Cysteinesulfenic acid

It is generally assumed, however, that cysteinesulfenic acid, in which the sulfur is in a lower oxidation state, is the first oxidation product of cysteine. In addition to the production of the sulfenic acid by oxidation, it is also proposed that it is formed by hydrolytic cleavage, as showm in equation 2. 

No convincing evidence is available at present for the formation of cysteinesulfenic acid by either hydrolysis or oxidation in biological systems. If the sulfenic acid is formed, it is unstable under physiological conditions and undergoes spontaneous dismutation into cysteine and cysteinesulfinic acid, according to equation 3 (see reaction 3, Fig. 2). 

The hydrolysis reaction discussed above leads to a primary direct oxidation of cystine sulfur, which is converted to cysteinesulfenic acid. We have discussed the fate of this acid on page 386. 

Contrary to what takes place in the case of methionine, the direct oxidation of the sulfur of cysteine plays a very important role in the metabolism of this compound. This oxidation leads, depending on conditions, to cystine, cysteinesulfenic and cysteinesulfinic acids, or to the corresponding sulfonic acid, cysteic acid. 

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