Structure of Heavy Chain from Strain 13 Guinea Pig Immunoglobulin-G(2). I. Isolation of Cyanogen Bromide Fragments

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.