Taurine synthesis from cysteine

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. 

For studies of taurine synthesis from cysteic acid incubations were conducted according to MacDonnell and Greengard (1975) in the presence of 20mM S-cysteic acid. For studies of taurine synthesis from S-cysteine the incubations were done according to Misra and Olney (1975). Pyridoxal - 5 - phosphate (0.5inM) was present in all incubations. Taurine synthesis was evaluated by thin layer chromatography as previously described (Schmidt, Berson, Watson and Huang 1977). 

Homocysteine (Hey) metabolism is closely linked to that of the essential amino acid methionine and thus plays a central role in several vital biological processes. Methionine itself is needed for protein synthesis and donates methyl groups for the synthesis of a broad range of vital methylated compounds. It is also a main source of sulphur and acts as the precursor for several other sulphur-containing amino acids such as cystathionine, cysteine and taurine. In addition, it donates the carbon skeleton for polyamine synthesis [1,2]. Hey is also important in the metabolism of folate and in the breakdown of choline. Hey levels are determined by its synthesis from methionine, which involves several enzymes, its remethylation to methionine and its breakdown by trans-sulphuration. 

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.

Chick embryo utilizes 35S-sulphate for the synthesis of 35S-taurine, but the amount of sulphate-sulphur present in the unincubated egg is insufficient to furnish S necessary for the taurine synthesized. Injection167 of L-methionine-35S or of L-cysteine HCl-35S into the egg white, and determination of the distribution of 35S in the chick hatched from the incubated egg revealed that with [35S]cysteine 10% of the administered 35S is located in the chick as taurine, 12% as sulphate, 1.3% as methionine and 49% as cystine. With methionine-35S 9.1% was recovered as taurine, 10% as sulphate, 43% as cystine and 35% as methionine. These findings indicate that methionine is converted to cystine during embryonic development, but significant amounts of cysteine-35S were incorporated also into methionine. Possibly, trans-sulphonation from methionine to cysteine is reversible to some extent in the chick embryo, similarly to what has been found in young rats168. 

The conjugation of free bile acids to taurine and glycine is a synthesis of an amide bond comparable to the synthesis of gluthatliione from glutamic acid, cysteine, and glycine, with the synthesis of hippuric acid from benzoic acid and glycine or with tlie synthesis of protein from amino acids. 

---