Comparison of Fch and Fc Fragments of IgG3: Evidence of an Intra-Chain S-S Bridge in the Hinge of γ3 Chains

The Fc and Fch fragments of human IgG₃, appearing late and early during papain digestion, respectively, were antigenically and structurally compared. Only the Fch fragment reacted with an anti-Fh (hinge region-specific) antiserum; thus the main portion of the hinge region is split off during the formation of Fc fragments The Fch fragment has a higher content of inter-chain S-S bridges than the Fc fragments. The Fc fragments are probably a mixture of three related fragments. The Fch fragment appears to be more homogeneous, although there was hydrolysis at the C-terminal site of the hinge region in some of the Fch chains. Evidence is presented that the γ³ chain bridge has an extra intra-heavy-chain disulphidc bridge at the C-terminal end of the hinge region.