The Thiol and Sulfide Quantitation Kit provides an ultrasensitive colorimetric assay for quantitating both protein and nonprotein thiols (also called sulfhydryls or mercaptans). In this assay, thiols or inorganic sulfides reduce a disulfide-inhibited derivative of papain, stoichiometrically releasing the active enzyme. The activity of the enzyme is then measured using the chromogenic papain substrate N-benzoyl-L-argining, p-nitroanilide (L-BAPNA). The enzymatic amplification step in this kit enables researchers to detect as little as 0.2 μM thiol - a sensitivity that is about 100-fold greater than that achieved using Ellman's reagent.