Evidence for presence of an internal thiolester bond in third component of human complement

Treatment of the third component of human complement (C3) with methylamine results in a loss of hemolytic function and the appearance of a thiol group. Studies with [14C]methylamine have indicatd a stoichiometric and covalent reaction with the native protein. Hydrazine-inactivated C3 and C3b prepared with bovine trypsin were unreactive with [14C]-methylamine. Alkylation experiments with [1-14C]iodoacetamide have further established a 1:1 correspondence between methylamine incorporation and expression of the reactive thiol. Autoradiographic analyses of [14C]methylamine-treated C3 and methylamine-inactivated [1-14C]carboxyamidomethylated C3 after NaDodSO4/polyacrylamide gel electrophoresis have shown a specific incorporation of each radiolabel into the alpha polypeptide chain. [14C]Methylamine-treated C3 was immobilized on Sepharose 4B by reaction of the protein thiol with a mixed disulfide. Digestion with bovine trypsin in 4 M urea released 96% of the bound absorbance (at 280 nm) units; the radiolabel remained associated with the Sepharose beads. Peptide material labeled with 14C was eluted with dithiothreitol, carboxymethylated with [3H]iodoacetic acid, and chromatographed on Sephadex G-75. On Edman degradation S-[3H]carboxymethylcysteine was released at step 9 and gamma-glutamyl[14C]-methylamide was released at step 12. We interpret these data to indicate the presence of an internal thiolester bond in native C3. In addition, evidence is presented for an identical reactive site in alpha 2-macroglobulin.