Tryptophan

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Editor-In-Chief: C. Michael Gibson, M.S., M.D. [1]


Tryptophan (abbreviated as Trp or W)[1] is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in genetic code as the codon TGG. Only the L-stereoisomer of tryptophan is used in structural or enzyme proteins, but the D-stereoisomer is occasionally found in naturally produced peptides (for example, the marine venom peptide contryphan).[2] The distinguishing structural characteristic of tryptophan is that it contains an indole functional group.

Isolation

The isolation of tryptophan was first reported by Sir Frederick Hopkins in 1901 [3] through hydrolysis of casein. From 600 grams of crude casein one obtains 4-8 grams of tryptophan.[4]

Biosynthesis and industrial production

Plants and microorganisms commonly synthesize tryptophan from shikimic acid or anthranilate.[5] The latter condenses with phosphoribosylpyrophosphate (PRPP), generating pyrophosphate as a by-product. After ring opening of the ribose moiety and following reductive decarboxylation, indole-3-glycerinephosphate is produced, which in turn is transformed into indole. In the last step, tryptophan synthase catalyzes the formation of tryptophan from indole and the amino acid, serine.

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The industrial production of tryptophan is also biosynthetic and is based on the fermentation of serine and indole using either wild-type or genetically modified E. coli. The conversion is catalyzed by the enzyme tryptophan synthase.[6]

Function

File:Tryptophan metabolism.png
Metabolism of L-tryptophan into serotonin and melatonin (left) and niacin (right). Transformed functional groups after each chemical reaction are highlighted in red.

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. The principal function of amino acids including tryptophan are as building blocks in protein biosynthesis. In addition, tryptophan functions as a biochemical precursor for the following compounds (see also figure to the right):

The disorder Fructose Malabsorption causes improper absorption of tryptophan in the intestine, reduced levels of tryptophan in the blood[11] and depression.[12]

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 sources

Tryptophan is a routine constituent of most protein-based foods or dietary proteins. It is particularly plentiful in chocolate, oats, bananas, mangoes, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, sunflower seeds, pumpkin seeds, spirulina, and peanuts.[13] It is also found in turkey at a level typical of poultry in general.[14]

Tryptophan (Trp) Content of Various Foods[14][15]
Food Protein
[g/100 g of food] 		Tryptophan
[g/100 g of food] 		Tryptophan/Protein [%]
turkey
21.89
0.24
1.11
cheese, cheddar
24.90
0.32
1.29
chicken
20.85
0.24
1.14
beef
20.13
0.23
1.12
lamb, chop
18.33
0.21
1.17
pork, chop
19.27
0.25
1.27
salmon
19.84
0.22
1.12
perch, Atlantic
18.62
0.21
1.12
milk
3.22
0.08
2.34
egg
12.58
0.17
1.33
wheat flour, white
10.33
0.13
1.23
potatoes, russet
2.14
0.02
0.84
rice, white
7.13
0.08
1.16

Use as a dietary supplement

For some time, tryptophan was available in health food stores as a dietary supplement, although it is common in dietary protein. 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)[16] and/or melatonin (a sleep-inducing hormone secreted by the pineal gland in response to darkness or low light levels).[17][18]

Clinical research tends to confirm tryptophan's effectiveness as a sleep aid[19][20][21] and for a growing variety of other conditions typically associated with low serotonin levels or activity in the brain[22] such as premenstrual dysphoric disorder [23] and seasonal affective disorder.[24][25] In particular, tryptophan has been showing considerable promise as an antidepressant alone,[26] and as an "augmenter" of antidepressant drugs.[26][27] However, the reliability of these clinical trials has been questioned.[28][29]

Metabolites

5-Hydroxytryptophan (5-HTP), a metabolite of tryptophan, has been suggested as a treatment for epilepsy[30] and depression, although clinical trials are regarded inconclusive and lacking.[31]

5-HTP readily crosses the blood-brain barrier and in addition is rapidly decarboxylated to serotonin (5-hydroxytryptamine or 5-HT)[32] 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 and Tript-OH.

In the United States, 5-HTP does not require a prescription, as it is covered under the Dietary Supplement Act. However, since the quality of dietary supplements is not regulated by the FDA, the quality of dietary and nutritional supplements tends to vary, and there is no guarantee that the label accurately depicts what the bottle contains.

Tryptophan supplements and EMS

Although currently available for purchase, in 1989 a large outbreak (1500 cases of permanent disability including at least 37 deaths) of a disabling autoimmune illness called eosinophilia-myalgia syndrome (EMS) was traced by some epidemiological studies[33][34][35] to L-tryptophan supplied by a Japanese manufacturer, Showa Denko KK.[36] It was further hypothesized that one or more trace impurities produced during the manufacture of tryptophan may have been responsible for the EMS outbreak.[37][38] However, many people who consumed Showa Denko L-tryptophan did not develop EMS and cases of EMS have occurred prior to and after the 1989 epidemic. Furthermore the methodology used in the initial epidemiological studies has been criticized.[39][40] 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.[41]

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. A Rutgers Law Journal article observed, "Political pressures have played a role in the FDA's decision to ban L-tryptophan as well as its desire to increase its regulatory power over dietary supplements."[42]

At the time of the ban, the FDA did not know, or did not indicate, that EMS was caused by a contaminated batch,[43][44] 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."[36]

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.

Turkey meat and drowsiness

One widely-held belief is that heavy consumption of turkey meat (as for example in a Thanksgiving feast) results in drowsiness, which has been attributed to high levels of tryptophan contained in turkey.[45][46][47] While turkey does contain high levels of tryptophan, the amount is comparable to that contained in most other meats.[14] Furthermore, postprandial Thanksgiving sedation may have more to do with what is consumed along with the turkey, in particular carbohydrates, rather than the turkey itself.

It has been demonstrated in both animal models[48] and in humans[49][50][51] 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 with other amino acids for the large neutral amino acid transporter protein for uptake of tryptophan across the blood-brain barrier into the central nervous system (CNS).[52][53] Once inside the CNS, tryptophan is converted into serotonin in the raphe nuclei by the normal enzymatic pathway.[48][50] The resultant serotonin is further metabolised into melatonin by the pineal gland.[9] Hence, these data suggest that "feast-induced drowsiness," and in particular, the common 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 serotonin and melatonin in the brain.[48][49][50][51]

Fluorescence

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. 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 [54] Hence, protein fluorescence may be used as a diagnostic of the conformational state of a protein.[55] Furthermore, tryptophan fluorescence is strongly influenced by the proximity of other residues (i.e., nearby protonated acidic 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 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. This could be avoided by the use of time-resolved fluorescence, but would not really make the analysis much easier.

References

  1. ↑ IUPAC-IUBMB Joint Commission on Biochemical Nomenclature. "Nomenclature and Symbolism for Amino Acids and Peptides". Recommendations on Organic & Biochemical Nomenclature, Symbols & Terminology etc. Retrieved 2007-05-17.
  2. ↑ Pallaghy PK, Melnikova AP, Jimenez EC, Olivera BM, Norton RS (1999). "Solution structure of contryphan-R, a naturally-occurring disulfide-bridged octapeptide containing D-tryptophan: comparison with protein loops". Biochemistry. 38 (35): 11553–9. PMID 10471307.
  3. ↑ Hopkins FG, Cole SW (1901). "A contribution to the chemistry of proteids: Part I. A preliminary study of a hitherto undescribed product of tryptic digestion". J. Physiol. (Lond.). 27 (4–5): 418–28. PMID 16992614.
  4. ↑ Cox GJ, King H (1943). "L-Tryptophane" (PDF). Organic Syntheses. Collected Volume 2: 612–616.
  5. ↑ Radwanski ER, Last RL (1995). "Tryptophan biosynthesis and metabolism: biochemical and molecular genetics". Plant Cell. 7 (7): 921–34. doi:10.1105/tpc.7.7.921. PMID 7640526.
  6. ↑ Ikeda M (2002). "Amino acid production processes". Adv. Biochem. Eng. Biotechnol. 79: 1–35. PMID 12523387.
  7. ↑ Fernstrom JD (1983). "Role of precursor availability in control of monoamine biosynthesis in brain". Physiol. Rev. 63 (2): 484–546. PMID 6132421.
  8. ↑ Schaechter JD, Wurtman RJ (1990). "Serotonin release varies with brain tryptophan levels" (PDF). Brain Res. 532 (1–2): 203–10. doi:10.1016/0006-8993(90)91761-5. PMID 1704290.
  9. ↑ 9.0 9.1 Wurtman RJ, Anton-Tay F (1969). "The mammalian pineal as a neuroendocrine transducer" (PDF). Recent Prog. Horm. Res. 25: 493–522. PMID 4391290.
  10. ↑ Ikeda M, Tsuji H, Nakamura S, Ichiyama A, Nishizuka Y, Hayaishi O (1965). "Studies on the biosynthesis of nicotinamide adenine dinucleotide. II. A role of picolinic carboxylase in the biosynthesis of nicotinamide adenine dinucleotide from tryptophan in mammals". J. Biol. Chem. 240: 1395–401. PMID 14284754.
  11. ↑ Ledochowski M, Widner B, Murr C, Sperner-Unterweger B, Fuchs D (2001). "Fructose malabsorption is associated with decreased plasma tryptophan". Scand. J. Gastroenterol. 36 (4): 367–71. PMID 11336160.
  12. ↑ Ledochowski M, Sperner-Unterweger B, Widner B, Fuchs D (1998). "Fructose malabsorption is associated with early signs of mental depression". Eur. J. Med. Res. 3 (6): 295–8. PMID 9620891.
  13. ↑ Tryptophan background
  14. ↑ 14.0 14.1 14.2 Joanne Holden, Nutrient Data Laboratory, Agricultural Research Service. "USDA National Nutrient Database for Standard Reference, Release 20". United States Department of Agriculture. Retrieved 2007-10-02.
  15. ↑ Rambali B, Andel I van, Schenk E, Wolterink G, Werken G van de, Stevenson H, Vleeming W (2002). "[The contribution of cocoa additive to cigarette smoking addiction]" (PDF). RIVM (report 650270002/2002).- The National Institute for Public Health and the Environment (Netherlands)
  16. ↑ Wurtman RJ, Hefti F, Melamed E (1980). "Precursor control of neurotransmitter synthesis" (PDF). Pharmacol. Rev. 32 (4): 315–35. PMID 6115400.
  17. ↑ Wurtman RJ, Larin F, Axelrod J, Shein HM, Rosasco K (1968). "Formation of melatonin and 5-hydroxyindole acetic acid from 14C-tryptophan by rat pineal glands in organ culture". Nature. 217 (5132): 953–4. doi:10.1038/217953a0. PMID 5300432.
  18. ↑ Ruddick JP, Evans AK, Nutt DJ, Lightman SL, Rook GA, Lowry CA (2006). "Tryptophan metabolism in the central nervous system: medical implications". Expert reviews in molecular medicine. 8 (20): 1–27. doi:10.1017/S1462399406000068. PMID 16942634.
  19. ↑ Hartmann E (1982). "Effects of L-tryptophan on sleepiness and on sleep". Journal of psychiatric research. 17 (2): 107–13. doi:10.1016/0022-3956(82)90012-7. PMID 6764927.
  20. ↑ Schneider-Helmert D, Spinweber CL (1986). "Evaluation of L-tryptophan for treatment of insomnia: a review". Psychopharmacology (Berl.). 89 (1): 1–7. doi:10.1007/BF00175180. PMID 3090582.
  21. ↑ Wyatt RJ, Engelman K, Kupfer DJ, Fram DH, Sjoerdsma A, Snyder F. (1970 Oct 24). "Effects of L-tryptophan (a natural sedative) on human sleep". Lancet. 1970 Oct 24,2 (7678): 842–6. ISSN 0140-6736. PMID 4097755. Check date values in: |year= (help)
  22. ↑ "research summary of Dr. Richard Wurtman, MIT". Retrieved 2007-08-12.
  23. ↑ Steinberg S, Annable L, Young SN, Liyanage N (1999). "A placebo-controlled clinical trial of L-tryptophan in premenstrual dysphoria". Biol. Psychiatry. 45 (3): 313–20. doi:10.1016/S0006-3223(98)00005-5. PMID 10023508.
  24. ↑ Lam RW, Levitan RD, Tam EM, Yatham LN, Lamoureux S, Zis AP (1997). "L-tryptophan augmentation of light therapy in patients with seasonal affective disorder". Canadian journal of psychiatry. Revue canadienne de psychiatrie. 42 (3): 303–6. PMID 9114947.
  25. ↑ Jepson TL, Ernst ME, Kelly MW (1999). "Current perspectives on the management of seasonal affective disorder". J Am Pharm Assoc (Wash). 39 (6): 822–9. PMID 10609448.
  26. ↑ 26.0 26.1 Thomson J, Rankin H, Ashcroft GW, Yates CM, McQueen JK, Cummings SW (1982). "The treatment of depression in general practice: a comparison of L-tryptophan, amitriptyline, and a combination of L-tryptophan and amitriptyline with placebo". Psychological medicine. 12 (4): 741–51. PMID 7156248.
  27. ↑ Levitan RD, Shen JH, Jindal R, Driver HS, Kennedy SH, Shapiro CM (2000). "Preliminary randomized double-blind [[placebo]]-controlled trial of tryptophan combined with fluoxetine to treat major depressive disorder: antidepressant and hypnotic effects". Journal of psychiatry & neuroscience : JPN. 25 (4): 337–46. PMID 11022398. URL–wikilink conflict (help)
  28. ↑ Meyers S (2000). "Use of neurotransmitter precursors for treatment of depression" (PDF). Alternative medicine review : a journal of clinical therapeutic. 5 (1): 64–71. PMID 10696120.
  29. ↑ Shaw K, Turner J, Del Mar C (2002). "Tryptophan and 5-hydroxytryptophan for depression". Cochrane database of systematic reviews (Online) (1): CD003198. doi:10.1002/14651858.CD003198. PMID 11869656.
  30. ↑ Kostowski W, Bidzinski A, Hauptmann M, Malinowski JE, Jerlicz M, Dymecki J (1978). "Brain serotonin and epileptic seizures in mice: a pharmacological and biochemical study". Pol J Pharmacol Pharm. 30 (1): 41–7. PMID 148040.
  31. ↑ Turner EH, Loftis JM, Blackwell AD (2006). "Serotonin a la carte: supplementation with the serotonin precursor 5-hydroxytryptophan". Pharmacol Ther. 109 (3): 325–38. PMID 16023217.
  32. ↑ Hardebo JE, Owman C (1980). "Barrier mechanisms for neurotransmitter monoamines and their precursors at the blood-brain interface". Ann NeurolAnn Neurol. 8 (1): 1–31. PMID 6105837.
  33. ↑ Slutsker L, Hoesly FC, Miller L, Williams LP, Watson JC, Fleming DW (1990). "Eosinophilia-myalgia syndrome associated with exposure to tryptophan from a single manufacturer". JAMA. 264 (2): 213–7. PMID 2355442.
  34. ↑ Back EE, Henning KJ, Kallenbach LR, Brix KA, Gunn RA, Melius JM (1993). "Risk factors for developing eosinophilia myalgia syndrome among L-tryptophan users in New York". J. Rheumatol. 20 (4): 666–72. PMID 8496862.
  35. ↑ Kilbourne EM, Philen RM, Kamb ML, Falk H (1996). "Tryptophan produced by Showa Denko and epidemic eosinophilia-myalgia syndrome". The Journal of rheumatology. Supplement. 46: 81–8, discussion 89-91. PMID 8895184.
  36. ↑ 36.0 36.1 FDA Information Paper on L-tryptophan and 5-hydroxy-L-tryptophan
  37. ↑ Mayeno AN, Lin F, Foote CS, Loegering DA, Ames MM, Hedberg CW, Gleich GJ (1990). "Characterization of "peak E," a novel amino acid associated with eosinophilia-myalgia syndrome". Science. 250 (4988): 1707–8. doi:10.1126/science.2270484. PMID 2270484.
  38. ↑ Ito J, Hosaki Y, Torigoe Y, Sakimoto K (1992). "Identification of substances formed by decomposition of peak E substance in tryptophan". Food Chem. Toxicol. 30 (1): 71–81. doi:10.1016/0278-6915(92)90139-C. PMID 1544609.
  39. ↑ Shapiro S (1996). "Epidemiologic studies of the association of L-tryptophan with the eosinophilia-myalgia syndrome: a critique". The Journal of rheumatology. Supplement. 46: 44–58, discussion 58-9. PMID 8895181.
  40. ↑ Horwitz RI, Daniels SR (1996). "Bias or biology: evaluating the epidemiologic studies of L-tryptophan and the eosinophilia-myalgia syndrome". The Journal of rheumatology. Supplement. 46: 60–72. PMID 8895182.
  41. ↑ Smith MJ, Garrett RH (2005). "A heretofore undisclosed crux of eosinophilia-myalgia syndrome: compromised histamine degradation". Inflamm. Res. 54 (11): 435–50. doi:10.1007/s00011-005-1380-7. PMID 16307217.
  42. ↑ Beisler JH (2000). "Dietary Supplements and Their Discontents: FDA Regulation and the Dietary Supplement Health and Education Act of 1994 (L-tryptophan Section)". Rutgers Law Journal.
  43. ↑ FDA Tryptophan Recall
  44. ↑ Raphals P (2000). "Does medical mystery threaten biotech?". Science. 250: 4981. doi:10.1126/science.2237411. PMID 2237411.
  45. ↑ "About.com: Does Eating Turkey Make You Sleepy?". Retrieved 2007-08-17.
  46. ↑ "Howstuffworks.com: Is there something in turkey that makes you sleepy?". Retrieved 2007-08-17.
  47. ↑ "Chemistry.org: Thanksgiving, Turkey, and Tryptophan". Retrieved 2007-08-17.
  48. ↑ 48.0 48.1 48.2 Fernstrom JD, Wurtman RJ (1971). "Brain serotonin content: increase following ingestion of carbohydrate diet". Science. 174 (13): 1023–5. doi:10.1126/science.174.4013.1023. PMID 5120086.
  49. ↑ 49.0 49.1 Lyons PM, Truswell AS (1988). "Serotonin precursor influenced by type of carbohydrate meal in healthy adults" (PDF). Am. J. Clin. Nutr. 47 (3): 433–9. PMID 3279747.
  50. ↑ 50.0 50.1 50.2 Wurtman RJ, Wurtman JJ, Regan MM, McDermott JM, Tsay RH, Breu JJ (2003). "Effects of normal meals rich in carbohydrates or proteins on plasma tryptophan and tyrosine ratios". Am. J. Clin. Nutr. 77 (1): 128–32. PMID 12499331.
  51. ↑ 51.0 51.1 Afaghi A, O'Connor H, Chow CM (2007). "High-glycemic-index carbohydrate meals shorten sleep onset". Am. J. Clin. Nutr. 85 (2): 426–30. PMID 17284739.
  52. ↑ Pardridge WM, Oldendorf WH (1975). "Kinetic analysis of blood-brain barrier transport of amino acids". Biochim. Biophys. Acta. 401 (1): 128–36. doi:10.1016/0005-2736(75)90347-8. PMID 1148286.
  53. ↑ Maher TJ, Glaeser BS, Wurtman RJ (1984). "Diurnal variations in plasma concentrations of basic and neutral amino acids and in red cell concentrations of aspartate and glutamate: effects of dietary protein intake". Am. J. Clin. Nutr. 39 (5): 722–9. PMID 6538743.
  54. ↑ Intrinsic Fluorescence of Proteins and Peptides
  55. ↑ Vivian JT, Callis PR (2001). "Mechanisms of tryptophan fluorescence shifts in proteins". Biophys. J. 80 (5): 2093–109. PMID 11325713.

See also

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