Proteinase K is a highly active stable endopeptidase with a broad spectrum of action was isolated by E. Merk's Darmstadt Biochemical Research Department in 1970 from a culture filtrate of the fungus, Tritirachium album Limber. This fungus is able to grow on Keratin (e.g., wool, horn particles) as the sole source of carbon and nitrogen. The isolated protease was, therefore, given the K designation.
Proteinase K is a stable and highly reactive serine protease. Evidence from crystal and molecular structure studies indicates the enzyme belongs to the subtilisin family with an active-site catalytic triad (Asp39-His69-Ser224). It is stable in a broad range of environments: pH, buffer salts, detergents (SDS), and temperature. In the presence of 0,1 to 0,5% SDS, proteinase K retains activity and will digest a variety of proteins and nucleases in DNA preparations without compromising the integrity of the isolated DNA.

• Foreign activity: 7,5 to 12,00 (denatured hemoglobin as substrate) (Lit.)
• RNase <5×10⁻⁴ U/mg
• DNase <5×10⁻⁴ U/mg
• Soluble in water

Proteinase K is useful for the proteolytic inactivation of nucleases during the isolation of DNA and RNA. It is also useful in removal of endotoxins bound to cationic proteins such as lysozyme and ribonuclease A. It is reported to be useful for the isolation of hepatic, yeast, and mung bean mitochondria. It is useful in determination of enzyme localisation on membranes. It is also used in treatment of paraffin embedded tissue sections to expose antigen binding sites for antibody labeling. Proteinase K is used in digestion of proteins from brain tissue samples for prions in Transmissible Spongiform Encephalopathies (TSE) research.