Limited papain hydrolysis of immunoglobulin M (IgM) produces a subunit-like proteolytic fragment designated IgMp (Inman & Hazen, 1968). In the presence of mercaptans, IgMp partially dissociated into Fcμ-like and Fabμ fragments. Treatment of residual IgM (that remaining after a papain digestion) with 2mm-mercaptoethylamine resulted in fragmentation of the same type that occurs in a routine limited digestion of IgM with papain, although exogenous enzyme was not added to the mixture. When IgM was hydrolysed with 14C-labelled papain, a small quantity of the enzyme was found to be associated with the residual IgM and IgMp fractions. IgM and IgM 7S subunit (IgMs) that had been exposed to papain in the absence of activating mercaptan and separated from the enzyme by gel filtration also fragmented when subsequently treated with 2mm-mercaptoethylamine. The fragments resembled those produced during a typical limited papain digestion of IgM. It was concluded that mercaptoethylamine induced fragmentation of IgMp by activating adsorbed papain.

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