Characterization of a large fragment produced by proteolysis of human immunoglobulin M with papain.

The controlled digestion of human immunoglobulin M (IgM) (Patient Dau.) with papain in the presence of 0.002 M mercaptoethylamine resulted in the release of a basic subunit-like fragment which we designated IgM,. It resembled the subunit (IgM,) produced by mild reduction of IgM in the following ways. The molecular weight (186,000) and sedimentation coefficient (6.2 to 6.5 S) of IgM, were similar to those of IgM,. IgM, contained light chains and a large portion of each of its p chains. IgM, was antigenically identical with IgM, and IgM when tested with specific antiIgM antiserum. The mobilities of IgM, and IgM, in agarose seemed to be virtually identical. In contrast, IgM, did not possess the free sulfhydryl groups which were found on IgM,. It was concluded that IgM, was formed from IgM by proteolysis of the p chains near the carboxyl terminus. The fragment retained intact its interchain disulfide bonds.