Cathepsin G from human polymorphonuclear leukocytes cleaves human IgM

Abstract

Cathepsin G, the chymotrypsin-like enzyme from human polymorphonuclear leukocytes, cleaves human IgM and produces two major fragments that closely resemble those released by leukocyte elastase digestion of IgM. An F(ab) 2μ-like fragment, mol. wt 140,000, retains some reactivity with an anti-Fcμ antiserum and is antigenically deficient with respect to the IgM subunit. The other fragment is an Fabμ-like product with a mol. wt of 54,000. Both cathepsin G fragments are distinguishable from the elastase counterparts by immunochemical analysis. An Fcμ fragment could not be recovered. The kinetic course of the cleavage shows that cathepsin G produces Fabμ fragments at a higher rate than F(ab) 2μ, whereas the contrary is valid for elastase. Beside the two major fragments and low mol. wt peptides, cathepsin G releases also a product with the same mol. wt and immunological reactivity as the IgM subunit. The biological significance of the interaction between IgM and leukocyte proteinases is discussed.