Guinea pig serum contains two isotypes of immunoglobulin G: IgGl and IgG2. These immunoglobulins are antigenically distinct from each other and they mediate different biologic processes in the same guinea pig, although they share the ability to bind the same antigen. An attempt was made to study the primary structure of the γ1-heavy chain from IgGl in comparison with the largely known primary structure of the γ2-heavy chain from IgG2, with the aim of demarcating the structural differences between these molecules. IgGl was isolated from the serum of immune strain 13 guinea pigs. Both IgGl and the γ1 chain were digested with CNBr, Nine fragments were isolated from both digests by gel filtration procedures before and after reductive cleavage of disulfide bonds. These fragments appear to account for the entire ~444 residues in the γ1 chain. Amino acid composition data of CNBr fragments suggest that at least the amino terminal ~ 182 residues of the γ1 and γ2 chains are very similar. Two of the fragments which have been isolated have amino acid compositions suggesting their derivation from the “hinge” region and carboxyl terminus of the γ1 chain.
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