Chemical properties of functional groups of mouse immunoglobulins of the IgA, IgG2a, and IgM classes

Anionic lipid-induced conformational changes in human phagocyte flavocytochrome b precede assembly and activation of the NADPH oxidase complex

Phagocyte NADPH oxidases generate superoxide at high rates in defense against infectious agents, a process regulated by second messenger anionic lipids using incompletely understood mechanisms. We reconstituted the catalytic core of the human neutrophil NADPH oxidase, flavocytochrome b (Cyt b) in 99% phosphatidylcholine vesicles in order to correlate anionic lipid-dependent conformational changes in membrane-bound Cyt b and oxidase activity. The anionic lipid 10:0 phosphatidic acid (10:0 PA) specifically induced conformational changes in Cyt b as measured by a combination of fluorescence resonance energy transfer methods and size exclusion chromatography. The fluorescence lifetime of a complex between Cyt b and Cascade Blue-derivatized anti-p22(phox) antibody (CCB-CS9), increased after exposure to 10:PA by ∼50% of the change observed when the complex is dissociated, indicating a structural rearrangement of p22(phox) and/or the Cyt b heme prosthetic groups. Half of the quenching relaxation occurred at 10:0 PA concentrations permissive to less than 10% full NADPH oxidase activity, but saturated near the saturation in activity in a matched cell-free oxidase assay. We conclude that anionic lipids modulate the conformation of Cyt b in the membrane and suggest they may serve to modulate the structure of Cyt b as a control mechanism for the NADPH oxidase.

Functional mapping of the anti-idiotypic antibody anti-TS1 scFv using site-directed mutagenesis and kinetic analysis

Recombinant antibodies may be engineered to obtain improved functional properties. Functional mapping of the residues in the binding surfaces is of importance for predicting alterations needed to yield the desired properties. In this investigation, 17 single mutation mutant single-chain variable fragments (scFvs) of the anti-idiotypic antibody anti-TS1 were generated in order to functionally map amino acid residues important for the interaction with its idiotype TS1. Residues in anti-TS1 determined to be very important for the interaction were identified, Y32L, K50L, K33H, and Y52H, and they were distributed adjacent to a centrally located hydrophobic area, and contributed extensively to the interaction energy (≥2.5 kcal/mol) in the interaction. Quantitative ELISA assays, BIAcore technologies and three-dimensional surface analysis by modeling were employed to visualize the consequences of the mutations. The expression levels varied between 2 - 1,800 nM as determined by ELISA. All the 17 scFvs displayed higher dissociation rates (60 - 1,300 times) and all but two of them also faster association rates (1.3 - 56 times). The decrease in affinity was determined to be 1.6 - 12,200 times. Two of the mutants displayed almost identical affinity with the wild type anti-TS1, but with a change in both association and dissociation rates. The present investigation demonstrates that it is possible to generate a large panorama of anti-idiotypic antibodies, and single out a few that might be of potential use for future clearing and pre-targeting purposes of idiotypic-anti-idiotypic interactions.

Anionic amphiphile and phospholipid-induced conformational changes in human neutrophil flavocytochrome b observed by fluorescence resonance energy transfer

The integral membrane protein flavocytochrome b (Cyt b) comprises the catalytic core of the human phagocyte NADPH oxidase complex and serves to initiate a cascade of reactive oxygen species that participate in the elimination of infectious agents. Superoxide production by the NADPH oxidase complex has been shown to be specifically regulated by the enzymatic generation of lipid second messengers following phagocyte activation. In the present study, a Cyt b-specific monoclonal antibody (mAb 44.1) was labeled with Cascade Blue (CCB) and used in resonance energy transfer (RET) studies probing the effects of a panel of lipid species on the structure of Cyt b. The binding of CCB-mAb 44.1 to immunoaffinity-purified Cyt b was both highly specific and resulted in significant quenching of the steady state donor fluorescence. Titration of the CCB-mAb 44.1:Cyt b complex with the anionic amphiphile lithium dodecyl sulfate (LDS) resulted in a saturable relaxation of fluorescence quenching due to conformational changes in Cyt b at concentrations of the amphiphile required for maximum rates of superoxide production by Cyt b in cell-free assays. Similar results were observed for the anionic amphiphile arachidonic acid (AA), although no relaxation of fluorescence quenching was observed for arachidonate methyl ester (AA-ME). Saturable relaxation of fluorescence quenching was also observed with the anionic, 18:1 phospholipids phosphatidic acid (DOPA) and phosphatidylserine (DOPS), while no relaxation was observed upon addition of the neutral 18:1 lipids phosphatidylcholine (DOPC), phosphatidylethanolamine (DOPE) or diacylglycerol (DAG) at similar levels. Further examination of a variety of phosphatidic acid (PA) species demonstrated DOPA to both potently induce conformational changes in Cyt b and to cause more dramatic conformational changes than PA species with shorter, saturated acyl chains. The data presented in this study support the hypothesis that second messenger lipids, such as AA and PA, directly bind to flavocytochrome b and modulate conformational states relevant to the activation of superoxide production.

Studies of the interactions between the anticytokeratin 8 monoclonal antibody TS1, its antigen and its anti-idiotypic antibody alphaTS1

The monoclonal antibody TS1 against cytokeratin 8 and its antiidiotype alphaTS1 have been used for immunotargeting and therapy of carcinomas in experimental tumor model systems. The interaction surfaces between mab TS1, the cytokeratin 8 epitope, and its anti-idiotypic antibody, alphaTS1, were studied in detail in order to make future veneering of the interactions possible. The V-genes of TS1 and alphaTS1 were cloned and sequenced and the CDRs and the framework residues were identified. Amino acids participating in the interactions were identified following chemical modifications of residues in non-protected and protected molecules of cytokeratin 8, alphaTS1 and TS1. From the sequences, the three-dimensional structures were generated using computer modelling of the antibody variable regions. Several charged amino acid, histidine and tyrosine residues were displayed in the antibody surfaces implicated in the interactions and chemical modification confirmed the importance of these amino acids. The cytokeratin 8 epitope has previously been identified by Johansson et al. and it displays negatively charged amino acid residues which could be identified in the chemical modification. It was also revealed that the TS1 binding to cytokeratin 8 and alphaTS1 respectively are partly overlapping; a histidine identified in TS1 is probably involved only in the interaction with alphaTS1. Furthermore, the chemical modification demonstrated that exchanging aspartic-glutamic acids to asparagine-glutamine residues in TS1 increased the binding of TS1 to cytokeratin 8, indicating that there is at least one acidic amino acid that is an obstacle in the TS1-CK8 binding. The detailed assembly of the interaction surfaces will facilitate the future use of site directed mutagenesis to improve the TS1-CK8 association rate and the clearing of TS1 with alphaTS1 in vivo.

Characterisation of residues in antibody binding sites by chemical modification of surface-adsorbed protein combined with enzyme immunoassay

Specific functional group modification of an antibody adsorbed to microtitre plates has been used to probe the binding site residues that determine antigen specificity. Chemical modification of adsorbed protein in tandem with enzyme immunoassay (termed CMAP-EIA) consumes only modest amounts of antibody, while allowing a variety of reagents to be rapidly screened in situ. Modification of tyrosine and arginine residues with 1-fluoro-2,4-dinitrobenzene, and p-hydroxyphenylglyoxal resulted in reduced binding of polysaccharide antigen from Yersinia enterocolitica O-polysaccharide to its homologous monoclonal antibody, YsT9-1. Modification with 1-ethyl-3-(3-dimethylaminopropyl) carbodiimide under various conditions indicated that carboxylate groups may also be involved. Parallel experiments with diethylpyrocarbonate and acetic anhydride were used to rule out the involvement of histidine and lysine residues respectively. In all cases, binding of an anti-idiotypic antibody, AJ5, could only be reduced at concentrations of modifying reagent substantially higher than those required to reduce polysaccharide antigen binding to YsT9-1. The results are discussed with regard to the structure of the combining site of YsT9-1 as determined by X ray crystallography and by modelling, and the role of particular residues in complex formation with antigen and in the idiotope.

Determination of chemical properties of individual histidine and tyrosine residues of concanavalin A by competitive labeling with 1-fluoro-2,4-dinitrobenzene

A selective peptide-mapping procedure was devised to purify peptides containing histidine or tyrosine residues from proteolytic digests of concanavalin A (Con A). The protein was modified with maleic anhydride followed by 1-fluoro-2,4-dinitrobenzene (Dnp-F) and then digested with thermolysin. The resulting labeled peptides were separated by high-performance liquid chromatography, and the Dnp-histidine and Dnp-tyrosine peptides were identified by their spectral characteristics. From their amino acid compositions, the labeled peptides could all be assigned within the known sequence. Peptides representing five of the six histidines and all seven tyrosines were obtained. With the same peptide-mapping procedure, the chemical properties (pK and reactivity) of these residues were determined. Samples of concanavalin A at various pH values were labeled with trace amounts of [3H]Dnp-F, in the presence of Gln-Gly as an internal standard. To each sample was added an aliquot of a mixture of [14C]Dnp-Gln-Gly and [14C]Dnp-maleyl-Con A. Portions of each sample were removed, [14C]Dnp-Ala-Ala and epsilon-[14C]Dnp-lysine were added, and the mixtures were hydrolyzed. The various Dnp amino acid derivatives were purified by HPLC. The remainder of each [3H]Dnp sample was maleylated, dinitrophenylated, and digested with thermolysin and separated by HPLC as above. From the 3H/14C ratios of the Dnp amino acid derivatives and the Dnp peptides relative to the ratio of the internal standard, pK and reactivity data were obtained for (a) the average behavior of the lysine, histidine, and tyrosine residues and (b) the individual behavior of the N-terminal alanine residue and the five histidine and seven tyrosine residues in the protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Structural mapping of membrane-bound immunoglobulin E-receptor complexes: use of monoclonal anti-IgE antibodies to probe the conformation of receptor-bound IgE

Previous resonance energy-transfer measurements have suggested that immunoglobulin E (IgE) may bend near the junction of its Fc and Fab segments in order to bind to its high-affinity receptor on rat basophilic leukemia cells. In order to test this possibility, two monoclonal antibodies were employed that bind specifically to rat IgE (IgER) when IgER is in solution and when it is bound to receptors on the plasma membrane. The F(ab')2 fragment of one monoclonal (B5) that is specific for the Fab region of IgER was labeled with donor probes and bound to IgER, and the quenching of the fluorescence of these donors due to simultaneous binding of the Fab' fragment of an anti-Fc monoclonal (A2) that was labeled with an acceptor probe at its interchain disulfide bond was measured. Significantly less energy transfer between these probes was observed when IgER was bound to its receptor on membrane vesicles than when it was free in solution, and this result is interpreted in light of other energy-transfer measurements using A2 and B5 that were preferentially labeled near their combining sites with donors and acceptors, respectively, as well as measurements of the distance of closest approach between these sites and the membrane surface. These results along with previous energy-transfer measurements and other biochemical information form the basis for a working model of the conformation and orientation of receptor-bound IgE. This study demonstrates the use of fluorescently labeled monoclonal antibodies as highly selective energy-transfer probes in assessing structures of macromolecular complexes on the plasma membrane.

The thiol groups of mouse immunoglobulin A. Incomplete formation of the C alpha 1-domain disulphide bridge

The BALB/c IgA (immunoglobulin A) myeloma protein M167 contained on average 5.7 free SH groups per IgA dimer. These groups were preponderantly on the heavy chains and comprised two distinct populations: 3.3 exposed SH groups per dimer in the Fc region, and 2.4 buried SH groups per dimer in the Fd region, detectable o only after denaturation. To locate the cysteine residues involved, labelled peptides were purified from thermolysin digests of radioalkylated IgA by high-performance liquid chromatography. From the amino acid compositions of the peptides, the exposed thiol groups were assigned to Cys-307 in the C alpha 2 domain, which thus existed in the reduced form to an extent exceeding 80%. This residue may allow attachment of secretory component to dimer IgA in the mouse to proceed via thiol-disulphide exchange. The buried thiol groups were assigned to Cys-150 and Cys-208, in the C alpha 1 domain, each being in the reduced form to the extent of approx. 30%. This pair of residues would normally give rise to the characteristic intradomain disulphide bridge. It appears that disulphide formation is not a crucial event during folding of the C alpha 1 domain in IgA biosynthesis. The sequence in the region 140-151 was re-investigated, and residue 142 was shown to be serine, not cysteine, helping explain the lack of heavy-chain-light chain bonding in BALB/c mouse IgA. A disulphide-bond model for mouse IgA is proposed on the basis of these assignments and other features of the mouse alpha-chain sequence.

Chemical properties of the N-termini of human haemoglobin

The chemical properties, namely pK and reactivity, of the N-termini of oxyhaemoglobin and deoxyhaemoglobin toward acetic anhydride and 1-fluoro-2,4-dinitrobenzene (Dnp-F) were determined by the competitive-labelling approach [Kaplan, Stevenson & Hartley, (1971) Biochem. J. 124, 289-229; Duggleby & Kaplan (1975) Biochemistry 14, 5168-5175]. At physiological pH and temperature, the valine-1 alpha and valine-1-beta amino groups had unusually low pK values, but showed only minimal changes in their pK values on deoxygenation. Between pH 7.5 and pH 8.0 a deviation was observed in the pH-reactivity profiles and the apparent pK values became markedly pH-dependent. It was found that Dnp-F, but not acetic anhydride, had an abnormally high reactivity toward the N-termini. It is concluded that the valine-1 alpha and valine-1 beta N-termini make little or no contribution to the alkaline Bohr effect at physiological pH values. The high reactivity toward Dnp-F is attributed to an interaction or binding near the N-terminal region, and the discontinuity in the pH-reactivity profile at moderate alkaline pH values to a conformational change which alters the environment of these groups.