FAQ: Why is the unit definition assay for Thermolabile Proteinase K (NEB #P8111) different from the unit definition assay for Proteinase K, Molecular Biology Grade (NEB # P8107)?

Proteinase K, Molecular Biology Grade (#P8107) uses a traditional denatured hemoglobin substrate to measure activity (1). This traditional assay allows comparison of Proteinase K activity to the current and historical market.  Over the years, casein has also been used as a substrate with less precision then hemoglobin. 

Thermolabile Proteinase K (#P8111) uses a more modern approach in which a nitroanilide substrate is used to measure activity by visible absorbance. This assay is preferred for its direct readout of proteolysis, sensitivity and 96-well plate compatibility. For the development of the direct assay see Pozgay et al. (2).



  1. Anson, M. L. et al. (1938) J. Gen. Physiol., 22, 78-89. “The estimation of pepsin, trypsin, papain and cathepsin with hemoglobin.” 
  2. Pozsgay, M. et al. (1979) Eur. J. Biochem., 95, 115-119 “A method for designing peptide substrates for proteases. Tripeptidyl-p-nitroanilide substrates for Subtilisin Carlsberg”