A colorimetric method for cysteine determination based on the peroxidase-like activity of ficin

The concentration of L-cysteine (L-Cys) provides important information about specific physiological functions and disease diagnosis. In this work, we provided a facile colorimetric approach for L-Cys detection with high sensitivity and specificity. Ficin acted as a peroxidase mimic and catalyzed the chromogenic reaction of TMB and H2O2. However, after the introduction of L-Cys, L-Cys competed with TMB for H2O2. The competition is generally won by the more-reactive L-Cys resulting in fading of the chromogenic reaction. The detection limit of L-Cys can be as low as 20 nM (3σ). In addition, the assay can also be successfully applied to detect L-Cys in biological fluids, indicating its potential in biological sample analysis. Our work could promote the application of the enzyme mimicking activity of ficin in medical diagnostics and biochemistry.