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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 to niacin. Hartnup disease is an inborn, autosomal
recessive, metabolic disorder involving amino acid tryptophan
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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.
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ryptophan --> 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.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 |
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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 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 syndrome (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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