L-Tryptophan
Overview:
Tryptophan (IUPAC-IUBMB abbreviation: Trp or W; IUPAC abbreviation: L-Trp or D-Trp; sold for medical use as Tryptan)[1] is one of the 20 standard amino acids, as well as an essential amino acid in the human diet. It is encoded in the standard genetic code as the codon UGG. 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). The distinguishing structural characteristic of tryptophan is that it contains an indole functional group. It is an essential amino acid as defined by its growth effects on rats.
Isolation
The isolation of tryptophan was first reported by Sir Frederick Hopkins in 1901 through hydrolysis of casein. From 600 grams of crude casein one obtains 4-8 grams of tryptophan.
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.
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 bacteria such as Corynebacterium glutamicum, Bacillus subtilis, Bacillus amyloliquefaciens or E. coli. These strains carry either mutations that prevent the reuptake of aromatic amino acids or multiple/overexpressed trp operons. The conversion is catalyzed by the enzyme tryptophan synthase.
Function
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.Serotonin, in turn, can be converted to melatonin (a neurohormone), via N-acetyltransferase and 5-hydroxyindole-O-methyltransferase activities.
-
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 and depression.
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, durians, mangoes, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, sunflower seeds, pumpkin seeds, spirulina, and peanuts. Despite popular belief to the contrary, the level found in turkey is at a level typical of poultry in general.
Tryptophan (Trp) Content of Various Foods
| Food |
Protein |
Tryptophan |
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 supplement
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)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.Furthermore tryptophan has shown some effectiveness for treatment of a variety of other conditions typically associated with low serotonin levels in the brain such as premenstrual dysphoric disorder and seasonal affective disorder.In particular, tryptophan has shown considerable promise as an antidepressant alone and as an "augmenter" of antidepressant drugs. However, the reliability of these clinical trials has been questioned.
Metabolites
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 may be a significant risk of heart valve disease from serotonin's effect on the heart.In Europe, 5-HTP is prescribed with carbidopa to prevent the conversion of 5-HTP into serotonin until it reaches the brain.
5-HTP readily crosses the blood-brain barrier and in addition is rapidly decarboxylated to serotonin (5-hydroxytryptamine or 5-HT) 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-HTP is usually converted to serotonin before it can reach the brain, elevating blood serotonin levels greatly, which may cause diarrhea and heart problems, while only slightly increasing brain serotonin. Therefore, 5-HTP is more effectively used when in conjunction with a dopa decarboxylase inhibitor such as Carbidopa, which slows its conversion to serotonin, allowing more of supplement to reach the brain.[citation needed]
Tryptophan supplements and EMS
Although now available for purchase, there was a large tryptophan-related outbreak of eosinophilia-myalgia syndrome (EMS) in 1989 which caused 1,500 cases of permanent disability including at least thirty-seven deaths. Some epidemiological studies traced the outbreak to L-tryptophan supplied by a Japanese manufacturer, Showa Denko KK. It was further hypothesized that one or more trace impurities produced during the manufacture of tryptophan may have been responsible for the EMS outbreak.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.However, the methodology used in the initial epidemiological studies has been criticized.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.
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,[50][51] 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."
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.[citation needed]
Turkey meat and drowsiness
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. While turkey does contain high levels of tryptophan, the amount is comparable to that contained in most other meats.Furthermore, post-meal Thanksgiving drowsiness may have more to do with what else is consumed along with the turkey, in particular carbohydrates and alcohol.
It has been demonstrated in both animal models and in humans 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).Once inside the CNS, tryptophan is converted into serotonin in the raphe nuclei by the normal enzymatic pathway. The resultant serotonin is further metabolised into melatonin by the pineal gland. Hence, these data suggest that "feast-induced drowsiness," and in particular, the common post-Christmas and North 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.