Cystine cysteine equivalency

Oxidation is conducted in an acidic medium, usually an acetate or acetate-formate buffer, by using a controlled amount of NBS. Side reactions usually encountered with NBS and proteins, such as oxidation of methionine, cysteine, cystine, histidine and tyrosine, have been generally assumed to be minimal and not to interfere with the tryptophan determination. Experiments with model compounds confirm that the reactivity toward NBS of tryptophan is much higher than that of other amino acid residues. Oxidation of residues other than tryptophan is revealed by increased consumption of NBS by the protein. Usually, 2-4 equivalents of reagent are needed for complete oxidation of tryptophan, but with some proteins, an excess of up to 20 equivalents may be needed (230, 372,374). Some proteins possess tryptophan residues which react very slowly or not at all under the usual conditions of assay the reaction should then be carried out in 8 M urea. 

Simple inorganic vanadium(V) compounds are reduced by thiolates such as cysteine and cysteinemethyl ester (CysMe) to VO +. Further reduction to V by CysMe is achieved on addition of H2(edta) , which stabilises the state by formation of [V (edta)H20]. For this reduction to occur it is cmcial that less than 1 equivalent of edta is employed, apparently in order to provide a site of direct interaction between VO + and the thiol function. CysMe is oxidised to cystine. Cysteine does not reduce... 

The mass of a cystine-containing protein will be two mass units smaller than the equivalent cysteine-containing protein because two —SH groups are oxidized to —S—S—. To confirm this finding, it would be necessary to treat the protein with a reducing agent and compare the masses of the reduced and nonreduced samples. 

Oxidation of Cystine, Cystine has been reported to initiate a series of reactions in which the S—S bond remains intact. An oxidation product is cystine disulfoxide, RSOSOR. This compound can yield both cysteine and sulfate in the intact rat, but most of it is converted to taurine. Both carboxyl groups of cystine disulfoxide are removed by a decarboxylase different from cysteic decarboxylase. Oxidation and hydrolysis of the decarboxylation product would yield two equivalents of taurine, but the individual steps of this conversion have not been isolated. 

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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