ONPG

ONPG (ortho-Nitrophenyl-ß-galactoside) is a lactose analog and a chromogenic substrate for ß-galactosidase. In the presence of the enzyme, ONPG is hydrolyzed into galactose and ortho-nitrophenol, turning from colorless to yellow. In the colorimetric assay for ß-galactosidase activity, this color change can be measured spectroscopically between 405 nm to 420 nm. ONPG is suitable for use in reporter assays and immunoassays that involve a ß-galactosidase label, such as ELISA.