Hypotaurine from cysteine

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.

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) ... 

Hypotaurine is also found in the epididymis of bull, stallion and dog. The amounts, however, are considerable smaller than in boar, particularly in the dog, where more cysteine is found. The relative extinction of cysteine in testis is in boar as well as in bull and stallion much higher than in cauda epididymidis. This leads to the supposition that hypotaurine is formed from cysteine. 

Cysteine-U- C is taken up by the squid giant axon and metabolized to l C-cysteinesulfinate, C-cysteate, C-hypotaurine, C-taurine, and C02, but not to C-isethionate. When S-cysteine is taken up, S-isethionate is formed. S-Sulfide diffuses into the squid axon, probably as unionized H2 S, and is also converted to S-isethionate. These results are shown in Tables 1 and 2. While the amount of sulfide taken up by the squid axon is much less than the amount of cysteine taken up, the amount of sulfide converted to isethionate as a fraction of the uptake is much greater than the amount of sulfur from cysteine considered in the same way. This suggests that the sulfur of cysteine is metabolized to isethionate via sulfide, or that both may pass through another common intermediate. 

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). 

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. 

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 ... 

An alternate possibility is a direct oxidation of cysteine to sulfenic and sulfinic acid (16), Eq. (3). Medes and Floyd (17) observed that a particulate preparation of liver homogenate catalyzes the production of CO from cystine and it was suggested that cystine may be oxidized to the disulfoxide which is then decarboxylated to cystamine disulfoxide Eq. (4). The latter could be converted to hypotaurine and then to taurine. The sequence of these hypothetical reactions is as follows ... 

The oxidation rate was also dependent on the concentration of hypotaurine in the incubation medium. The reaction appeared to obey simple Michaelis-Menten kinetics (Fig. 2). The kinetic constants were estimated from a linear transformation of the Michaelis-Menten equation in a t against v/s plot. The apparent Michaelis constant ( ) was about 0.2 ramol/1 and the maximal velocity (F) about 0.1 ymol/s X kg. In our crude liver homogenate the apparent for hypotaurine oxidation was of the same order of magnitude as for the partially purified L-cysteine sulphinate decarboxylase (Jacobsen et al., 1964), the preceding enz3nne in the biosynthesis pathway. 

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