Taurine

Taurine, or 2-aminoethanesulfonic acid, is an organic acid. It is also a major constituent of bile and can be found in the lower intestine and in small amounts in the tissues of many animals and in humans as well. Taurine is a derivative of the sulfur-containing (sulfhydryl) amino acid, cysteine. Taurine is one of the few known naturally occurring sulfonic acids.

Taurine is named after the Latin taurus, which means bull or ox, as it was first isolated from ox bile in 1827 by German scientists Friedrich Tiedemann and Leopold Gmelin. It is often called an amino acid, even in scientific literature,  but as it lacks a carboxyl group it is not strictly an amino acid. 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 onto a tRNA.

Biosynthesis
The major pathway for mammalian taurine synthesis occurs in the liver 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.

Taurine in the pharmaceutical and lab setting is synthesized through a combination of cysteine, methionine and vitamin E. It is naturally produced in testicles of many mammals. Urban legends surrounding the source of taurine have included bull urine extract and bull semen. While it's true that taurine is found in both sources, it is not the source of taurine in the pharmaceutical or food industry. And while taurine is sometimes extracted from the intestines of cattle, many food industry sources, including the popular energy drink Red Bull, make efforts to use synthesized sources that are vegetarian friendly.

Physiological roles
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 (1.5) of taurine's sulfonic acid group ensures that 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, which can be found in many energy drinks today. 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 bursts, adipose tissue regulation, calcium homeostasis, recovery from osmotic shock and prevention of epileptic seizures. It also acts as an antioxidant and protects against toxicity of various substances. Supplementation with taurine has been shown to prevent oxidative stress induced by exercise.

It is believed that prematurely born infants lack the enzymes needed to convert cystathionine to cysteine and may therefore become deficient in taurine. Thus, taurine is thought to be a dietary essential nutrient in these individuals 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 beneficial, or even necessary. 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). Recent studies have also shown that taurine can influence (and possibly reverse) defects in nerve blood flow, motor nerve conduction velocity, and nerve sensory thresholds in experimental diabetic neuropathic rats. 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 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. But unlike prescription diuretics, it is not a cellular poison. Because it aids the movement of potassium, sodium, and calcium in and out of the cell, taurine has been used as a supplementation 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 anti-anxiety agent in the central nervous system.

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" that cardiac function showed as largely normal (given various other studies about effects of taurine on the heart).

Taurine is also used in some contact lens solutions.

Taurine has also been shown in diabetic rats to decrease weight and decrease blood sugar. According to one study, daily administration of 1.5g taurine had no significant effect on insulin secretion or insulin sensitivity in humans. However it is possible that an effect may occur at higher dosages. 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 that taurine treated diabetic rats had a decrease in the formation of advanced glycation end products (AGEs) and AGEs content.

Taurine and cats
Taurine is essential for feline health, as 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. It was discovered in 1987 that taurine deficiency can also cause feline dilated cardiomyopathy,. Unlike CRD, the condition is reversible with and 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.

Taurine and bird development
Recent research has provided evidence that taurine is essential in early bird development of passerines. Many passerines, regardless of spider availability, seek out many taurine-rich spiders to feed their young particularly in their youngest stages of life. Researchers later compared the behaviors and development of birds fed a taurine-supplemented diet to a control diet and found that juveniles that were fed taurine-rich diets as neonates were much larger risk takers and more adept at spatial learning tasks.

Synthesis and production
In 1993, approximately 5,000–6,000 t. of taurine was produced; 50% for pet food manufacture, 50% in pharmaceutical applications. 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.

As a functional food
Taurine is used as a functional food in many energy drinks and energy products Despite being present in many energy foods, it 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. However, it has been proven effective in removing fatty liver deposits in humans, preventing liver disease, and reducing cirrhosis in rats.