Taurine production pathway

The inorganic pathway,the enzymes of which have recently been isolated by Martin et al.(2),should be considered scarcely efficient in taurine production if it can be calculated that only one molecule of labeled taurine over a total of 40,000 has been found after administration of 20 pCi of labeled sulfate(3).In this route sulfate is firstly activated to adenosylphosphosulfate(APS), then to phosphoadenosylphosphosulfate(PAPS)which is transferred to aaminoacrylate(aAA)derived from serine. 

The cysteinesulfinic acid(CSA)pathway has been till now considered the most important route for taurine production in animal tissues.However this claim needs some criticism for the following reasons ... 

These observations allow to conclude that the CSA pathway should be never unique nor the most important metabolic route for taurine production. 

Identification of the endogenous substrate(s) of an enzyme has an immediate effect on the biochemical characterization of the protein, but it can also have a broader impact on downstream metabolic and signaling studies. For example, the discovery of NATs naturally implies the presence of a previously unappreciated biosynthetic pathway responsible for the production of these compounds. Indeed, follow-up experiments confirmed the existence of a membrane-bound enzyme activity in the liver and kidney capable of producing NATs from acyl-CoAs and taurines [29] (Fig. 7a). 

Cysteine is considered a nonessential nutrient because it can be synthesized from methionine via the transsulfuration pathway (Figs. 21-1 and 21-2). Production of cysteine is metabolically important because it serves as a source of sulfur for incorporation into proteins and detoxification reactions. A lack of cysteine needed for incorporation into the structural protein collagen may be responsible for the musculoskeletal abnormalities seen in patients with CBS deficiency. A major metabolic use of cysteine is in the production of glutathionine (y-glutamylcysteinylglycine), an important antioxidant. Another important pathway for cysteine metabolism is its oxidation to cysteinesulfinate, which serves as a precursor for taurine, an amino acid that stabilizes cell membranes in the brain. 

Another metabolic pathway directly from cystine which yields taurine has been proposed by Medes and co-workers. The steps involved are given in reactions 9 to 12 of Fig. 2. The initial oxidation product, cystine disulfoxide, is readily oxidized to sulfate in the animal organism. The oxidation step from cystine to the disulfoxide appears to be reversible, as Bennett found that this compound was capable of replacing cystine in the diet of the rat in equimolar amounts. Decarboxylation of cystine disulfoxide was deduced from the active decarboxylation of cystine with the simultaneous absorption of O2 by an enzyme present in liver brei. 

One of the pathways for the removal of cholesterol from the blood is by degradation to bile acids (Gordon et al., 1957 Lewis, 1958). A deficiency of the sulfur amino acids can be important in limiting the production of taurine, which is required for the formation of taurocholic acid (Filios and Mann, 1954). 

It is known from in vivo experiments that cysteinesulfinic acid (10) taurine, and hypotaurine (11-13) are intermediate products in the metabolic pathway of cysteine. The pathway between cysteine and cysteinesulfinic acid has not been clarified, although a hypothetical mechanism, based on chemical considerations (14,16) is available ... 

The experimental results reported demonstrate that in beef kidney cortices is present a small protein operating a one-side decarboxylation of lanthionine with the production of aminoethylcysteine which is furtherly degraded into cysteamine,hypotaurine and taurine as previously reported(21).At the present time cannot be claimed that this decarboxylase is specific toward lanthionine its activity on lanthionine could be an ancillary activity of another decarboxylase.However this activity on lanthionine demonstrate the presence in animal tissues of a new route which,via cysteamine,contributes to taurine pool.These results further confirm that cysteamine pathway appears to be the most important route... 

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