A 2·2-2·5 S, Fab-related fragment (tFab′) was isolated from trypic digests of normal human IgG and of IgG1 myeloma proteins. Reductive cleavage of this gragment led to dissociation into polypeptide constituents of a mol. wt corresponding to one half of a light chain or of an Fd piece. Indirect evidence is presented that the disulphide bridge linking a κ to a γ 1 heavy chain is contained by tFab′. Heterologous anti-Fab immune sera precipitated fragment tFab′ as well as Fab (produced by papain) or the 3·9-4·4 S tryptic Fab fragment (tFab). Fragment tFab′ from a myeloma protein was found to be deficient in individual specific antigenic determinants of the parent protein. 'Trypsin-site' (determinant of IgG revealed by tryptic proteolysis) was found to be fully retained by tFab′, while the Gm(f) allotype-determinant was not detectable on it serologically.
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