Taurine

Abstract

Taurine, a sulfur-containing amino acid (Figure), has been termed a functional nutrient that could be used to protect against, among others, diabetes mellitus and atherosclerosis.1 Indeed, an increasing body of literature supports the use of taurine supplements. Because taurine has very diverse functions, notably, intracellular osmoregulation and bile acid formation, and is abundantly present in several organs, multiple pathways could be involved. Some of these are discussed in this editorial. Figure. A simplified biosynthesis pathway from methionine to taurine. Methionine is converted to homocysteine in 3 steps. Homocysteine is used as a substrate to form cysteine in 2 steps but can also be converted back to methionine by 2 different mechanisms. Cysteine, precursor for the important antioxidant glutathione and the gaseous transmitter/signaling molecule H2S, can be further converted to cysteine sulfinate. Cysteine sulfinate is converted to taurine in 2 steps via 2 different pathways. Taurine can decrease methionine by decreasing uptake. Whether taurine decreases homocysteine directly or via a reduction of methionine is unclear. Of the 20 canonical amino acids, 18 are composed of carbon, hydrogen, nitrogen, and oxygen only. The remaining 2, methionine and cysteine, also contain 1 atom of sulfur. Because sulfur is not as electronegative as oxygen, the sulfur-containing amino acids play a key role in protein structure and synthesis.2 Methionine and cysteine also play important roles in cell metabolism. For instance, methionine serves as a substrate for S-adenosylmethionine, which is vital for methylation of nucleic acids, proteins, lipids, etc. In proteins, cysteine easily forms double bonds with other cysteine residues, thus determining tertiary structure and binding sites. Moreover, cysteine is substrate for glutathione, an important intracellular antioxidant, and H2S, a gas that can induce endothelial-dependent relaxation.3 Cysteine is considered a nonessential amino acid because it is synthesized …