Thionitrobenzoate was prepared from 5,5′-dithiobis-(2-nitrobenzoic acid), and its reaction with several disulphide-containing proteins was examined under various conditions. No evidence for any cleavage of disulphide bonds was obtained. The claim by Robyt et al. [Arch. Biochem. Biophys. (1971) 147, 262–269] that thionitrobenzoate allows the quantitative determination of disulphide bonds in proteins is not substantiated.
