules, easily crosses blood- brain barrier and effectively
increases CNS synthesis of serotonin. Therapeutic administration
of 5-HTP is effective in treating conditions such as depression,
insomnia, fibromyalgia.
Precursor of Melatonin
Melatonin is a neurohormone produced in the brain by the pineal
gland from the amino acid tryptophan. Melatonin controls the
body’s circadian rhythm an internal 24-hour time keeping system.
The biosynthesis of melatonin is initiated by the uptake of
tryptophan into pineal paranchymal cells. Serotonin
concentrations synthesized from tryptophan are higher in the
pineal than in any other than in any other organ or in any brain
region.They exhibit a striking diurnal rhythm remaining at a
maximum level during daylight hours and falling by more than 80%
soon after onset of darkness as serotonin is converted into
melatonin.
Tryptophan --> 5-Hydroxytryptophan (5-HTP) --> Serotonin -->
N-Acetyl-serotonin --> Melatonin.
Thus tryptophan acts as a natural sedative with a calming effect
in the brain and plays a role in sleep.
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L- tryptophan is normally found in
turkey meat and many people believe it to be the cause of
sleepiness that is commonly experienced after consuming it. L-tryptophan
may be found in turkey and other dietary proteins but it’s
actually a carbohydrate rich meal that increases the level of
this amino acid in the brain and leads to serotonin synthesis.
Carbohydrates stimulate the pancreas to secrete insulin when
some amino acid that compete with tryptophan leave the blood
stream and enter the muscle cells. This causes an increase in
the relative concentration of tryptophan in the bloodstream.Thus
eating carbohydrates along with protein can increase the level
of tryptophan available to the brain, promoting feelings of calm
and well-being. It has been found that in any real meal
comprising of a mixture of proteins and carbohydrates the
effects of proteins predominate which decreases the ratio of
tryptophan to other amino acids. A comprehensive way to increase
tryptophan concentration in the brain is with dietary |
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supplements. Taken in pure form
tryptophan works in a qualitatively different way than when obtained
from food source. Concentration of tryptophan becomes high enough to
compete against other amino acid, and plenty of tryptophan crosses
into the brain. In 1989, a large outbreak of a disabling, and in
some cases deadly autoimmune illness called eosinophilia-myalgia
synd rome (EMS) was traced to L-tryptophan. Eosinophilia-myalgia
syndrome (EMS) is an incurable and sometimes fatal flu-like
neurological condition that is believed to have been caused by
ingestion of L-tryptophan supplements. EMS is a rare,
multi-systemic, and chronic autoimmune disease. Eosinophilia is an
elevated level of a type of white blood cell called an eosinophil.
Myalgia refers to muscle pain. EMS is a painful and progressive,
multi-system disease which causes permanent scarring and fibrosis to
nerve and muscle tissues, continuing inflammation, and provokes a
permanent change in the body's immune system. Eosinophilia-myalgia
syndrome was first recognized after an outbreak of the condition in
1989. Epidemologists first traced the cause to consumption of a
single brand of tryptophan supplement sold as sleep aid.
There was some evidence that new batches of this brand had been
improperly prepared which allowed contamination with chemical
impurities. Although the precise etiological agent remains unknown
evidence suggests that either a chemical contaminant or a metabolite
of L-tryptophan is responsible. Tryptophan is degraded to kynurenine
which may be metabolized to quinolinic acid an endogenous
neurotoxin. High performance liquid chromatography of EMS associated
L-tryptophan reveals several peaks that correspond to impurities.
Tryptophan has been implicated as a possible cause of schizophrenia
in people who cannot metabolize it properly. When improperly
metabolized, it creates a waste product in the brain that is toxic,
causing hallucinations and delusions. Tryptophan has also been
indicated as an aid for schizophrenic patients.As an essential amino
acid dietary deficiency of tryptophan may cause symptoms
characteristic of protein deficiency which includes weight loss and
impaired growth in infants and children. High dietary intake of
tryptophan from food sources is not known to cause any symptoms of
toxicity.
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Article Contributed By: Shaonli
Dasgupta
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