Fixation of the first component of complement by immune complexes: effect of reduction and fragmentation of antibody

Complement Biosensors Identify a Classical Pathway Stimulus in Complement-Mediated Hemolytic Uremic Syndrome

Complement-mediated hemolytic uremic syndrome (CM-HUS) is a thrombotic microangiopathy characterized by germline variants or acquired antibodies to complement proteins and regulators. Building upon our prior experience with the modified Ham (mHam) assay for ex vivo diagnosis of complementopathies, we have developed an array of cell-based complement "biosensors'' by selective removal of complement regulatory proteins (CD55 and CD59, CD46, or a combination thereof) in an autonomously bioluminescent HEK293 cell line. These biosensors can be used as a sensitive method for diagnosing CM-HUS and monitoring therapeutic complement blockade. Using specific complement pathway inhibitors, this model identifies IgM-driven classical pathway stimulus during both acute disease and in many patients during clinical remission. This provides a potential explanation for ~50% of CM-HUS patients who lack an alternative pathway "driving" variant and suggests at least a subset of CM-HUS is characterized by a breakdown of IgM immunologic tolerance.

Key Points

CM-HUS has a CP stimulus driven by polyreactive IgM, addressing the mystery of why 40% of CM-HUS lack complement specific variantsComplement biosensors and the bioluminescent mHam can be used to aid in diagnosis of CM-HUS and monitor complement inhibitor therapy.

Disulfide bond structures of IgG molecules: structural variations, chemical modifications and possible impacts to stability and biological function

The disulfide bond structures established decades ago for immunoglobulins have been challenged by findings from extensive characterization of recombinant and human monoclonal IgG antibodies. Non-classical disulfide bond structure was first identified in IgG4 and later in IgG2 antibodies. Although, cysteine residues should be in the disulfide bonded states, free sulfhydryls have been detected in all subclasses of IgG antibodies. In addition, disulfide bonds are susceptible to chemical modifications, which can further generate structural variants such as IgG antibodies with trisulfide bond or thioether linkages. Trisulfide bond formation has also been observed for IgG of all subclasses. Degradation of disulfide bond through β-elimination generates free sulfhydryls disulfide and dehydroalanine. Further reaction between free sulfhydryl and dehydroalanine leads to the formation of a non-reducible cross-linked species. Hydrolysis of the dehydroalanine residue contributes substantially to antibody hinge region fragmentation. The effect of these disulfide bond variations on antibody structure, stability and biological function are discussed in this review.

Effect of the light chain C-terminal serine residue on disulfide bond susceptibility of human immunoglobulin G1λ

The light chain cysteine residue that forms an interchain disulfide bond with the cysteine residue in the heavy chain in IgG1κ is the last amino acid. The cysteine residue is followed by a serine residue in IgG1λ. Effect of the serine residue on the susceptibility of disulfide bonds to reduction was investigated in the current study using a method including reduction, differential alkylation using iodoacetic acid with either natural isotopes or enriched with carbon-13, and mass spectrometry analysis. This newly developed method allowed an accurate determination of the susceptibility of disulfide bonds in IgG antibodies. The effect of the serine residue on disulfide bond susceptibility was compared using three antibodies with differences only in the light chain last amino acid, which was either a serine residue, an alanine residue or deleted. The results demonstrated that the presence of the amino acid (serine or alanine) increased the susceptibility of the inter light and heavy chain disulfide bonds to reduction. On the other hand, susceptibility of the two inter heavy chain disulfide bonds and intrachain disulfide bonds was not changed significantly.

Preparation of fcgamma for addition to sulfhydryl-expressing ligands with minimal disturbance of the hinge

We described previously a scheme for linking functionally intact human normal Fcgamma1, via a thioether linkage emerging from its hinge, to any molecule expressing a free sulfhydryl group (SH). The scheme entails reducing the Fc to release four SH from the two inter-gamma disulfide bonds (SS) in the hinge, blocking one SH by a stochastic alkylation, restoring by SS-interchange the inter-gamma SS whose two SH are still available, and attaching a bismaleimide linker to the one remaining SH. One thereby obtains Fc with a single maleimide group (Fc-maleimide) for attachment to the SH-displaying partner. Restoration of the inter-gamma SS is necessary if the final chimeric construct is to be able to activate the classical complement pathway. However, during this preparation of Fc-maleimide, there is apparently some SS-formation between non-homologous SH, so that not all hinges emerge with a reconstituted inter-gamma SS. To reduce this error we have modified the preparative procedure after investigating an initial partial reduction of the hinge, and reviewing the conditions for stochastic alkylation. During partial reduction by dithiothreitol, the two hinge SS were cleaved apparently randomly: there was no evidence for one bond being more susceptible to reduction than the other, and little indication that the reduction of one bond enhanced the susceptibility of the other. By limiting reduction to an average of one SS per molecule, and alkylation to 0.8 SH per molecule, a final Fc-maleimide product is obtained in which most of the molecules have passed through the entire preparation with one of their hinge SS, and by inference much of the hinge conformation, remaining intact.

Multistep denaturation and hierarchy of disulfide bond cleavage of a monoclonal antibody

A humanized, monoclonal antibody of the IgG4 class was treated with SDS at room temperature. Analysis using SDS-PAGE revealed the progressive formation of a series of denaturation intermediates, of increasing apparent molecular weights. The detectable species migrated with apparent molecular weights of 137, 152, 162, 182, 196, and 210 kDa. Antibody not incubated in SDS had an apparent molecular weight of 137 kDa, while boiling in SDS provoked the immediate conversion of all antibody to the slowest migrating form (210 kDa). A study designed to test if all rungs of the ladder were equally cleavable by mild treatment with mercaptoethanol revealed small amounts of a novel form, and conditions were devised to generate large amounts of this novel form. The novel form, before full denaturation, migrated at the position of unheated, intact antibody (137 kDa), while full denaturation in SDS converted the novel form to a species migrating at about 190 kDa. The novel form, before full denaturation, was identified as HHL ***L, where the asterisks indicate noncovalent binding of one light chain to HHL. This noncovalent complex remains intact during SDS-PAGE. The novel form, after full denaturation, was identified as HHL. This study, with earlier studies, reveals that different pathways of reductive cleavage are taken by different antibodies, and that the most sensitive disulfide bond is different for different antibodies. Our results on antibody denaturation serve as a warning to those who use two-dimensional gels for the analysis of antibodies.

The Fab/c fragment of IgG produced by cleavage at cyanocysteine residues

The intra- and inter-heavy chain disulfides of rabbit IgG were cleaved by mild reduction with either dithiothreitol or sulfite and cyanocysteines generated by treatment with either 2-nitro-5-thiocyanobenzoic acid or KCN, respectively. When cleavage occurs at a cyanocysteine residue in the hinge region of one heavy chain alone the Fab/c fragment is produced. Fab/c was also produced by papain digestion of IgG. Fab/c made by papain digestion was able to active complement in haemolytic assays; this activity was lost after cleavage of its accessible disulfide bonds. Fab/c made by cyanylysis of sulfite-reduced IgG was also active in these assays, but Fab/c made by cyanylysis of dithiothreitol-reduced IgG was not. Treatment of the latter fragment with cysteine and cystine resulted in partial reformation of cleaved disulfide bonds. Fab/c was also made from human IgG and from murine IgG2a and IgG2b.

Carbodiimide crosslinking of human C1q and rabbit IgG

The water soluble carbodiimide 1-ethyl-3-(3-dimethylaminopropyl)carbodiimide was used to covalently link carboxyl groups on rabbit IgG to lysyl groups on complement protein C1q. The interaction between C1q and IgG was disrupted by varying the pH, modifying essential residues in the IgG binding site of C1q and by reducing the interchain disulfides of IgG. Under each of these conditions the correlation found between binding and crosslinking indicated a strong requirement for the proteins to bind normally in order for crosslinking to occur. SDS-PAGE analysis of the crosslinked material showed a 210 kDa band consistent with one IgG crosslinked to two disulfide linked C1q chains. Blotting and autoradiography showed the crosslinking involved the A and/or B and C chains of C1q. The lysines flanking the intrachain half cystines are proposed as the likely candidates for crosslinking to IgG, thus delineating the immunoglobulin binding site of C1q.

Quantitative analysis of the interaction between immune complex and C1q complement subcomponent. The role of interdomain interactions in rabbit IgG in binding of C1q to immune precipitates

A novel method was developed for the analysis of the interaction of large multivalent ligands with surfaces (matrices) to analyse the binding of complement subcomponent C1q to immune precipitates. Our new evaluation method provides quantitative data characteristic of the C1q-immune-complex interaction and of the structure of the immune complex as well. To reveal the functional role of domain-domain interactions in the Fc part of IgG the binding of C1q to different anti-ovalbumin IgG-ovalbumin immune complexes was studied. Immune-complex precipitates composed of rabbit IgG in which the non-covalent or covalent bonds between the heavy chains had been eliminated were used. Non-covalent bonds were abolished by splitting off the CH3 domains, i.e. by using Facb fragments, and the covalent contact was broken by reduction and alkylation of the single inter-heavy-chain disulphide bond. The quantitative analysis of the binding curves provides a dissociation constant (K) of 200 nM for the interaction between C1q and immune precipitate formed from native IgG. Surprisingly, for immune precipitates composed of Facb fragments or IgG in which the inter-heavy-chain disulphide bond had been selectively reduced and alkylated, stronger binding (K = 30 nM) was observed. In this case, however, changes in the structure of the immune-complex matrix were also detected. These structural changes may account for the strengthening of the C1q-immune-complex interaction, which can be strongly influenced by the flexibility and the binding-site pattern of the immune-complex precipitates. These results suggest that domain-domain interactions in the Fc part of IgG affect the segmental mobility of IgG molecules and the spatial arrangement of the immune-complex matrix rather than the affinity of individual C1q-binding sites on IgG.

Co-operation between the pair of C gamma 2 domains in Clq-binding by rabbit IgG

The single site binding constants of rabbit IgG and its plasmin-derived fragments F(acb)2, Facb and F(ab)2 for human C1q were measured by the sedimentation velocity method. The intact IgG and F(acb)2 having the paired C gamma 2 domains gave an identical association constant at 20 degrees C (Ka) of 3.02 X 10(4) M-1 in the presence of a physiological concn of salt and on the basis of six sites per C1q. The C1q-binding affinity was found to be decreased to 1.04 X 10(4) M-1 in the reduced, monomerized fragment Facb. Under the same conditions F(ab)2, which is completely unable to activate the classical complement cascade, gave an apparent C1q-affinity of 0.36 X 10(4) M-1. The results, together with previous observations, led us to the conclusion that the C1q-binding site of rabbit IgG is constituted associatively by the pair of C gamma 2 domains, each of which providing a limited, complementary part of the binding free energy between IgG and C1q.

An examination of the structural and biological properties of three intravenous immunoglobulin preparations

Three commercial preparations of immunoglobulin G prepared for administration by the i.v. route were tested for their physical integrity and in vitro biological activity. Size exclusion chromatography by HPLC in native and denaturing buffers together with SDS-PAGE analysis were used to determine whether covalent-bond cleavage had occurred as a result of procedures used in their preparation. C1 complement binding assays and measurements of competitive binding to an Fc receptor-bearing promonocyte cell line U937 were used to assess whether such changes had altered the capacity of these preparations to engage biological effector functions. A purified IgG1 myeloma protein was used as a reference standard. WinRho, an unmodified IgG, consisted almost wholly of monomeric IgG by HPLC size exclusion and showed no evidence of proteolytic fragments in denaturing buffers or on SDS-PAGE. Sandoglobulin, a product treated at pH 4 with pepsin, contained about 10% dimeric protein and, as revealed under denaturing conditions, about 2% fragments. Relative affinity of binding to U937 cells was similar to WinRho. C1 binding by Sandoglobulin showed normal activity with 50% inhibition at 2.8 nM. Gamimune, modified by partial reduction and alkylation, contained about 15% dimers. Between 20 and 30% of the preparation retained covalent interchain disulfides. Binding to U937 cells was two-fold weaker than the other preparations and binding to C1 was also diminished and modified. This accords well with previous reports of the deleterious effect of reduction and alkylation on Fc function.

Activation of complement pathways by univalent antibody derivatives with intact Fc zones

The ability of two univalent antibody derivatives to invoke complement-mediated lysis of guinea-pig L2C leukemic lymphocytes was investigated. The derivatives were Fab/c from rabbit IgG antibody, in which only one Fab arm is removed from the parent molecule and FabFc in which Fab gamma, from peptic digestion of sheep IgG antibody, is disulfide-bonded to the Fc gamma yielded by papain digestion of an arbitrary IgG. Antibody activity was directed against surface IgM on the target cells. Both derivatives could invoke lysis via the classical pathway in the presence of rabbit complement. Exposure of the cells to the derivatives at 37 degrees C before introducing complement yielded no protective antigenic modulation. At low complement concns the derivatives were more efficient than the parent antibodies at invoking lysis, apparently due to the fact that the derivatives do not cause modulation: it appears that cells can undergo a useful degree of modulation when confronted simultaneously by bivalent antibody and low levels of complement. The Fab/c preparations were also able to invoke lysis by guinea-pig complement. Lysis occurred under conditions where activation took place only via the classical pathway (in dilute complement) or only via the alternative pathway (in the absence of calcium ions, or in C4-deficient guinea-pig serum). The results demonstrate that there is no need for two antigen-binding Fab arms in antibody activation of either the classical or alternative complement pathways. They favour models requiring clustering of Fc regions rather than steric changes which might follow binding of antigen.

Decreased protective efficacy of reduced and alkylated human immune serum globulin in experimental infection with Haemophilus influenzae type b

Conventionally prepared immune serum globulin frequently produces severe side effects when administered intravenously. A modified preparation in which 4 to 5 interchain disulfide bonds have been reduced and alkylated has been made for intravenous use. However, reduction and alkylation may affect Fc-mediated functions of immunoglobulin G, particularly its ability to fix complement by the classical pathway. To determine whether reduction and alkylation alters the protective activity of immune serum globulin in vivo we compared it with two less harshly prepared globulins (pH 4 treated or ultrafiltered) in an infant rat model of Haemophilus influenzae b infection. Antibody binding to the capsular and noncapsular components of H. influenzae b and in vitro bactericidal activity were similar in the globulin preparations. Infant rats were treated with various doses of globulins adjusted to provide identical concentrations of anticapsular antibodies as measured by the Farr radioactive antigen binding assay. At high doses of anticapsular antibody (greater than 1,500 ng per pup), all preparations protected well. At marginal doses (750 ng per pup), however, rats given reduced and alkylated globulin had a significantly greater incidence of bacteremia (P less than 0.05), meningitis (P less than 0.01), and death (P less than 0.05) and a higher magnitude of bacteremia (P less than 0.02) than rats who received pH4-treated or ultrafiltered globulins. These differences were not due to differences in anticapsular antibody concentrations achieved in the serum. The 50% protective serum concentrations of anticapsular antibody in this model were 200 to 300 ng/ml for reduced and alkylated globulin and 100 to 200 ng/ml for acid-treated globulin. Absorption of the globulins with purified H. influenzae b capsule reduced in vitro bactericidal activity and rat protective activity. However, the magnitude of bacteremia was lower in rats receiving absorbed pH 4-treated globulin than in those receiving absorbed reduced and alkylated globulin (P less than 0.05). We conclude that reduced and alkylated immunoglobulin G provides significantly less protective activity against H. influenzae b infection in this model than globulins not so modified, and we suggest that the altered Fc function of the immunoglobulin G, such as the decreased ability to fix complement by the classical pathway or decreased Fc-mediated opsonization, may be responsible for this impairment.

The effects of cleavage of the inter-chain disulphide bonds of rabbit IgG on its ability to bind C1q

Immune complexes prepared from rabbit anti-ovalbumin IgG in which the interchain disulphide bonds had been reduced and then blocked with N-( iodoacetylaminoethyl )-8-naphthylamine-1-sulphonic acid retained the ability to bind 125I-labelled C1q. This ability was lost when a small alkylating agent (iodoacetamide) was used to block the cleaved disulphide bonds. The ability of the IgG to form insoluble immune complexes was partially compromised when iodoacetamide was used to block the disulphide bonds, but was unimpaired when N-( iodoacetylaminoethyl )-8-naphthylamine-1-sulphonic acid was used. These data are consistent with the suggestion that access to the C1q binding site in IgG in immune complexes is modulated by movement of the Fab arms, which may block access to the site.

Effect of reduction and alkylation on structure and function of rabbit IgG antibody—II. Effects on classical pathway C3 convertase formation

Anaerobic reduction of purified rabbit IgG antibody (Ab) with 1.5 moles of dithiothreitol per mole of Ab at pH 8.0, followed by alkylation, cleaves 39% of the inter-heavy-chain (H-H) disulfide (SS) bonds. This treatment has the following effects on the ability of the Ab to activate the classical pathway of complement. Compared to control Ab, reduced and alkylated (RA) Ab retained 4-5.6% of overall hemolytic activity and 55% of complement-fixing activity at 0 degrees C. Complexes of RA Ab and equivalent amounts of soluble Ag consumed C4, C2 and C3 at 37, 51 and 44%, respectively, of the rate at which these components were consumed by equal concns of complexes containing control Ab. Complexes made with RA Ab bound 18% as much C-1 as those made with native Ab. These data indicate that the principal, if not the only, effect of RA is on C-1 binding. Measurements of the ability of complexes of Ab with cell-bound Ag to bind C-1 showed at most a 20% loss of C-1 binding sites and a ca two-fold decrease in affinity for C-1. Similar results were obtained with purified (activated) C-1 and with native C1 in serum. No significant difference could be detected in the rate of activation of bound C1. Normal rabbit IgG which was reduced and alkylated under the same conditions retained 52% of its H-H SS bonds and 30% of its ability to bind C-1. This finding suggests that the impairment in C-1 binding results from an effect on the C1 binding site itself, rather than from an effect on the ability of the RA Ab to transmit a putative conformational "signal" from the Ag-binding site to the C1 binding site. Finally, our data show that the observed functional effect of reduction and alkylation depends strongly on the assay used to evaluate that effect.

Destruction of antibody idiotopes with ultra-low concentrations of reducing agents

The nature of the idiotopes present on F(ab')2 fragments prepared from rabbit anti-micrococcal carbohydrate antibodies and the loss of idiotypic reactivity of these F(ab')2 fragments upon iodination were examined. Rabbit anti-micrococcal idiotopes were shown to be exquisitely sensitive to treatment with very low concentrations of sodium metabisulfite or 2-mercaptoethanol. The treatment destroyed anti-micrococcal idiotopes, as shown by the loss of idiotopes on F(ab')2 fragments after reduction; the allotype epitopes and the antigen binding capacity of the F(ab')2 fragments were unaffected. The destruction of the idiotopes by very low concentrations of reducing agents indicated that an extremely labile disulfide bond is involved in the structure of the idiotope or in the maintenance of the conformation of the anti-micrococcal idiotopes. Identical reduction-sensitive anti-micrococcal idiotopes have been demonstrated in a number of related outbred rabbits, and in each case they induced a natural auto-anti-idiotype (AAI) antibody response. Recognition of the existence of these reduction-sensitive idiotopes and their properties could provide a basis for further study of these idiotopes and may lead to a better understanding of the idiotope network.

The hemolytic activity of (Fab-Fc) recombinant immunoglobulins with specificity for the sheep red blood cells

Antibody-like molecules were formed by artificial recombination of proteolytic IgG fragments (Fab' with anti-SRBC activity, and Fc) and used for studies concerning the complement fixation. Such molecules, when composed of single Fab' bound to Fc fragment appeared inactive, while species containing two Fab' fragments revealed the hemolytic activity. The results were discussed and interpreted, assuming that the interaction of Fab domains with CH2 domains in the Fc fragment is a main structural effect influencing the binding of the complement.

A new preparation of modified immune serum globulin (human) suitable for intravenous administration. I. Standardization of the reduction and alkylation reaction

Immune serum globulin (ISG) prepared by Cohn cold alcohol fractionation of pooled human plasma was reduced with dithiothreitol (DTT) and alkylated with iodoacetamide and other alkylating agents. Our results show that there are a few labile interheavy chain disulfide bonds in ISG which react rapidly under mild, nondissociating conditions. The extent of disulfide cleavage is controlled primarily by the ratio of DTT to ISG until about 4-5 disulfide bonds have been reduced. We report detailed studies on the variables of ISG concentration, DTT to ISG ratio, pH, and time, leading to a chemically modified ISG that has a controlled and limited number of reduced and alkylated disulfide bonds.

Importance of the integrity of the inter-heavy-chain disulphide bond of rabbit IgG in the activation of the alternative pathway of human complement by the F(ab')2 region of rabbit IgG antibody in immune aggregates

Immune aggregates formed from either rabbit IgG antibody or F(ab')2 antibody, and antigen, caused exactly the same extent of incubated with human serum under conditions when incubated with human serum under conditions allowing only alternative pathway activation. These observations confirm that the F(ab')2 region of the antibody molecule can cause alternative pathway activation and that this activation is not affected by the presence of the Fc region of the molecule when only alternative pathway activation is permitted. Under conditions allowing activation of both the classical and alternative pathways, increased alternative pathway activation was obtained with IgG antibody aggregates compared to that obtained with F(ab')2 antibody aggregates. On reduction and alkylation of principally he inter-heavy-chain disulphide bond in the IgG antibody, prior to aggregate formation, it was found that no activation of the alternative pathway by IgG aggregates took place under conditions allowing only alternative pathway activation. Treatment of the IgG antibody with reducing agent alone, or alkylating reagent alone, followed by dialysis and aggregate formation, yielded aggregates which caused alternative pathway activation values close to those obtained for untreated IgG aggregates. These results indicate that the integrity of the inter-heavy-chain disulphide bond of rabbit IgG antibody in immune aggregates is necessary to allow the F(ab')2 region of the IgG molecule to activate the alternative pathway of human complement.

Study of congenital nystagmus: optokinetic nystagmus

A severe defect of optokinetic nystagmus (OKN) was found in 46 patients with congenital nystagmus. Abnormal patterns of OKN, such as superimposition of pendular oscillations on the optokinetic slow component and inversion of OKN, were observed. Optokinetic gain (eye movement velocity/drum velocity) was decreased compared to that in normal subjects, and optokinetic after-nystagmus, or transient persistence of OKN after cessation of visual stimulation, was absent. These findings suggest that there is a defect in the subcortical optokinetic system in congenital nystagmus. Optokinetic responses did not clearly differentiate patients who had ocular lesions that impair vision, such as ocular albinism and opacities of the ocular media, from patients who did not have such lesions. Similar abnormal patterns of OKN were found in both groups of patients, although optokinetic gain tended to be lower in patients with ocular lesions than in those without lesions.

Dimeric, trimeric and tetrameric complexes of immunoglobulin G fix complement

The binding of pure dimers, trimers and tetramers of randomly cross-linked non-immune rabbit immunoglobulin G to the first component and subcomponent of the complement system, C1 and C1q respectively, was studied. These oligomers possessed open linear structures. All three oligomers fixed complement with decreasing affinity in the order: tetramer, trimer, dimer. Complement fixation by dimeric immunoglobulin exhibited the strongest concentration-dependence. No clear distinction between a non-co-operative and a co-operative binding mechanism could be achieved, although the steepness of the complement-fixation curves for dimers and trimers was better reflected by the co-operative mechanism. Intrinsic binding constants were about 10(6)M-1 for dimers, 10(7)M-1 for trimers and 3 X 10(9)M-1 for tetramers, assuming non-co-operative binding. The data are consistent with a maximum valency of complement component C1 for immunoglobulin G protomers in the range 6-18. The binding of dimers to purified complement subcomponent C1q was demonstrated by sedimentation-velocity ultracentrifugation. Mild reduction of the complexes by dithioerythritol caused the immunoglobulin to revert to the monomeric state (S20,w = 6.2-6.5S) with concomitant loss of complement-fixing ability.

Effect of cleaving interchain disulfide bridges on the radius of gyration and maximum length of anti-poly(D-alanyl) antibodies before and after reaction with tetraalanine hapten

The small-angle x-ray scattering of solutions of rabbits IgG antibodies and their derivatives has been investigated. The reduction and alkylation of the native antibody cause a small increase of the molecular parameters, indicating a limited expansion of the molecule. Binding of native antipoly(D-alanyl) antibodies with hapten (80% saturation) causes a significant change of the quaternary structure, expressed by a decrease in the maximum diameter of about 2 nm, of the radius of gyration by 5.5%, and of the volume. The same antibodies, in which the single inter-heavy-chain disulfide bridge was opened by reduction and carboxamidomethylation, do not show any significant decrease in the overall molecular parameters upon reaction with hapten, except for a local structural change in a part of the molecule. These data lend further support to the notion that binding of hapten induces a conformational transition in its specific antibodies and suggest that the opening of the interchain disulfide bridges affects that transition. The dimensions of the intact antibodies calculated from measurements of small-angle x-ray scattering at low concentrations agree closely with those obtained from crystallographic studies.

Changes in quaternary structure of IgG upon reduction of the interheavy-chain disulfide bond

Reduction of the single disulfide bond between heavy chains in the hinge region of rabbit IgG antibody causes destabilization of the CH2 region of the molecule. Our studies indicate that reduced antibody molecules undergo a large change in quaternary structure in the CH2 region upon aggregation with a small bivalent hapten. The conformational change was observed both in hydrodynamic studies and by electron microscopy. The sizes of native and reduced antibody complexes were measured from electron micrographs. These measurements show that reduction of the hinge disulfide allows the CH2 domains of the antibody to separate under the strain induced by complex formation. The Fab arms, which are clearly seen in the electron micrographs of the native complexes, are extended by a portion of the Fc region to effectively become Facb arms in the reduced complexes. The length of the arms is effectively increased by 23 A. This results in a massive alteration in the quaternary structure of the CH2 region of the molecule, and this may be the basis of many of the effects of mild reduction on the various effector functions of the antibody molecule. These findings also support the open structure of the CH2 region proposed on the basis of crystallographic analyses, and they demonstrate how the interheavy-chain hinge disulfide restricts segmental flexibility in the Fc fragment of the IgG molecule.

The role of interchain disulphide bridges in the conformational stability of human immunoglobulin G1 subclass. Hydrogen-deuterium exchange studies

The hydrogen-deuterium exchange data of human immunoglobulin G1 (IgG1) are interpreted by assuming fast fluctuations of the protein conformation, through which the peptide groups become exposed to the solvent. The probability of solvent exposure of peptide hydrogens reflects a rather loose conformation for native IgG in comparison with other globular proteins. The probability of solvent exposure is greater than 10(-3) for half of the peptide groups, which shows that the conformational transitions by which these groups are exposed to the solvent are accompanied by changes in standard free energy less than 17 kJ/mol (4 kcal/mol). In the range of pH 6.2-8.45, at 25 degrees C no gross conformational changes are reflected in the hydrogen-deuterium exchange behaviour of the native, the reduced-nonalkylated-reassociated and the reduced-S-alkylated-reassociated IgG1. No difference could be detected in the conformational stability of the native and reoxidised reassociated IgG1 proteins. The lack of inter-subunit disulphide bridges in S-alkylated-reassociated molecules results in an increased conformational motility. This destabilization of protein conformation affects about 90% of the peptide groups covered by the measurements, and corresponds to changes in standard free energy of 8 kJ/mol on the average.

Structure and function of the constant regions of immunoglobulins

Immunoglobulins are somewhat unusual in that they are biologically active proteins capable of carrying out several different functions. Thus, they can bind specifically with antigen, activate the complement system, mediate many cytotropic reactions, and act as antigen receptors on lymphocyte membranes.