Use of 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole for determining cysteine and cystine in cereal and legume seeds

A new colorimetric method has been developed for the determination of chemically available cysteine and half-cystine in maize and legume seeds. The method is based on the reduction of cystine with aqueous NaBH(4)/urea/EDTA solution and the reaction of cysteine with 7-chloro-4-nitrobenzo-2-oxa-1,3-diazole (NBD-Cl) in 0.2 M sodium acetate/HCl buffer (pH 2.0) to form a greenish product showing maximum absorbance at 410 nm. The method is simple, accurate, and highly specific for cysteine, in the presence of other naturally occurring amino acids. The method was applied to the determination of cysteine and of cysteine plus half-cystine in some seed meals. There was a good correlation between the results obtained using this method and those obtained using Ellman's reagent 5, 5-dithiobis(2-nitrobenzoic acid)]. There was also a good correlation between the results obtained using this method and cysteic acid values calculated from amino acid analysis of the samples.