Cathepsin G and neutrophil elastase activities in cathepsin G-deficient mice. The activity of cathepsin G and neutrophil elastase was determined by the ability of total bone marrow protein extracts to cleave colorometric peptide substrates in vitro, as described in Materials and Methods. (A) A peptide substrate for cathepsin G (N-Succinyl-Ala-Ala-Pro-Phe-pNA) is used. Cathepsin G−/− mice have no detectable conversion of this substrate even after 30 minutes of incubation at 37°C. (B) The conversion of the neutrophil elastase-specific peptide (N-Methoxysuccinyl-Ala-Ala-Pro-Val-pNA) is shown. Wild-type and cathepsin G−/− mice have equivalent amounts of neutrophil elastase activity. These experiments were performed 3 times with identical results.