The determination of the amino acid sequence of heavy chain (γ2 chain) from the major immunoglobulin in the serum of strain 13 guinea pigs, IgG(2), was undertaken. The aim is to demarcate all sections of this chain having a single amino acid sequence and then to compare the remaining regions showing sequence variability from antibodies of different specificities. γ2 chain from strain 13 guinea pig IgG(2) was purified by gel filtration after mild reduction and radioalkylation. Four half-cystines were thus radiolabeled. Isolated γ2 chain was subjected to CNBr cleavage, and five fragments, accounting for 303 residues from the C-terminal three-quarters of the chain, were isolated by gel filtration. Among these fragments were the C-terminal octadecapeptide (C-5), the “hinge region” fragment (C-l-c), the predominant carbohydrate-containing fragment (C-3), and two others (C-l-b and C-4). Fragment C-l-c contains three of the four radiolabeled half-cystines. In addition to these five sections, other CNBr fragments have been found which seem to come from the N-terminal one-quarter of γ2 chain. The fourth radioactive half-cystine is also found in this region.
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