Ortho-Nitrophenyl-β-galactoside

Chemical compound

ortho-Nitrophenyl-β-galactoside (ONPG) is a colorimetric and spectrophotometric substrate for detection of β-galactosidase activity. This compound is normally colorless. However, if β-galactosidase is present, it hydrolyzes the ONPG molecule into galactose and ortho-nitrophenol. The latter compound has a yellow color that can be used to check for enzyme activity by means of a colorimetric assay. β-Galactosidase is required for lactose utilization, so the intensity of the color produced can be used as a measure of the enzymatic rate.

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