A simple and specific colorimetric determination of cysteine with p-dimethylaminocinnamaldehyde.

A simple and specific assay was developed for the determination of cysteine. Cysteine was reacted with p-dimethylaminocinnamaldehyde in methanol at 60 degrees C for 2 h in the presence of sulphuric acid as a catalyst. Absorbance was measured at 587 nm; the chromogen was stable at 60 degrees C for at least 5 h. The colour reaction was specific for cysteine, and was negative for other amino acids but positive for cysteamine. The absorbance followed Beer's law in the range from 0.825 to 413 mumol/l, and the apparent molar lineic absorbance was calculated to be 5000 m2 . mol-1. This procedure is applicable to the determination of cysteine and cystine in amino acid mixtures and in plasma, since cystine can be quantitatively reduced to cysteine with dithiothreitol under alkaline conditions. Recovery of cysteine and cystine from human plasma was 98.5 to 103% and 94.2 to 102%, respectively.