Enzymatic fragmentation of an unusual human IgG2 (Kva) myeloma protein.

The Kva IgG2(k) myeloma protein showed a complete resistance to papain in the presence of cysteine at neutral pH, and a higher resistance to trypsin and alpha-chymotrypsin digestion than other IgG2 proteins. On the other hand, the Kva molecule was cleaved by pepsin at low pH to give the expected F(ab')2 fragment. When the cleavage conditions were altered, it was possible to obtain Fab, Fc, and Fc' fragments from this molecule as well. The Fab/c fragment and FacbFc' complex were also obtained, which have not previously been reported from human IgG2 molecules. Incubation at elevated temperatures (45-50 degrees C) and/or lower pH resulted mainly in enzymatic attack on the C terminal side of the hinge. It was necessary to destroy the hinge by reduction or to expose the Kva molecule at 70 degrees C or at lower pH (2.5) prior to digestion to facilitate enzyme digestion on the NH2 terminal side of the hinge. These results indicate that the hinge region of the Kva molecule has an unusually compact structure, which makes it extremely resistant to proteolysis.