Limited Proteolysis of a Chemically Modified Third Component of Human Complement, C3, by Cathepsin G of Human Leukocytes

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Abstract
We have investigated the limited proteolysis of the third component of complement, C3, by a human leukocyte protease, cathepsin G, by using a chemically modified C3, which was prepared by treatment of C3 with methylamine and a fluorescent thiol reagent, N-(dimethylamino-4-methylcoumarinyl)-maleimide (DACM) and was thus named DACM-C3me. Although native C3 was hardly cleaved by cathepsin G, DACM-C3me was cleaved by cathepsin G into three major fragments, which were termed C3c-G (150,000 daltons, 150 kd), C3d-G (25 kd), and C3a-G (10 kd). C3c-G was composed of four disulfide-linked polypeptide chains of 75 kd, 35 kd, and two 25 kd. C3d-G and C3a-G were single-chain fragments derived from the a chain. The N-terminal sequence of C3d-G was determined as Thr-Glu-Asp Ala-Val-, suggesting that cathepsin G released C3d-G by cleaving a Met-Thr peptide bond which is located at 19 residues toward the N-terminal from the cysteinyl residue forming an internal thiolester linkage in native C3. C3d-G, like C3d-K (a C3d fragment produced by the action of plasma kalli-krein), was found to have bioactivities such as leukocytosis-inducing and immuno-suppressive activities.