Papain Fragmentation of the Subunits of Human Macroglobulin

Abstract

Papain fragmentation of the subunits (IgMs) produced by mild reduction and alkylation of IgM that had been isolated from a patient with macroglobulinemia was studied. A stable fragment (Ms pap I) was isolated after papain digestion of IgMs for 16 hr. The approximate molecular weight of the Ms pap I fragment was 50,000. It was composed of light chain (κ chain) and a part of heavy chain (µ chain) and obtained in a yield of 10 fragments/molecule of IgM or 2/molecule of the subunit. Some chromatographic and electrophoretic heterogeneity was observed with this fragment although there seemed to be no difference in chain composition and size between chromatographically separated Ms pap I fragments. Another fragment, Ms pap III, was isolated from a short period digest. The sedimentation coefficient of this fragment was 2.9 S and it was composed of a part of the µ chain which was not included in the Ms pap I fragment. Although it may not represent all of the µ chain other than included in the Ms pap I fragment, it carries the major antigenic determinants of the µ chain and most of the hexose of IgMs. The former fragment, Ms pap I, is considered as a counterpart of Fab fragment of IgG and the latter, Ms pap III, as that of Fc fragment and, thus, the formula of the papain fragmentation of IgMs is similar to that of IgG except that the counterpart of the Fc portion is hydrolyzed to dialyzable peptides in a short period.