Taurine synthesis

Taurine was discovered in 1827 in ox hUe, where it is conjugated with the bile acids. It was later shown to be a major excretory product of the sulfur amino acids methionine and cysteine. Until about 1976, it was assumed that it was a metabolic end-product whose only function was the conjugation of bile acids. In the rat, taurine synthesis accounts for 70% to 85% of total cysteine catabolism. 

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. 

The increased glycine/taurine ratio in bile from hamsters with amorphous pigment stones is not unexpected, considering the composition of the stones, but the reason for this change is unclear. The increase ratio results from a relative deficiency of taurine and could be produced by a decreased protein intake, which supplies the sulfur-containing amino acids used for taurine synthesis, or by an excessive fecal loss of taurine conjugates, as in... 

Transsulfuration is facilitated by the action of two vitamin Be-dependent enzymes, cystalhionine-p-synthase (CBS), the enzyme deficient in homocystinuria, and cystathionine-Y-lyase (CTH). CBS catalyzes the condensation of homocysteine and serine to cystathionine, and CTH subsequently catalyzes the hydrolysis of cystathionine to cysteine and a-ketobutyrale. Cysteine is important in protein synthesis and taurine synthesis and is a precursor to glutathione, a strong antioxidant and essential compound in detoxification of many xenobiotics [8,10,11]. 

The presence of the enzymic activities Involved In taurine synthesis In the nuclear fraction Is Indicative of their location at level of photoreceptors. 

Administration of methoxamine, an a-agonist and vasoconstrictor, to rodents also produced a decrease in cardiac tissue and an increase in blood levels of taurine. On the other hand, taurine levels in the liver increased approximately 2.5- to 3-fold. While it is known that liver is capable of sequestering exogenous taurine, it can be calculated that the loss of taurine from the heart was quantitatively not sufficient to account for the increase in the liver. Thus the source of the increased taurine content of the liver is not known. Whether methoxamine stimulated de novo taurine synthesis in the liver or caused a loss of taurine from tissues other than the heart (such as skeletal muscle) which in turn was taken up by the liver requires further experimentation. 

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). 

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