Cystine sulfur, oxidation, direct

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. 

These 3 sulfur-containing amino acids and their derivatives are susceptible to oxidation and other destructive reactions. Even when great care has been taken to remove all oxygen from hydrolysis tubes, considerable losses of cysteine and cystine are found after acid hydrolysis, and this usually prevents direct quantitation of these amino acids in proteins. However, total cysteine plus half-cystine content may be determined as cysteic acid after performic acid... 

Although methionine is the only sulfur-containing essential amino acid, it has not been considered as part of supplementation regimes. Since cysteine easily oxidizes to cystine, which has a poor solubility, it is generally supplemented in the form of n-acetylcysteine (NAG). Both directly and indirectly, as a precursor for glutathione, NAG has attracted attention as a potentially protective agent against... 

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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