Cysteic acid decarboxylation

To the methods reviewed in an earlier volume 1 may be added the preparation by the oxidation of cystamine 2 and by the decarboxylation of cysteic acid.3 The method given in the procedure has appeared recently in the literature.4... 

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). 

Oxidation to cysteic acid, followed by decarboxylation to taurine. Cysteic acid and cysteine sulfinic acid decarboxylase activities occur in constant ratio in various tissues, and it is likely that both substrates are decarboxy-lated by the same enzyme. In general, cysteine sulfinic acid is the preferred substrate, and there is little formation of taurine by way of cysteic acid. 

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... 

Enzymic decarboxylations of [1 -uC]cysteinesulphinic acid and /I -uC]cysteic acid in mammalian tissues... 

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. 

HT is oxidized to TA, probably with the help of HT oxidase70. Recently Fellman presented evidence that HT is first oxidized by a hydroxyl radical to bis-aminoethyl-a-disulfone 26. The hydroxyl radical is generated by a liver microsomal NADPH oxidase. 26 has been prepared from HT in the presence of chemically or enzymatically generated radicals and was characterized by NMR and mass spectrometry. It has been found in male sexual tissue which contains HT and TA in respectively high concentrations83. A relatively minor pathway consists of oxidation of CSA to cysteic acid (cysteinesulfonic acid, CA 27) and its decarboxylation to TA under the influence of CSA decarboxylase70. 

Pharmacological evidence was obtained several years ago that indicated that tryptophan is decarboxylated to tryptamine by both animal and bacterial enzymes. More recent studies have failed to detect this reaction, but instead have shown decarboxylation to occur only after oxidation of the indole nucleus to yield 5-hydroxytryptophan. Decarboxylation of 5-hydroxytryptophan produces 5-hydroxytryptamine, serotonin, which has important, though incompletely defined functions in animal physiology. In some animal livers there is an enzyme that decarboxylates cysteic acid to taurine. Glutamic decarboxylase has been found in animal brain, where it is responsible for the formation of 7-aminobutyric acid. This product has been implicated in nervous function as an inhibitor of synaptic transmission. ... 

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). 

A second pathway of cysteinesulfinic acid involves decarboxylation to hypotaurine (II). This reaction is followed by oxidation to taurine. This pathway appears to be more important than the cysteic acid pathway. 

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. ... 

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). 

J.G. Jacobsen and L.H. Smith, Jr., Comparison of decarboxylation of cysteine sulphinic acid-l-C and cysteic acid-l-C by hximan, dog and rat liver and brain. 

Cysteine, whose synthesis we have just seen, is degraded chiefly through these two reaction sequences Either to pyruvate + H2S + NH3 (perhaps through 0-mercaptopyruvate, or via the mechanism of a,/3-elimination, Section 4) or through oxidation to cysteic acid and subsequent decarboxylation to taurine ... 

The work by Virtue and Doster-Virtue (135) has shown that formation of taurine takes place. These authors experimented with dogs which were properly starved and provided with bile fistulas. They introduced cholic acid and at the same time cysteic acid by ingestion and concluded that formation of taurocholic acid was unquestionable. Such a conversion of cysteic acid to taurine obviously implies a decarboxylation ... 

Medes and Floyd (87) studied the action of different tissue slices in the presence of air and ascertained the presence of cysteic acid decarboxylase in spleen, but, in contrast to Blaschko s findings, did not find it in the liver, heart, or muscles. The decarboxylating and deaminating activities of the intestinal mucosa appear at the same time the nature of the resulting product is not known to date. On the other hand, comparison of the action of slices of spleen with ground spleen, under aerobic as well as anaerobic conditions, shows that slices of spleen under both conditions, and ground spleen under anaerobic conditions, convert cysteic acid into... 

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 ... 

---