Stepwise cleavage of rabbit immunoglobin G by papain and isolation of four types of biologically active Fc fragments

Four types of Fc fragments of different sizes were isolated by papain treatment of rabbit immunoglobulin G under various conditions and by subsequent chromatographic procedures. 1. Brief digestion at neutral pH without reduction produced a molecule in which the Fab and Fc fragments were still linked by a pair of labile disulphide bridges, and the Fc fragment released by cleaving these bonds, called 1Fc fragment, contained a portion of the ;hinge' region including an interchain disulphide bridge. Both complement-binding and guinea-pig skin-binding activities were retained by this fragment, which had mol. wt. 48000. 2. Prolonged digestion at neutral pH of immunoglobulin G whose labile inter-heavy-chain disulphide bridges had been reduced removed the ;hinge' region, giving mFc fragments (mol. wt. 46000), which lacked the capacity to bind guinea-pig skin but retained the antigenic as well as the complement-binding activities of 1Fc fragment completely. 3. Digestion at pH5.0 yielded a smaller fragment, sFc (mol. wt. 40000), which was no longer able to bind complement. Though the antigenic structure was intact, sFc fragment was curiously unable to precipitate with antibodies to the N-terminal determinants. 4. Fragment stFc (mol. wt. 25000), representing the C-terminal portion of Fc fragment, was formed from all the larger fragments by digestion at pH4.5. Only the C-terminal antigenic determinants were retained by stFc fragment.