ONPG is the common abbreviation for (ortho) 2-Nitrophenyl-β-D-galactopyranoside.
ONPG is used in colourimetric and spectrophotometric assays for the detection of β-galactosidase activity which are of interest in clinical, environmental, food and molecular biology arenas. ONPG is hydrolysed by β-galactosidase to yield galactose and ortho-Nitrophenol, which is yellow in colour and absorbs light at 420 nm. The intensity of the colour can be used to determine the rate at which β-galactosidase can hydrolyse lactose. The impact of experimental factors on the reproducibility of β-galactosidase assays has been reported.
In common with several other β-galactosidase substrates such as X-Gal, ONPG does not act as an inducer of genes under the control of the lac operon in bacteria such as E. coli. In the absence of a lactose analogue such as IPTG (Product Code: E10593 I) transcription of β-galactosidase will not take place and ONPG will not be hydrolysed.