Papain-susceptibility of 50 IgG myeloma proteins was investigated. 34 per cent of the proteins proved to be papain-resistant. A definite correlation between papain-susceptibility and heavy chain subclass was found in the cases of myeloma proteins belonging to subclasses IgG2, IgG3, and IgG4, as the IgG2 and IgG4 proteins were papain-resistant, whereas the IgG3 proteins were papain-sensitive. There exists, however, a heterogeneity in the subclass IgGl.
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