Taurine, from cysteine

Some pyridoxal phosphate-dependent enzymes are normally fuUy saturated with cofactor and show the same activity on assay in vitro whether additional pyridoxal phosphate is present in the incubation medium or not. Examples of this class of enzymes include liver cysteine sulfinate decarboxylase (which is involved in the synthesis of taurine from cysteine Section 14.5.1) and the brain and liver glutamate and aspartate aminotransferases. 

Figure 14.7. Pathways for the synthesis of taurine from cysteine. Cysteine sulfinate decarboxylase, EC 4.1.1.29 cysteic acid decarboxylase, EC 4.1.1.29 (glutamate decarboxylase, EC 4.1.1.15) cysteine oxidase, EC 1.13.11.20 cysteamine oxygenase, EC 1.13.11.19 and hypotaurine oxidase, EC 1.8.1.3. Relative molecular masses (Mr) cysteine, 121.2 cysteamine, 77.2 cysteine sulfinic acid, 153.2 cysteic acid, 169.2 hypotaurine, 109.1 and taurine, 125.1.

Fig. 3. Synthesis of taurine from cysteine. The major pathway for the formation of taurine is via hypotaurine.

Tests to determine the nature of the compounds formed from the decarboxylation of cystine that could undergo oxidation led to the observation that cystamine disulfoxide was oxidized with ease, whereas cystamine was completely resistant. From these observations, it was hypothesized that two oxidizing enzymes may be involved in the reaction, one oxidizing cystine to the disulfoxide, and one continuing the oxidation of the cystamine disulfoxide subsequent to the decarboxylation of the cystine disulfoxide. If it is assumed that the cystamine disulfoxide is simultaneously oxidized and cleaved to 2-aminoethanesulfinic acid, the oxidation scheme leads to the intermediate that was established for the biosynthesis of taurine from cysteine by Awapara. ... 

The problem of regulation is complicated by the metabolic complexity of sulfur amino acids, and the wide variation in organ taurine concentrations between species. The major putative metabolic routes to taurine from cysteine are three These involve the intermediacy respectively of cysteine sulfinic acid, cysteic acid, and cysteamine. The first two utilize the enzyme cysteine sulfinic acid decarboxylase (CSAD), and the latter the enzyme cysteamine dioxygenase (CD). The distribution of these enzymes differ both quantitatively and qualitatively in corresponding organs of various species. Other pathways of taurine biosynthesis have also been proposed. For... 

CJH4O5, H02CCH(0H)C02H. Colourless crystals with IH O lost at 60 C. M.p. IhO C (decomp.). Prepared by heating dinitrotartaric acid in aqueous alcohol, taurine, aminoethylsulpbonic acid, C2H7NO3S, NHj CHj CH SOjH. Crystallizes in columns, decomposing at 317 C. In combination with cholic acid it forms one of the bile acids. It is formed in the liver from cysteine. 

The thioethanolamine of coenTyme A and the taurine of taurocholic acid arise from cysteine. 

Taurine (2-aminoethanesulfonic acid 4.235) is an inhibitory neurochemical that probably acts primarily as a neuromodulator rather than a neurotransmitter. It is formed from cysteine, and its accumulation can be prevented by the cardiac glycoside ouabain. Although receptor sites and specific actions cannot be elucidated without an antagonist, taurine has been implicated in epilepsy and, potentially, in heart disease. There are a large number of physiological effects attributed to taurine, among them cardiovascular (antiarrythmic), central (anticonvulsant, excitability modulation), muscle (membrane stabilizer), and reproductive (sperm motility factor) activity. Analogs of taurine, phthalimino-taurinamide (4.236) and its iV-alkyl derivatives, are less polar than taurine and are potent anticonvulsant molecules. 

As shown in Figure 14.7, taurine is a /3-amino sulfonic acid (2-aminoethane sulfonic acid) and can be synthesized from cysteine by three pathways ... 

Taurine is a dietary essential in the cat, which is an obligate carnivore with a limited capacity for taurine synthesis from cysteine. On a taurine-free diet, neither supplementary methionine nor cysteine will maintain normal plasma concentrations of taurine, because cats have an alternative pathway of cysteine metabolism reaction with mevalonic acid to yield felinine (3-hydroxy-1,1-dimethylpropyl-cysteine), which is excreted in the urine. The activity of cysteine sulfinic acid decarboxylase in cat liver is very low. 

It is not known to what extent taurine may be a dietary essential for human beings. There is little cysteine sulfinic acid decarboxylase activity in the human liver and, like the cat, loading doses of methionine and cysteine do not result in any significant increase in plasma taurine. This may be because cysteine sulfinic acid can also undergo transamination to /3-sulfhydryl pyruvate, which then loses sulfur dioxide nonenzymically to form pyruvate, thus regulating the amount of taurine that is formed from cysteine. There is no evidence of the development of any taurine deficiency disease under normal conditions. 

Taurine is a rather specialized topic in the nutritional biochemistry of digestion. Taurine, 2 aminoethane sulfonic acid, is synthesized from cysteine, as shown in Figure 2AI). I hc taurine molecule contains an amino group as well as an acidic group. Taurine has not been found in proteins, although some evidence suggests that it occurs in certain polypeptides. Taurine t ccurs as a component of bile salts and plays an important role in the transport and absorption of lipids. 

Taurine is a sulfur-containing amine synthesized from cysteine. With the excep- QUESTION 15.3... 

The free amino acid fraction also contains 0.02-0.1% taurine (I). As such, taurine should be regarded as a major constituent of this fraction. It is obtained biosynthetically from cysteine through cysteic acid and/or from a side pathway involving cysteamine and hypotaurine (II) ... 

In animal tissues taurine is formed from cysteine. The enzymic decarboxylation of cysteine sulfinate offers the most probable mechanism of this process 22, 26). An alternate possibility is the direct decarboxylation of cysteic acid 26). The participation of cysteate itself in the sulfur metabolism of animal tissues is however uncertain. Acyl CoA derivatives of bile acids can react with either taurine or glycine in the presence of microsomal enz3mies to form tauro- or glycochohc acids 27). 

Scandurra, R., Politi, L., Dupre, S., Moriggi, M., Barra, D., and Cavallini, D., 1977, Comparative biological production of taurine from free cysteine and from cysteine bound to phosphopantothenate. Bull, mol. Biol. Med., 2 172. 

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