Tryptophan

Tryptophan has the molecular formula of C11H12N2O2 Tryptophan is one of the 20 standard amino acids. It is also an essential amino acid within the human diet. It is encoded in the standard genetic code as the codon UGG. However, only the L-stereoisomer of trytophan is used in structural or enzyme proteins. The D-stereoisomer is occasionally found in naturally produced peptides such as the m arine venom peptidecontryphan. The distinguishing structural characteristic of tryptophan is that it contains an indole functional gro up.

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): The disorders fructose malabsorption and lactose intolerance cause improper absorption of tryptophan in the intestine, reduced levels of tryptophan in the blood and depression.
 * 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.

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, dried dates, milk, yogurt, cottage cheese, red meat, eggs, fish, poultry, sesame, chickpeas, sunflower seeds, pumpkin seeds, corn, spirulina, bananas, and peanuts. Despite popular belief that turkey has a particularly high amount of tryptophan, the amount of tryptophan in turkey is typical of most poultry.

There is evidence that blood tryptophan levels are unlikely to be altered by changing the diet but for some time, tryptophan has been available in health food stores as a dietary supplement. Clinical research has shown mixed results with respect to tryptophan's effectiveness as a sleep aid, especially in normal patients. Tryptophan has shown some effectiveness for treatment of a variety of other conditions typically associated with low serotonin levels in the brain In particular, tryptophan has shown some promise as an antidepressant alone and as an "augmenter" of antidepressant drugs. However, the reliability of these clinical trials has been questioned because of lack of formal controls and repeatability. In addition, tryptophan itself may not be useful in the treatment of depression or other serotonin-dependent moods, but may be useful in understanding the chemical pathways that will give new research directions for pharmaceuticals.

Lifespan:
Rats fed a low tryptophan diet showed reduced blood levels of triiodothyronine, which was suggested to retard the aging process Rats on tryptophan-reduced diets have shown increased maximum life span and improved biomarkers of aging (although the rate of initial deaths was higher than in controls) The result was attributed to harmful effects of the age-related increase in brain serotonin. Cancers upregulate the liver enzyme tryptophan dioxygenase because its primary product, kynurenine, increases tumor growth. That would be especially relevant to lifespan studies in rodents, insofar as most rodents die of cancer[citation needed]. Additionally, excess dietary tryptophan has been shown to induce insulin resistance in pigs, a physiological condition that has a deleterious effect on many organ systems.