Cystine-cysteine interconversion

The RSH/RSSR interconversion is well known in organic chemistry due to its biological importance in systems like cysteine-cystine, glutathione and the corresponding disulfide, and Q -lipoic acid and related dithiol. 

There are at least two types of enzyme systems involved in the formation and breakage of disulfide bonds of cystine residues in proteins. A thiol-disulfide interchange enzyme (protein disulfide-isomerase EC 5.3.4.1 other name, S-S-rearrangase) was first described in 1963 ( 47) and was subsequently purified from beef liver (48,49). The molecular weight of the enzyme is 42,000. The enzyme contains three half-cystine residues, one of which must be cysteine in order for the enzyme to be active. The enzyme catalyzed the rearrangement of random incorrect pairs of half-cystine residues to the native disulfide bonds in several protein substrates. Low levels of mercaptoethanol were required for activity unless the enzyme was reduced prior to use. The efficiency of the enzyme in catalyzing the interconversion of disulfide bonds was found to be a function of the number of disulfide bonds in the substrate. Purification of a thiol-disulfide interchange enzyme from Candida claussenii has been described recently (50). 

Sulfhydryl-Disidfide Interconversion. Cysteine is readily oxidized to the disulfide, cystine. This oxidation is catalyzed very efficiently by traces HjC—SH HS—CHj H2C—S—S—CH2... 

The generally accepted dietary equivalency of cystine and cysteine indicates that tissues must contain an efficient enzyme system which catalyzes the interconversion of the thiol and disulfide forms of this amino acid. The biological role of cystine reductase may be further emphasized by the current view that reduction of the disulfide to the thiol is a necessary intermediary step for its further metabolism. 

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