Taurine

Taurine, or 2-aminoethanesulfonic acid, is an organic acid. It is a major constituent of bile and can be found in the large intestine and in the tissues of many animals, including humans. Taurine has a ubiquitous distribution and accounts for approximately 0.1% of total body weight. Taurine has many fundamental biological roles such as conjugation of bile acids, antioxidation, osmoregulation, membrane stabilization and modulation of calcium signaling. Taurine is essential for cardiovascular function, and development and function of skeletal muscle, the retina and the central nervous system. While taurine is often wrongly described as an amino acid, it is in fact a sulfonic acid. All amino acids contain a carboxy group; taurine, however, does not.

Discovery and etymology
Taurine is named after the Latin taurus (a cognate of the Greek ταύρος) which means bull or ox, as it was first isolated from ox bile in 1827 by German scientists Friedrich Tiedemann and Leopold Gmelin.

Structure
Taurine is a derivative of cysteine, an amino acid which contains a sulfhydryl group. Taurine is one of the few known naturally occurring sulfonic acids. In the strict sense, it is not an amino acid, as it lacks a carboxyl group, but it is often called one, even in scientific literature. It does contain a sulfonate group and may be called an amino sulfonic acid. Small polypeptides have been identified which contain taurine, but to date no aminoacyl tRNA synthetase has been identified as specifically recognizing taurine and capable of incorporating it into a tRNA.

Dietary intake
Taurine occurs naturally in food, especially in seafood and meat. The mean daily intake from omnivore diets was determined to be around 58 mg (range from 9 to 372 mg) and to be low or negligible from a strict vegan diet. In another study, taurine intake was estimated to be generally less than 200 mg/day, even in individuals eating a high-meat diet. According to another study, taurine consumption was estimated to vary between 40 and 400 mg/day.

Energy drinks
Despite being present in many energy foods, taurine has not been proven to be energy-giving. A study of mice hereditarily unable to transport taurine suggests that it is needed for proper maintenance and functioning of skeletal muscles. In addition, it has been shown to be effective in removing fatty liver deposits in rats, preventing liver disease, and reducing cirrhosis in tested animals. There is also evidence that taurine is beneficial for adult human blood pressure and possibly, the alleviation of other cardiovascular ailments (in humans suffering essential hypertension, taurine supplementation resulted in measurable decreases in blood pressure).

Taurine is regularly used as an ingredient in energy drinks, with many containing 1000 mg per serving, and some as much as 2000 mg. A 2003 study by the European Food Safety Authority found no adverse effects for up to 1,000 mg of taurine per kilogram of body weight per day.

A review published in 2008 found no documented reports of negative or positive health effects associated with the amount of taurine used in energy drinks, concluding that "The amounts of guarana, taurine, and ginseng found in popular energy drinks are far below the amounts expected to deliver either therapeutic benefits or adverse events".

Biosynthesis
Taurine is a major constituent of bile and can be found in the large intestine and in the tissues of many animals, including humans. Mammalian taurine synthesis occurs in the pancreas via the cysteine sulfinic acid pathway. In this pathway, the sulfhydryl group of cysteine is first oxidized to cysteine sulfinic acid by the enzyme cysteine dioxygenase. Cysteine sulfinic acid, in turn, is decarboxylated by sulfinoalanine decarboxylase to form hypotaurine. It is unclear whether hypotaurine is then spontaneously or enzymatically oxidized to yield taurine.

Chemical synthesis and commercial production
Synthetic taurine is obtained from isethionic acid (2-hydroxyethanesulfonic acid), which in turn is obtained from the reaction of ethylene oxide with aqueous sodium bisulfite. Another approach is the reaction of aziridine with sulfurous acid. This leads directly to taurine.

In 1993, approximately 5,000–6,000 tons of taurine were produced for commercial purposes; 50% for pet food manufacture, 50% in pharmaceutical applications. As of 2010, China alone has more than 40 manufacturers of taurine. Most of these enterprises employ the ethanolamine method to produce a total annual production of about 3,000 tons.

Physiological functions
Taurine is essential for cardiovascular function, and development and function of skeletal muscle, the retina and the central nervous system.

Taurine is conjugated via its amino terminal group with chenodeoxycholic acid and cholic acid to form the bile salts sodium taurochenodeoxycholate and sodium taurocholate. The low pKa of taurine's sulfonic acid group ensures this moiety is negatively charged in the pH ranges normally found in the intestinal tract and, thus, improves the surfactant properties of the cholic acid conjugate. Taurine crosses the blood-brain barrier  and has been implicated in a wide array of physiological phenomena including inhibitory neurotransmission, long-term potentiation in the striatum/hippocampus, membrane stabilization, feedback inhibition of neutrophil/macrophage respiratory burst, adipose tissue regulation and possible prevention of obesity,  calcium homeostasis, recovery from osmotic shock, protection against glutamate excitotoxicity and prevention of epileptic seizures. It also acts as an antioxidant and protects against toxicity of various substances (such as lead and cadmium). Additionally, supplementation with taurine has been shown to prevent oxidative stress induced by exercise. In a 2008 study, taurine has been shown to reduce the secretion of apolipoprotein B100 and lipids in HepG2 cells. High concentrations of serum lipids and apolipoprotein B100 (essential structural component of VLDL and LDL) are major risk factors of atherosclerosis and coronary heart disease. Hence, taurine supplementation is possibly beneficial for the prevention of these diseases. In a 2003 study, Zhang et al. have demonstrated the hypocholesterolemic (blood cholesterol-lowering) effect of dietary taurine in young overweight adults. Furthermore, they reported body weight also decreased significantly in the taurine supplemented group. These findings are consistent with animal studies. Taurine has also been shown to help people with congestive heart failure by increasing the force and effectiveness of heart-muscle contractions.

Taurine levels were found to be significantly lower in vegans than in a control group on a standard American diet. Plasma taurine was 78% of control values, and urinary taurine was 29%.

In the cell, taurine keeps potassium and magnesium inside the cell, while keeping excessive sodium out. In this sense, it works like a diuretic. Because it aids the movement of potassium, sodium, and calcium in and out of the cell, taurine has been used as a dietary supplement for epileptics, as well as for people who have uncontrollable facial twitches.

According to animal studies, taurine produces anxiolytic effect and may act as a modulator or antianxiety agent in the central nervous system by activating the glycine receptor.

Taurine is necessary for normal skeletal muscle functioning. This was shown by a 2004 study using mice with a genetic taurine deficiency. They had a nearly complete depletion of skeletal and cardiac muscle taurine levels. These mice had a reduction of more than 80% of exercise capacity compared to control mice. The authors expressed themselves as "surprised" their cardiac function showed as largely normal (given various other studies about effects of taurine on the heart).

Studies have shown taurine can influence (and possibly reverse) defects in nerve blood flow, motor nerve conduction velocity, and nerve sensory thresholds in experimental diabetic neuropathic rats. In another study on diabetic rats, taurine significantly decreased weight and decreased blood sugar in these animal models. Likewise, a 2008 study demonstrated taurine administration to diabetic rabbits resulted in 30% decrease in serum glucose levels. According to the single study on human subjects, daily administration of 1.5g taurine had no significant effect on insulin secretion or insulin sensitivity. There is evidence that taurine may exert a beneficial effect in preventing diabetes-associated microangiopathy and tubulointerstitial injury in diabetic nephropathy. Taurine acts as a glycation inhibitor. Studies have shown taurine-treated diabetic rats had a decrease in the formation of advanced glycation end products (AGEs) and AGEs content. The United States Department of Agriculture has found a link between cataract development and lower levels of vitamin B6, folate, and taurine in the diets of the elderly.

Taurine has been investigated in animal studies as an alternative to glucose as osmotic agent for use in peritoneal dialysis solutions.

Toxicity
Taurine is involved in a number of crucial physiological processes. However, the role of taurine in these processes is not clearly understood and the influence of high taurine doses on these processes is uncertain. A substantial increase in the plasma concentration of growth hormone was reported in some epileptic patients during taurine tolerance testing (oral dose of 50 mg/kg bw/day), suggesting a potential to stimulate the hypothalamus and to modify neuroendocrine function. There is an indication that taurine (2 g/day) has some function in the maintenance and possibly in the induction of psoriasis. It may also be necessary to take into consideration that absorption of taurine from beverages may be more rapid than from foods.

In animal nutrition
Taurine is an essential dietary requirement for feline health, since cats cannot synthesize the compound. The absence of taurine causes a cat's retina to slowly degenerate, causing eye problems and (eventually) irreversible blindness — a condition known as central retinal degeneration (CRD), as well as hair loss and tooth decay. Decreased plasma taurine concentration has been demonstrated to be associated with feline dilated cardiomyopathy. Unlike CRD, the condition is reversible with supplementation. Taurine is now a requirement of the Association of American Feed Control Officials (AAFCO) and any dry or wet food product labeled approved by the AAFCO should have a minimum of 0.1% taurine in dry food and 0.2% in wet food.

Research suggests taurine is essential to the normal development of passerine birds. Many passerines seek out taurine-rich spiders to feed their young, particularly just after hatching. Researchers compared the behaviors and development of birds fed a taurine-supplemented diet to a control diet and found the juveniles fed taurine-rich diets as neonates were much larger risk takers and more adept at spatial learning tasks.

Other uses
Lately, cosmetic compositions containing taurine have been introduced, possibly due to its antifibrotic properties. It has been shown to prevent the damaging effects of TGFB1 to hair follicles. It also helps to maintain skin hydration.

Prematurely born infants are believed to lack the enzymes needed to convert cystathionine to cysteine, and may, therefore, become deficient in taurine. Taurine is present in breast milk, and has been added to many infant formulas, as a measure of prudence, since the early 1980s. However, this practice has never been rigorously studied, and as such it has yet to be proven to be necessary, or even beneficial.

Taurine is also used in some contact lens solutions.