Taurine, from cysteic acid

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). 

Fig. 24-25), another component of nervous tissue. Cysteic acid can arise in an alternative way from O-acetylserine and sulfite (reaction 1, Fig. 24-25), and taurine can also be formed by decarboxylation of cysteine sulfinic acid to hypotaurine and oxidation of the latter (reaction m). Cysteic acid can be converted to the sulfolipid of chloroplasts (p. 387 Eq. 20-12). 

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.

Growing chicken128 and hens129 utilize sulphate sulphur for cystine synthesis. Biological radio-tracer experiments130 with Na235S04 (10 fiCi) have shown that over 65% of the 35S administered to a 24-hour-old embryo is incorporated into taurine of the chick. No radioactive cystine, methionine or cysteic acid was detected in the hydrolysate obtained from the embryo and only a small portion of total taurine-35S occurs as taurocholic acid. The embryo is unable to utilize sulphate sulphur for cystine synthesis. 

The in vivo mechanism of 35S-taurine formation from 35S-cysteine in the rat has been studied by Awapara and Wingo148. Ten minutes after injection, large amounts of 35S cysteine and traces of sulphate-35S were found only in the liver. After 20 minutes small amounts of 2-aminoethanesulphinic-3 5 S acid were also found (equation 81). Taurine began to appear in the liver 30 minutes after the injection. Two hours after administration, analyses for [35S]taurine, alanine, [35S]cysteic acid and 2-aminoethanesulphinic acid were carried out in liver, kidney, heart and spleen of the rats. [3 5S]Cysteic acid had been detected only when large amounts of 35S-labelled cysteine were injected. It has been suggested that the degradation of [35S]cysteine in vivo proceeds in rats according to equation 81. Formation of 2-aminoethanesulphinic acid and its oxidation to taurine is a preferred pathway. Much less [35S]sulphate than 2-aminoethanesulphinic-35S acid and taurine-35S had been found one hour after incorporation of 35S-labelled cysteine. 

The aerobic metabolism of cystine-3 5S by chicken embryo, investigated166 both in vivo and in vitro, resulted in the formation of cystinedisulphoxide-35S (in vitro only), [35S] cystinesulphinic acid, [35S] cysteic acid, [35S]taurine and sulphate-35S. Hypotaurine has been detected neither in vivo nor in vitro. This indicates that, contrary to what had been observed in mammalian liver, hypotaurine is not the precursor of taurine in chicken embryo (equation 86). The enzyme decarboxylase, which effectively decarboxylates [35S]cysteic acid, does not act on cysteinesulphinic acid. Sulphate-35S may be produced also by the desulphination of cysteinesulphinic acid (equation 87) or from some other... 

Enzymatic decarboxylation169 of L-cysteic acid-35S (equation 86) by the tissues of chicken embryo has been investigated by Fromageot and coworkers170. Enzyme preparations from liver appeared to be the most active, and the authors determined [35S]taurine as well as unreacted 35S-cysteic acid, 35S-/Lsulphonylpyruvic acid and 35S-sulphate. The reaction is inhibited by L-cysteine sulphinic acid, by DL-a-methylcysteic acid, CH2ICOONa, NaCN and by hydroxylamine. The enzyme is activated by pyridoxal phosphate. 

The metabolism of 35S-labelled sulphur amino acids in marine and fresh water invertebrates has been studied and reviewed by Awapara and coworkers179 180. The general conclusion drawn from these studies was that the metabolism of sulphur-bearing amino acids in two molluscs studied is qualitatively the same as in mammals. Taurine, which serves as an osmoregulator in marine molluscs, is formed either by decarboxylation of cysteic acid (in Rangia cuneata) or by oxidation of hypotaurine (in Mytilus edulis), derived from cysteinesulphinic acid by decarboxylation. In Arenicola cristata only the terminal reactions are different. Methionine and cysteine sulphur incorporates into taurocyamine by transamidation between taurine and arginine. 

Cysteinesulfinic Acid. Cysteine is oxidized by enzyme systems present in bacteria and in liver to the corresponding sulfinic acid. It has been suggested that the unstable sulfenic acid is an intermediate in this oxidation. The nature of the reaction that produces cysteinesulfinic acid is not known. The subsequent metabolism of the sulfinic acid may proceed by any of three pathways. One involves further oxidation to cysteine-sulfonic acid, cysteic acid. The enzyme responsible has not been separated from the system responsible for the formation of cysteinesulfinic acid. Cysteinesulfonic acid is decarboxylated to taurine (I) by the decarboxylase mentioned previously (p. 284). 

Enzyme preparations that oxidize cysteine to cysteic acid have been obtained from liver. Conversion of cysteic acid to taurine was indicated by Virtue and Doster-Virtue, who observed an increase in the output of taurocholic acid in the bile of dogs upon administering cysteic acid along with cholic acid. 

In an extensive investigation of the metabolic fate of cysteic acid, Medes and Floyd " observed the anaerobic decarboxylation of cysteic acid to taurine by an enzyme preparation obtained from kidney. A similar enzyme had been reported to occur in liver by Blaschko, although this was not confirmed. ... 

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). 

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... 

In a spontaneous dismutation reaction between two molecules of this acid, one molecule of sulfinic acid is created and at the same time one molecule of cysteine is regenerated. The sulfinic acid thus formed can serve two different purposes. First, it may bind an additional oxygen atom and thus form cysteic acid. This oxidation would conform with the ability of the sulfinic acid to produce taurine, which was experimentally demonstrated in dogs bj Virtue and Doster-Virtue (135). Second, the sulfur of the cysteinesulfinic acid may be oxidized to sulfate which would entail a fission of sulfur from the carbon chain. The mechanism of the fission and of this oxidation has not been completely elucidated. 

Sulfur-containing amino acids related to cysteine and methionine are cysteic add and its decarboxylation product taurine. Lanthionine, with one sulfur atom less than cystine, has been isolated from wool hydrolyzates. Homocystdne is the de-methylation product of methionine. 

The latter is decarboxylated by the action of cysteic decarboxylase and the oxidized sulfur is excreted in the form of taurine. At that time it is of interest to compare the action of desulfinicase with the action of decarboxylase. There seems to exist a certain analogy between these enzymes, just as there is an analogy between the sulfinic and carboxyl groups on which they act. But the experiments carried out to date do not show whether the desulfinicase acts by splitting off sulfur dioxide from cysteine sulfinic acid according to the equation ... 

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