Human cathepsin G. Catalytic and immunological properties.

1. The specificity of cathepsin G, a neutral proteinase from human spleen, was examined by use of low-molecular-weight substrates. The enzyme was found to hydrolyse several synthetic substrates also hydrolysed by chymotrypsin, but with different kinetic constants. 2. Maximal activity against benzoyl-DL-phenylalanine 2-naphthol ester and azo-casein was in the range pH 7.5-8.0. 3. The sensitivity of cathepsin G to the action of potential inhibitors was determined, and compared with those of bovine chymotrypsin and subtilisin. Cathepsin G showed the characteristics of a serine proteinase, but was less affected by the chloromethyl ketone of tosylphenylalanine than was chymotrypsin. 4. A rabbit anti-(human cathepsin G) serum was raised, and precipitin lines formed in agarose gel were stained for activity of the enzyme. 5. Cathepsin G was shown to be immunologically identical with the chymotrypsin-like enzyme of the azurophil granules of the neutrophil granulocytes.