Taurine, also called 2-aminoethanesulfonic acid, is a non-proteinaceous amino acid that was first isolated from ox (Bos taurus) bile. Taurine is a sulfonic amino acid which means that rather than a carboxylic acid on the β-carbon, it has a sulfonic acid. The sulfonic acid gives taurine a high dissociation constant, similar to hydrochloric acid, and this means it exists almost entirely in the zwitterionic form at normal physiological pH. In comparison, some carboxylic amino acids remain unionised at the same pH. Because taurine exists as a zwitterion, taurine is highly water soluble and this prevents rapid passage of taurine through cell membranes (which are made of lipids). This allows the tissues to generate massive concentration gradients for taurine across cell membranes, and this can be as high as 400 to 1 in the retina. The zwitterionic nature of taurine also has unique isoelectric properties and this may allow it to regulate membrane electrical activity. Taurine has a large distribution in the human diet because it is widespread in animal foods. In animals, taurine is one of the most abundant low molecular chemicals, being widely distributed in all tissues. The large amount of taurine in humans, roughly 70 grams in a typical 70 kg adult, suggests that taurine has a significant and important role in human physiology. Taurine was initially considered to be required for correct osmoregulation, and since modern investigations of taurine which began in 1968, a large number of additional physiological functions have been found for taurine.
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