Oxidation Mechanisms of Cysteine Sulfur

OXIDATION MECHANISMS OF CYSTEINE SULFUR Direct Oxidation of Cysteine Sulfur 

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. 

Oxidation to Cystine. The types of oxidation of cysteine to cystine differ depending on the nature of the hydrogen acceptor in the system oxygen, sulfur, aminoacrylic acid, pyruvic acid, etc. The oxidation of cysteine in the presence of aminoacrylic acid and pyruvic acid will be discussed later in conjunction with the action of cysteine desulfhydrase. 

Oxidation in the Presence of Oxygen. One knows from Sakuma s experiments (102) that, contrary to what was believed previously, carefully purified cysteine in neutral solution, completely free of traces of metals, is practically not oxidized by oxygen from air. On the contrary, this oxidation is very strongly catalyzed by iron, copper, and manganese (71,102, 141,143) even if the metals present are traces. In animal tissues, the role of oxidation catalyst for cysteine is accomplished by cytochrome oxidase in the presence of cytochrome c. The action of this system on cysteine has been studied in detail by Keilin (66) and by Medes (85), and is represented by  

This action is inhibited by hydrogen cyanide, sodium azide, and carbon monoxide. The inhibition by carbon monoxide is suppressed by light (66). It should be pointed out that cytochrome oxidase, with or without cytochrome c, does not exert any oxidizing action on methionine or cysteine sulfinic acid. 

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