The differential enzyme susceptibility of bovine immunoglobulin G1 and immunoglobulin G2 to pepsin and papain.

Purified bovine immunoglobulins IgG1 and IgG2 were subjected to enzymatic degradation with pepsin and papain. Results were monitored using density gradient ultracentrifugation, acrylamide electrophosesis and immunodiffusion employing subclass- and light chain-specific antisera. The results indicated a marked enzymatic susceptibility of IgG1 to digestion with pepsin. This differential susceptibility can also be demonstrated in unfractionated bovine gamma-globulin. No striking differences between the two subclasses were observed during treatment with papain in the presence of cysteine and after 24 h, most IgG1 and IgG2 was degraded to Fc and Fab fragments. The pepsin Fc fragment generated from IgG2 was larger than that generated from IgG1 although the F(ab')2 fragments were simialr in size. These results are consistent with the hypothesis that the Fc region of IgG1 contains multiple cleavage sites for pepsin whereas IgG2 has few. Rabbits immunized with the first elution peak from a 30 h pepsin digest of bovine gamma-globulin fractionated on Sephadex G-150, responded primarily to common gamma-chain and IgG2-specific determinants. Thus, the differential susceptibility of bovine subclasses to pepsin provides a method for stimulating IgG2-specific antibodies in rabbits.