Proteinase K is a nonspecific serine protease. It is not inactivated by metal ions, chelating agents (e.g. EDTA), sulfyhydryl reagents or by trypsin or chymotrypsin inhibitors. It is stable over a wide pH (4-12.5), with optimal activity at pH 6.5-9.5. Proteinase K activity can be stimulated by addition of denaturing agents SDS and urea. The optimum temperature for the enzyme is 65°C; it is twelve times more active at 65°C than at 25°C. However, rapid denaturation of the enzyme will occur at temperatures above 65°C. Thus, protein digestion is usually carried out at 55°C