Gamma globulin and antibody obtained from inbred C3H mice are split by papain and cysteine into fragments roughly one-third the size of the original Molecule (S20,w = 3.5S). The papain digests were characterized by starch gel electrophoresis and immunological methods. The highly heterogeneous fragments could be divided into two groups with distinct antigenic determinants (S and F), which were separated by DEAE ion-exchange cellulose chromatography. Approximately two-thirds of the fragments had S antigenic groupings and one-third had F antigenic groupings. These data are consistent with the view that mouse gamma globulin is split by papain and cysteine into three major fragments, two of which are of the S type and one of the F type.
Antibody activity of the original molecule was present in the S fragments. Although the S fragments did not precipitate the antigen (hemocyanin) they were shown to bind antigen specifically in the manner of univalent antibodies.
The S fragments of normal γ-globulin were very heterogeneous with a broad spectrum of electrophoretic mobilities. Comparison of S fragments from slow and fast migrating globulins showed that the mobilities of the original γ-globulin samples were largely reflected in the mobilities of their S fragments. Additional observations indicated that the F fragments also may help to determine the electrophoretic mobility of intact γ-globulin molecules.
S fragments of differing electrophoretic mobility were shown to have the same antigenic determinants, indicating that the structural differences responsible for the electrophoretic mobility differences were not involved in the antigenic groupings identified with rabbit antisera.
The F fragments of normal γ-globulin migrated more rapidly than the S fragments, were less heterogeneous, and showed several bands on starch gel electrophoresis. The F fragments differed antigenically from the S fragments, and had no antibody activity. Two groups of F fragments (F and F') were detected with some antisera.
The γ-myeloma protein (5563) formed in a C3H plasma cell tumor and similarly fragmented by treatment with papain and cysteine, produced much more discrete S and F components than were found in the normal γ-globulin digest. The electrophoretic properties of the myeloma protein fragments were within the range observed for normal γ-globulin fragments. Although the γ-myeloma protein shares antigenic determinants with normal γ-globulins it lacks some of the antigenic groupings present in the γ-globulin preparation. Both S and F fragments from the myeloma protein share antigenic determinants with the corresponding fragments from normal γ-globulin. In addition, both S and F fragments of normal γ-globulin possess antigenic groupings not present in fragments of the γ-myeloma protein, accounting for the antigenic deficiency observed on comparison of the γ-myeloma protein with normal γ-globulins.
Masking of protein antigen by modification of amino groups with carbobenzoxychloride (benzyl chloroformate) and demasking by treatment with nonspecific protease.