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For many organisms (including humans), tryptophan is an essential amino acid. This means that it cannot be synthesized by the organism and therefore must be part of its diet. Amino acids, including tryptophan, act as building blocks in protein biosynthesis. In addition, tryptophan functions as a biochemical precursor for the following compounds (see also figure to the right):

  • Serotonin (a neurotransmitter), synthesized via tryptophan hydroxylase.
  • Niacin is synthesized from tryptophan via kynurenine and quinolinic acids as key biosynthetic intermediates.
  • Auxin (a phytohormone) when sieve tube elements undergo apoptosis tryptophan is converted to auxins.

The disorders Fructose Malabsorption and Lactose intolerance causes improper absorption of tryptophan in the intestine, reduced levels of tryptophan in the blood

In bacteria that synthesize tryptophan, high cellular levels of this amino acid activate a repressor protein, which binds to the trp operon. Binding of this repressor to the tryptophan operon prevents transcription of downstream DNA that codes for the enzymes involved in the biosynthesis of tryptophan. So high levels of tryptophan prevent tryptophan synthesis through a negative feedback loop and, when the cell's tryptophan levels are reduced, transcription from the trp operon resumes. The genetic organisation of the trp operon thus permits tightly regulated and rapid responses to changes in the cell's internal and external tryptophan levels.

Dietary sourcesEdit

Tryptophan is a routine constituent of most protein-based foods or dietary proteins. It is particularly plentiful in chocolate, oats, durians, mangoes, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, sunflower seeds, pumpkin seeds, spirulina, and peanuts.[1] It is found in turkey at a level typical of poultry in general.[2]

Tryptophan (Trp) Content of Various Foods
Food Protein
[g/100 g of food]
Tryptophan
[g/100 g of food]
Tryptophan/Protein [%]
egg, white, dried
81.10
1.00
1.23
spirulina, dried
57.47
0.93
1.62
cod, atlantic, dried
62.82
0.70
1.11
soybeans, raw
36.49
0.59
1.62
cheese, Parmesan
37.90
0.56
1.47
caribou
29.77
0.46
1.55
sesame seed
17.00
0.37
2.17
cheese, cheddar
24.90
0.32
1.29
sunflower seed
17.20
0.30
1.74
pork, chop
19.27
0.25
1.27
turkey
21.89
0.24
1.11
chicken
20.85
0.24
1.14
beef
20.13
0.23
1.12
salmon
19.84
0.22
1.12
lamb, chop
18.33
0.21
1.17
perch, Atlantic
18.62
0.21
1.12
egg
12.58
0.17
1.33
wheat flour, white
10.33
0.13
1.23
milk
3.22
0.08
2.34
rice, white
7.13
0.08
1.16
potatoes, russet
2.14
0.02
0.84
banana
1.03
0.01
0.87

Use as a dietary supplementEdit

For some time, tryptophan has been available in health food stores as a dietary supplement. Many people found tryptophan to be a safe and reasonably effective sleep aid, probably due to its ability to increase brain levels of serotonin (a calming neurotransmitter when present in moderate levels)[1] and/or melatonin (a sleep-inducing hormone secreted by the pineal gland in response to darkness or low light levels). Some users of MDMA (street name "ecstasy") will eat tryptophan-containing foods to shorten the 'come down' effect of having lower levels of serotonin than usual (due to an extra large release caused by the drug).

Clinical research has shown mixed results with respect to tryptophan's effectiveness as a sleep aid, especially in normal patients and for a growing variety of other conditions typically associated with low serotonin levels or activity in the brain. In particular, tryptophan has been showing considerable promise as an antidepressant alone, and as an "augmenter" of antidepressant drugs. However, the reliability of these clinical trials has been questioned.

MetabolitesEdit

5-Hydroxytryptophan (5-HTP), a metabolite of tryptophan, has been suggested as a treatment for epilepsy= and depression, although clinical trials are regarded inconclusive and lacking.

Due to the conversion of 5-HTP into serotonin by the liver, there is a significant risk of heart valve disease from serotonin's effect on the heart.[1][2] In Europe, 5-HTP is prescribed with carbidopa to prevent the conversion of 5-HTP into serotonin until it reaches the brain.[3]

5-HTP readily crosses the blood-brain barrier and in addition is rapidly decarboxylated to serotonin (5-hydroxytryptamine or 5-HT)[4] and therefore may be useful for the treatment of depression. However serotonin has a relatively short half-life since it is rapidly metabolized by monoamine oxidase, and therefore is likely to have limited efficacy. It is marketed in Europe for depression and other indications under the brand names Cincofarm, Tript-OH and Optimax (UK).

In the United States, 5-HTP does not require a prescription, as it is covered under the Dietary Supplement Act. Since the quality of dietary supplements is now regulated by the U.S. Food and Drug Administration there is now a guarantee that the label accurately depicts what the bottle contains. [5]

Tryptophan supplements and EMS Edit

Although currently available for purchase, in 1989 a large outbreak (1,500 cases of permanent disability including at least thirty-seven deaths) of a disabling autoimmune illness called eosinophilia-myalgia syndrome (EMS) was traced by some epidemiological studies[1][2][3] to L-tryptophan supplied by a Japanese manufacturer, Showa Denko KK.[4] It was further hypothesized that one or more trace impurities produced during the manufacture of tryptophan may have been responsible for the EMS outbreak.[5][6] The fact that the Showa Denko facility used genetically engineered bacteria to produce L-tryptophan gave rise to speculation that genetic engineering was responsible for such impurities.[7][8] However, the methodology used in the initial epidemiological studies has been criticized.[9][10] An alternative explanation for the 1989 EMS outbreak is that large doses of tryptophan produce metabolites which inhibit the normal degradation of histamine and excess histamine in turn has been proposed to cause EMS.[11]

Most tryptophan was banned from sale in the US in 1991, and other countries followed suit. Tryptophan from one manufacturer, of six, continued to be sold for manufacture of baby formulas. At the time of the ban, the FDA did not know, or did not indicate, that EMS was caused by a contaminated batch,[12][13] and yet, even when the contamination was discovered and the purification process fixed, the FDA maintained that L-tryptophan was unsafe. In February 2001, the FDA loosened the restrictions on marketing (though not on importation), but still expressed the following concern:

   "Based on the scientific evidence that is available at the present time, we cannot determine with certainty that the occurrence of EMS in susceptible persons consuming L-tryptophan supplements derives from the content of L-tryptophan, an impurity contained in the L-tryptophan, or a combination of the two in association with other, as yet unknown, external factors."[4] 

Since 2002, L-tryptophan has been sold in the U.S. in its original form. Several high-quality sources of L-tryptophan do exist, and are sold in many of the largest health food stores nationwide. Indeed, tryptophan has continued to be used in clinical and experimental studies employing human patients and subjects.

In recent years in the U.S., compounding pharmacies and some mail-order supplement retailers have begun selling tryptophan to the general public. Tryptophan has also remained on the market as a prescription drug (Tryptan), which some psychiatrists continue to prescribe, particularly as an augmenting agent for people who are unresponsive to antidepressant drugs.Template:Fact

Turkey meat and drowsinessEdit

One belief is that heavy consumption of turkey meat (as for example in a Thanksgiving or Christmas feast) results in drowsiness, which has been attributed to high levels of tryptophan contained in turkey.[1][2][3] While turkey does contain high levels of tryptophan, the amount is comparable to that contained in most other meats.[4] Furthermore, postprandial Thanksgiving sedation may have more to do with what is consumed along with the turkey, in particular carbohydrates and alcohol, rather than the turkey itself.

It has been demonstrated in both animal models[5] and in humans[6][7][8] that ingestion of a meal rich in carbohydrates triggers release of insulin. Insulin in turn stimulates the uptake of large neutral branched-chain amino acids (LNAA) but not tryptophan (trp) into muscle, increasing the ratio of trp to LNAA in the blood stream. The resulting increased ratio of tryptophan to large neutral amino acids in the blood reduces competition at the large neutral amino acid transporter resulting in the uptake of tryptophan across the blood-brain barrier into the central nervous system (CNS).[9][10] Once inside the CNS, tryptophan is converted into serotonin in the raphe nuclei by the normal enzymatic pathway.[5][7] The resultant serotonin is further metabolised into melatonin by the pineal gland.[11] Hence, these data suggest that "feast-induced drowsiness," and in particular, the common post-Christmas and American post-Thanksgiving dinner drowsiness, may be the result of a heavy meal rich in carbohydrates which, via an indirect mechanism, increases the production of sleep-promoting melatonin in the brain.[5][6][7][8]

Fluorescence Edit

The fluorescence of a folded protein is a mixture of the fluorescence from individual aromatic residues. Most of the intrinsic fluorescence emissions of a folded protein are due to excitation of tryptophan residues, with some emissions due to tyrosine and phenylalanine; but be aware that di-sulfide bonds also have appreciable absorption in this wavelength range. Typically, tryptophan has a wavelength of maximum absorption of 280 nm and an emission peak that is solvatochromic, ranging from ca. 300 to 350 nm depending in the polarity of the local environment [1] Hence, protein fluorescence may be used as a diagnostic of the conformational state of a protein.[2] Furthermore, tryptophan fluorescence is strongly influenced by the proximity of other residues (i.e., nearby protonated groups such as Asp or Glu can cause quenching of Trp fluorescence). Also, energy transfer between tryptophan and the other fluorescent amino acids is possible, which would affect the analysis, especially in cases where the Förster acidic approach is taken. In addition, tryptophan is a relatively rare amino acid; many proteins contain only one or a few tryptophan residues. Therefore, tryptophan fluorescence can be a very sensitive measurement of the conformational state of individual tryptophan residues. The advantage compared to extrinsic probes is that the protein itself is not changed. The use of intrinsic fluorescence for the study of protein conformation is in practice limited to cases with few (or perhaps only one) tryptophan residues, since each experiences a different local environment, which gives rise to different emission spectra.

See also Edit

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