Covalent binding and hemolytic activity of complement proteins.

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Abstract
The inactivation of the 3rd component of complement (C3) by hydroxylamine is reported. C3 hemolytic and covalent binding activities decline with identical kinetics, demonstrating a direct correlation between the 2 activities. Apparently covalent, surface-bound C3b is hemolytically active. The inactivation of C3 is 1st order with respect to hydroxylamine. C3 inactivation with [14C]methylamine was also studied. The inactivation corresponds quantitatively with the labeling of C3 in the C3d domain. The data obtained support the following hypothesis: there is an internal thioester within C3 which becomes highly reactive on activation to C3b, and C3b binds to receptive surfaces by transfer of the acyl function of the thioester to a hydroxyl group on the receptive surface. This proposed model for the reaction of C3 with receptive surfaces also applies to C4, which binds to membrane surfaces covalently and is able to be inactivated by hydroxylamine and methylamine. C5 is not inactivated by treatment with the amines. [Complement components from human and guinea pig were used.].