Taurine may exert some of its beneficial physiological effects by binding to proteins. For example, it is known that taurine can bind to transport proteins and in this way affects the flux of ions such as calcium. However, it is unclear as to which other non-transport proteins taurine may bind to, and on which bonding sites this may occur. Further, many studies that have shown potential binding of taurine to proteins have not done so at physiologically relevant concentrations and so it is unclear as to what effects taurine is having on cellular proteins. One aspect of taurine binding that shows promise in terms of accumulating evidence is the ability of taurine to bind to receptors that are involved in inhibitory neurotransmission such as GABA or glycine. However, this binding may be indirect and taurine may not be a ligand for these receptors. However, that does not mean that taurine is ineffective at altering GABA and glycine neurotransmission, because evidence suggests that it can. For example, it is known that taurine stimulates the influx of chlorine ions and this hyperpolarises the cell membrane, making further neurotransmission less likely. Therefore taurine may directly or indirectly stimulate the chloride ion channel in cell membranes, possibly through interaction with inhibitory neurotransmission systems. 

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Huxtable, R. J. 1992. Physiological actions of taurine. Physiological Reviews. 72(1): 101-163

Taurine may have the ability to interact with phospholipids and this may affect cell membranes. Taurine binds to phospholipids with low affinity to a specific binding site in the membrane. This affinity is within physiological concentrations for taurine, and this suggests that taurine may bind to phospholipids in animals and humans. Taurine may also regulate calcium ion binding to the phospholipid phosphatidylserine. In this way taurine may play a key role in the electrical potential of membranes and in their stability. Taurine also inhibits the enzyme phospholipid N-methyltransferase, and this in turn alters the amount of phosphatidylcholine and phosphatidylethanolamine within membranes. This then has further effects by regulating the activity of proteins within the membrane, and these proteins can have significant functions in transport and metabolism of substances through or on the cell membrane. For example, taurine may regulate protein kinases and phosphatases, and in cardiomyocytes this may have significant effects on the regulation of heart muscle tissue. 

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Huxtable, R. J. 1992. Physiological actions of taurine. Physiological Reviews. 72(1): 101-163

One of the main physiological functions of taurine in humans and animals is that of osmoregulation. Osmoregulation is pivotal to cell survival in animals and humans because the plasma membrane is permeable to many solutes and water and this creates the risk that the cell can become desiccated or undergo lysis based on the solute concentrations of the internal and external environments. Cells need to maintain osmoregulation such that pressure within the cells remains relatively constant and this is also vital because it is used to maintain the electrochemical gradient in the cell membrane that provides electrical activity to the membrane. Cells achieve this by maintaining distinct solute concentrations across plasma membranes using active and passive transport as well as diffusion of water molecules. 

Taurine is an effective osmoregulatory agent because its structure facilitates this role. Taurine is transported via specific β-amino acid transporters in a sodium ion dependent manner, and very high concentration gradients can be maintained because taurine is highly water soluble and does not cross the plasma membrane outside of its transport system. However, in mammals the role of taurine in osmoregulation is not as great as in other species, for example marine organisms such as fish and invertebrates, which face unique challenges due to the high saline conditions they live in. In contrast taurine in mammals likely plays an accessory role in osmoregulation, with the main role played by inorganic solutes such as sodium ions, potassium ions and chloride ions. 

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Huxtable, R. J. 1992. Physiological actions of taurine. Physiological Reviews. 72(1): 101-163

The importance of taurine in animals was not understood until the old dogma of it being a waste product of sulphur metabolism was overcome. Since this time a growing list of physiological functions have been uncovered. In animals taurine is extracted in an unmetabolised form, and this means the compound is regarded as being inert. However, the fact that it is inert does not mean that it does not play a highly important role in animal and human metabolism. In terms of metabolism, mammals are not capable of sulphur reduction (only oxidation), and therefore sources of reduced sulphur in the diet must supply the need for reduced sulphur for metabolic purposes. In mammals this reduced sulphur can be found in the dietary amino acids cysteine and methionine, both of which are sulphurous amino acids, and both of which are essential due to their ability to supply the sulphur required for metabolic purposes. The metabolism of cysteine and methionine results in the formation of taurine, which is either excreted unchanged, or can be converted to the bile salt taurocholate. 

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Huxtable, R. J. 1992. Physiological actions of taurine. Physiological Reviews. 72(1): 101-163

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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RdB

Huxtable, R. J. 1992. Physiological actions of taurine. Physiological Reviews. 72(1): 101-163