4-Nitrophenyl 2,3,4,6-tetra-O-acetyl-b-D-thiogalactopyranoside

4-Nitrophenyl 2,3,4,6-tetra-O-acetyl-b-D-thiogalactopyranoside (NPG) is a fluorogenic substrate that can be used in the detection of beta-galactosidase activity. It reacts with 4-nitrophenol to form an orange product at pH 6.0. The fluorescence intensity increases with increasing concentrations of the substrate and enzyme. The reaction is inhibited by borax, which competes for the enzyme active site. This product has been used as a chromogenic substrate for beta-galactosidase and as an enzyme substrate for beta-galactosidase in diagnostic tests for galactosemia and other carbohydrate metabolism disorders. NPG has also been used in chemiluminescent assays to detect beta-galactosidase activity in culture media and as a bioluminescence substrate in the detection of bacterial.