The invention concerns substituted dithio-bis-nitrobenzenes (dithio-bis-nitrobenzylamines, dithio-bis-nitrobenzamides or dithio-bis-nitrophenacyls) and their uses as alternative substrates for NAD(P)H-dependent disulphide-reductase enzymes, such as trypanothion reductase and thioredoxine reductase and as means for screening inhibitors of said enzymes. The invention also concerns an enzymatic test using said substrates and assay kits containing them. Said dithio-bis-nitrobenzenes correspond to general formula (I).