Further characterization of protein A reactive and non-reactive subfragments of Fc from human IgG.

Tryptic digests of acid-treated Fc from normal human IgG were separated into four peaks (I-IV) by gel filtration on Sephadex G-100. The second peak was further divided into two fractions (II and II'). Peak I was indistinguishable from intact Fc on electrophoresis, immunodiffusion, and reactivity to protein A. The protein A reactive fragments of fractions II, II', and III were shown to contain antigenic determinants of both the CH2 and CH3 domains, to interact with the anti-Gm (1) specific rheumatoid factor, and to fix complement. These results, together with SDS-electrophoresis, showed that protein A reactive fragments are all composed of an intact Fc chain with shorter chains covalently linked to it. The protein A non-reactive fragments of fractions II' and III were homogeneous, fixed complement and showed no interaction with the Gm (1) rheumatoid factor. These results, in addition to the observed antigenic determinants, localized the fragments to the CH2 region.