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 alpha 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 kallikrein), was found to have bioactivities such as
leukocytosis-inducing and immunosuppressive activities.