Rate assay of N-acetyl-beta-D-hexosaminidase with 4-nitrophenyl N-acetyl-beta-D-glucosaminide as an artificial substrate

A rapid and accurate rate assay method for N-acetyl-beta-D-hexosaminidase (EC 3.2.1.52, also known as N-acetyl-beta-D-glucosaminidase, or NAGase) using 4-nitrophenyl N-acetyl-beta-D-glucosaminide (NP-GlcNAc) as an artificial substrate was developed using diethylaminoethyl-alpha-cyclodextrin (DEn-CD, where n is the number of diethylaminoethyl groups introduced to alpha-cyclodextrin), as an additive to ionize 4-nitrophenol to yellow-colored 4-nitrophenoxide at pH near 5, where the Enzyme acts optimally. A possible recipe for the rate assay of NAGase is as follows. Prepare a stock solution containing 4.8 mmol/l NP-GlcNAc and 1% DEn-CD (n is preferably near 17) in 0.1 mol/l glycolate buffer, pH 5.50. Introduce the stock solution and a properly diluted sample (urine or other body fluid) to a reaction cell placed in a spectrophotometer at a ratio of 1:1, and monitor the absorbance at 400 or 420 nm. The reaction rate (enzymatic activity) can be conveniently read directly from calibration plots prepared for a given lot of DEn-CD sample, or can be calculated from the rate of the absorbance increase, ionization degree of 4-nitrophenol at pH 5.50, and the millimolar absorbance coefficient of 4-nitrophenoxide in the presence of 0.5% DEn-CD.