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Additional evidence has been obtained to show that different guinea pig anti-hapten antibodies differ in the structure of their L polypeptide chains. Antibodies from animals immunized with the same hapten conjugated to different carrier proteins gave similar starch gel electrophoretic patterns after dissociation of their chains. In a study of fine differences of specificity, cross-reacting antibodies were found to have some L chains with the same electrophoretic mobility. The multiplicity of L chain bands found in the characteristic starch gel electrophoretic patterns of dissociated anti-DNP antibodies was shown to be a reflection of the heterogeneity of antibodies of slightly different specificities. Reduction and alkylation of the active fragment produced by digestion of antibodies with papain yielded starch gel electrophoretic bands corresponding in mobility to L chains. The results are consistent with the notion that L chains are involved in the acquisition of immunologic specificity.

The antigenic properties of the polypeptide chains of human 7S gamma-globulin have been related to two major non-cross-reacting antigenic determinants of the whole molecule. These determinants, called S and F, were obtained by hydrolysis of gamma-globulin with papain. Antisera against whole gamma-globulin and against S and F fragments were used in techniques of immune diffusion. Light (L) chains of gamma-globulin showed reactions of partial identity with S fragments, and thus are antigenically deficient with respect to these fragments. Antisera directed against F determinants did not react with L chains but did react with heavy (H) polypeptide chain preparations. In addition, the major component of H chain fractions did not appear to contain determinants in common with the S fragments. L chains of a gamma-myeloma protein were shown to be antigenically deficient with respect to the whole myeloma molecule, and antigenically identical with the Bence-Jones protein of the same patient. Correlation of these results with those of previous investigations have led to the conclusions that the S fragment which is known to contain the combining region of antibody molecules, consists in part of L chains or portions of L chains, and that the F fragment, which mediates several other functions of the whole molecule, is composed in part of portions of H chains.

The L polypeptide chains of certain Bence-Jones proteins of group I have been found in three forms: monomers of molecular weight of about 20,000, dimers which monomerize in dissociating solvents, and dimers which are stable in such solvents. The L polypeptide chains of some Bence-Jones proteins of group II were found to occur naturally only as stable dimers. The L chains of normal human gamma-globulin have been obtained in a reduced unalkylated form, and a fraction of these chains was found to form stable dimers under oxidizing conditions. It is suggested that a single disulfide bond is involved in stabilization of the dimer. In experiments on the reconstitution of 7S gamma-globulin, it was found that stable dimers of L polypeptide chains did not associate appreciably with H(gamma) chains to form a soluble product. L chains in the monomeric form, both of a reduced alkylated Bence-Jones protein and of reduced unalkylated gamma-globulin, combined with H(gamma) chains to form a 7S product. After hydrolysis with papain, the 7S material containing the Bence-Jones L chains yielded fragments comparable to the fragments of papain-treated myeloma proteins. As indicated by spectrofluorometric measurements, dissociable dimers and stable dimers of the L chains of a Bence-Jones protein both underwent identical thermally induced transitions in the temperature range 48-58 degrees C. When L polypeptide chains were present in reduced alkylated gamma-globulin or reduced alkylated S fragments, no transition occurred until 65 degrees C, the coagulation temperature of gamma-globulin and S fragments. Above this temperature, L chains were released into solution. These experiments suggested that free L chains and L chains bound to H(gamma) chains have different conformational stabilities.

The relationships between the polypeptide chains of gammaG immunoglobulin and fragments of the molecule produced by papain and pepsin have been investigated. Specific procedures were employed including peptide mapping of tryptic hydrolysates and analysis of molecules reconstituted from chains labeled with different iodine isotopes. By these means, the Fab fragment was shown unequivocally to consist of the light chain and a portion of the heavy chain, the Fd fragment. The Fc fragment was found to be comprised of the residual portions of the heavy chain. These findings support the gross arrangement of chains embodied in recent models of the gammaG immunoglobulin molecule. The present studies have also provided additional information on the susceptibility of gammaG immunoglobulin to proteolytic cleavage. It was found that the portion of heavy chains corresponding to the Fd fragment was extensively cleaved by papain.

Specific precipitates and purified human antibodies were reduced and alkylated and subjected to starch gel electrophoresis in 8 M urea to dissociate and separate the L and H polypeptide chains. Dissociated antibodies to dextran, levan, teichoic acid, blood group A substance, and tetanus toxoid showed sharp bands corresponding to L polypeptide chains. The patterns differed for antibodies of different specificities. Some differences were also seen among antibodies of the same specificity from different individuals. Purified antidextran antibodies showed particularly simple patterns resembling those of purified human gamma myeloma proteins. In some cases only one L chain band was present.

L polypeptide chains of myeloma globulin and Bence-Jones protein isolated from the same patient were found to be identical after comparison of their tryptic hydrolysates by two-dimensional high voltage electrophoresis. The patterns of peptides from proteins belonging to antigenic group I differed markedly from those of proteins in antigenic group II. A partially purified H chain fraction was compared with L chains from the same myeloma protein. The tryptic hydrolysates yielded dissimilar patterns of peptides. These data indicate that gamma-myeloma proteins contain two kinds of polypeptide chains, Hgamma chains and either L(I) or L(II) chains. The L chains appear to be identical with those comprising the Bence-Jones protein from the same patient.

When human and rabbit 7S gamma-globulins were reduced in strong urea solutions by a number of procedures, their molecular weights fell to approximately (1/3) of the original values. Partial separation of the reduction products was achieved using chromatography and starch gel electrophoresis in urea solutions. One of the components of reduced human 7S gamma-globulin was isolated by chromatography, identified by starch gel electrophoresis, and subjected to amino acid analyses. The amino acid composition of this component differed from that of the starting material and also from that of the remaining components. A reduced pathological macroglobulin dissociated to components with an average molecular weight of 41,000. Several reduced human myeloma proteins, when subjected to starch gel electrophoresis, yielded individual patterns that nevertheless had features in common with those of reduced normal gamma-globulins. Reduction of normal and abnormal gamma-globulins was accompanied by the appearance of titratable sulfhydryl groups. Chemical treatments other than reduction were used to determine the type of bond holding the subunits together. It was tentatively concluded that they were linked by disulfide bonds. An hypothesis is presented to relate the structural features of the various gamma-globulins in terms of the multiplicity of polypeptide chains in these molecules.

Reconstitution of phage-neutralizing activity was observed after admixture of separated L and H polypeptide chains of purified antibodies obtained from single sheep or guinea pigs and directed against f1 and f2 phages. Hybrid mixtures of complementary chains of antiphage antibodies and gamma-globulins of the same animal showed less activity than the homologous mixtures of chains from antibodies of the same specificity. 7S interspecies molecular hybrids could be formed between L or H chains of sheep gamma-globulins and the complementary chains of guinea pig gamma-globulins. In contrast to the results obtained within a single animal species, mixtures of L or H chains of antiphage antibodies from one species with the complementary chains of antibodies to the same phage from the other species showed slight or negligible potentiation of neutralizing activity.

The chemical relations among Bence-Jones proteins, myeloma proteins, and normal gamma-globulins have been investigated by a variety of means. Starch gel electrophoresis in 8 M urea of reduced alkylated Bence-Jones proteins yielded patterns of bands corresponding to those of the light (L) polypeptide chains of the dissociated myeloma protein from the same patient. One instance in which this correspondence was found was chosen for extensive study. Chromatography on carboxymethylcellulose in 6 M urea was employed to isolate the light (L) polypeptide chains and heavy (H) polypeptide chains of the completely reduced and alkylated myeloma protein. Isolation of similarly treated Bence-Jones protein from the same patient corroborated the correspondence to the L chains of the myeloma protein. Amino acid analyses indicated that the compositions of the Bence-Jones protein and the L chains of the myeloma protein were identical. Moreover, the thermosolubility properties and spectrofluorometric behavior of the isolated L chains and Bence-Jones protein were similar. Ultracentrifugal analyses of the L chains of normal human 7S gamma-globulin showed that their molecular weight in 6 M urea was 20,000. In aqueous solution their molecular weight was 41,000, suggesting that they exist as dimers under these conditions. The L chains of normal human gamma-globulin were found to have reversible thermosolubility properties similar to those of Bence-Jones proteins. The H chains of normal human gamma-globulin did not share these properties. Using spectrofluorometric methods, characteristic molecular transitions were found upon heating Bence-Jones proteins and L chains. These transitions were indicated by an increase in the intensity of fluorescence at well defined temperatures as well as by reversible shifts in the wavelength of maximal emission. The findings suggest that Bence-Jones proteins are composed of L chains of the type found in normal and pathological gamma-globu]ins.

The amino acid sequence of the first 133 residues of the heavy (gamma) chain from a human gammaG immunoglobulin (He) has been determined. This gamma-chain is identical in Gm type to that of protein Eu, the complete sequence of which has been reported. Comparison of the two sequences substantiates the previous suggestion that there are subgroups of variable regions of heavy chains. The variable region of Eu has been assigned to subgroup I and that of He to subgroup II; on the other hand, the constant regions of the two proteins appear to be identical. Comparison of the sequence of the heavy chain of He with the heavy chain sequences determined in other laboratories suggests that the variable region of subgroup II is at least 118 residues long. The nature and distribution of amino acid variations in this heavy chain subgroup resemble those observed in light chain subgroups. These studies provide evidence that the translocation hypothesis applies to heavy as well as to light chains, viz., genes for variable regions (V) are somatically translocated to genes for constant regions (C) to form complete VC structural genes.

Admixture of separated L and H polypeptide chains of 7S gamma-globulins under appropriate conditions has been found to result in the reconstitution of 7S molecules. The chains were mixed in 0.5 N propionic acid and when dialyzed into neutral aqueous buffers interacted to form reconstituted material in greater than 30 per cent yield. This material had sedimentation coefficients of 6S to 7S, a weight average molecular weight of 160,000, and its antigenic structure and electrophoretic properties were the same as those of 7S gamma-globulin. By the use of I(131) and I(125) labels on the different types of chains, combined with ultracentrifugation of chain mixtures in sucrose density gradients, the 7S product was found to contain both isotopes in ratios consistent with the presence of two L and two H chains. After hydrolysis with papain, the reconstituted material yielded products resembling S and F fragments. All of the isotope corresponding to L chains was found in the counterpart of the S fragment; the isotope corresponding to the H chain fraction was present in both fragments. The activity reconstituted from chains of a purified guinea pig antibody to f1 phage was found to be associated mainly with the 7S material. Hybrid molecules containing rabbit L chains and human H chains and of human L chains and rabbit H chains were formed by the same techniques of reconstitution. It was found that the interchain disulfide bonds of native 7S gamma-globulins could be broken and reoxidized, as could those of reconstituted 7S material. Reduced L and H chains mixed in propionic acid, dialyzed against neutral buffers containing mercaptan, then against neutral buffers in the absence of mercaptan, formed stable 7S molecules of molecular weight 160,000, which were dissociable only after reduction.