Papain digestion fragments of human IgM globulins.
Papain digestion of two Waldenström IgM globulins produced a high amount of small peptides and resulted in the formation of two end products, the Fabmicro and Fcmicro fragments. The Fcmicro fragment is characterized by a fast electrophoretic mobility, a high content in carbohydrate, and a high molecular weight. It was demonstrated that this fragment is made of heavy chain pieces belonging to several disulfide-linked monomeric subunits, presumably representing the carboxy-terminal end of the micro-chains. Fc fragments from the two macroglobulins could not be distinguished immunologically. An appreciable proportion of IgM molecules apparently underwent degradation without the formation of a stable Fc fragment. An Fc-like fragment, analogous to the reduced Fc fragment, was obtained at early stages of papain digestion of the IgM subunits. The Fabmicro fragment, with slow and individually distinct electrophoretic mobility, bears many physicochemical and immunological similarities to the Fabgamma fragment. It consists of one light chain and one Fd piece, both of which were isolated. The interaction of these two constituents was demonstrated by gel diffusion studies. Fab fragments of both IgM globulins were resolved into two subpopulations with different electric charges. In addition to these fragments, intermediary split products were observed at early stages of the degradation process, together with a high yield of small peptides mainly derived from the papain-sensitive region of the heavy chains. Immunologic data strongly suggested that this segment of micro-chains is situated between the Fd piece and the portion included in the Fc fragment. Several experiments indicated the importance of conformational antigenic specificity in both Fab and Fc regions of the IgM globulins.