DNA-binding proteins in Yoshida ascites tumor fluid

Interaction of the antiphospholipid syndrome autoantigen beta-2 glycoprotein I with DNA and neutrophil extracellular traps

Beta-2 glycoprotein I (β2GPI) is a phospholipid-binding plasma protein and prominent autoantigen in antiphospholipid syndrome (APS). Here, we tested the hypothesis that β2GPI might bind to not only phospholipids, but also cell-free DNA and neutrophil extracellular traps (NETs). We report that β2GPI interacts with cell-free DNA from different species, as well as NETs, in a dose-dependent manner, retarding their migration in an agarose-gel electrophoretic mobility shift assay. We confirm the direct binding interaction of β2GPI with DNA and NETs by ELISA. We also demonstrate that β2GPI colocalizes with NET strands by immunofluorescence microscopy. Finally, we provide evidence that β2GPI-DNA complexes can be detected in the plasma of APS patients, where they positively correlate with an established biomarker of NET remnants. Taken together, our findings indicate that β2GPI interacts with DNA and NETs, and suggest that this interaction may play a role as a perpetuator and/or instigator of autoimmunity in APS.

Human plasma protein N-glycosylation

Glycosylation is the most abundant and complex protein modification, and can have a profound structural and functional effect on the conjugate. The oligosaccharide fraction is recognized to be involved in multiple biological processes, and to affect proteins physical properties, and has consequentially been labeled a critical quality attribute of biopharmaceuticals. Additionally, due to recent advances in analytical methods and analysis software, glycosylation is targeted in the search for disease biomarkers for early diagnosis and patient stratification. Biofluids such as saliva, serum or plasma are of great use in this regard, as they are easily accessible and can provide relevant glycosylation information. Thus, as the assessment of protein glycosylation is becoming a major element in clinical and biopharmaceutical research, this review aims to convey the current state of knowledge on the N-glycosylation of the major plasma glycoproteins alpha-1-acid glycoprotein, alpha-1-antitrypsin, alpha-1B-glycoprotein, alpha-2-HS-glycoprotein, alpha-2-macroglobulin, antithrombin-III, apolipoprotein B-100, apolipoprotein D, apolipoprotein F, beta-2-glycoprotein 1, ceruloplasmin, fibrinogen, immunoglobulin (Ig) A, IgG, IgM, haptoglobin, hemopexin, histidine-rich glycoprotein, kininogen-1, serotransferrin, vitronectin, and zinc-alpha-2-glycoprotein. In addition, the less abundant immunoglobulins D and E are included because of their major relevance in immunology and biopharmaceutical research. Where available, the glycosylation is described in a site-specific manner. In the discussion, we put the glycosylation of individual proteins into perspective and speculate how the individual proteins may contribute to a total plasma N-glycosylation profile determined at the released glycan level.

Beta-2-glycoprotein I is growth regulated and plays a role as survival factor for hepatocytes

Beta-2-glycoprotein I (beta(2)GPI) is mainly produced by the liver and is found in plasma partially associated to lipoproteins. Although various properties have been attributed to this protein, its physiological role remains still unclear. We investigated its expression in cultured liver cells and in regenerating liver. Expression studies in HepG2 cells demonstrate that beta(2)GPI mRNA is regulated in a cell cycle-dependent manner, with very low expression in low cycling conditions and increasing levels in proliferating cells. p21 WAF-dependent growth arrest, induced by butyrate treatment, down-regulate beta(2)GPI mRNA levels. Immunolocalization in normal rat liver shows a non-homogeneous pattern, being mainly present in the centrolobular area; post-hepatectomy regenerating rat liver is uniformly immunostained and mitotic elements show the highest protein expression. Albumin gene expression, studies as control liver specific product, was not affected by sodium butyrate induced growth arrest. As previously reported for endothelial cells, beta(2)GPI behaves as survival factor for HepG2 cells: when increasing amounts of the protein (10-50 microg) have been added to serum deficient cultured liver cells a progressive reduced cell loss was observed. In conclusion, the present data demonstrate that beta(2)GPI gene expression is strictly related to the proliferative status of hepatic cells and that this protein could play a role in maintaining liver cells vitality when exposed to different stress factors such as regeneration after partial hepatectomy or growth factors depletion.

The interaction of beta(2)-glycoprotein I domain V with chaperonin GroEL: the similarity with the domain V and membrane interaction

To clarify the mechanism of interaction between chaperonin GroEL and substrate proteins, we studied the conformational changes; of the fifth domain of human beta(2)-glycoprotein I upon binding to GroEL. The fifth domain has a large flexible loop, containing several hydrophobic residues surrounded by positively charged residues, which has been proposed to be responsible for the binding of beta(2)-glycoprotein I to negatively charged phospholipid membranes. The reduction by dithiothreitol of the three intramolecular disulfide bonds of the fifth domain was accelerated in the presence of stoichiometric amounts of GroEL, indicating that the fifth domain was destabilized upon interaction with GroEL. To clarify the GroEL-induced destabilization at the atomic level, we performed H/(2)H exchange of amide protons using heteronuclear NMR spectroscopy. The presence of GroEL promoted the H/(2)H exchange of most of the protected amide protons, suggesting that, although the flexible loop of the fifth domain is likely to be responsible for the initiation of binding to GroEL, the interaction with GroEL destabilizes the overall conformation of the fifth domain.

Conformation of beta 2-microglobulin amyloid fibrils analyzed by reduction of the disulfide bond

Beta2-microglobulin (beta2-m), a major component of dialysis-related amyloid fibrils, has an intrachain disulfide bond buried inside the native structure. We examined the conformation of beta2-m amyloid fibrils by analyzing the reactivity of the disulfide bond to a reducing reagent, dithiothreitol. Although the disulfide bond in the native structure was highly protected from reduction, the disulfide bonds in the amyloid fibrils prepared at pH 2.5 were progressively reduced at pH 8.5 by 50 mm dithiothreitol. Because beta2-m amyloid fibrils prepared under acidic conditions have been known to depolymerize at a neutral pH, we examined the relation between depolymerization and reduction of the disulfide bond. The results indicate that the disulfide bonds in the amyloid fibrils were protected from reduction, and the reduction occurred during depolymerization. On the other hand, the disulfide bonds of immature filaments, the thin and flexible filaments prepared under conditions of high salt at pH 2.5, were reduced at pH 8.5 more readily than those of amyloid fibrils, suggesting that the disulfide bonds are exposed to the solvent. Taken together, the disulfide bond once exposed to the solvent upon acid denaturation may be progressively buried in the interior of the amyloid fibrils during its formation.

Heparin inhibits the binding of beta 2-glycoprotein I to phospholipids and promotes the plasmin-mediated inactivation of this blood protein. Elucidation of the consequences of the two biological events in patients with the anti-phospholipid syndrome

The phospholipid-binding plasma protein beta2-glycoprotein I (beta2-GPI) is the primary antigen recognized by the circulating autoantibodies in patients with the "anti-phospholipid syndrome" (APS). Although heparin is routinely used in the treatment and prophylaxis of APS patients, the primary heparin-binding site within beta2-GPI has not been identified. More importantly, how heparin exerts its beneficial effects in vivo in APS patients has not been deduced at the molecular level. Using an expression/site-directed mutagenesis approach, we now show that the positively charged site that resides in the first domain of beta2-GPI is not the primary heparin-binding site. Rather it is the second positively charged site located within the fifth domain of the protein that also binds to phospholipids. Lys(284), Lys(286), and Lys(287) in this domain are essential for the interaction of beta2-GPI with heparin. These data indicate that beta2-GPI binds to heparin in a relatively specific manner even though the affinity for the interaction is rather low. Lys(317) resides in the center of the high affinity phospholipid-binding site. Surprisingly, heparin at concentrations that can be achieved in vivo during anticoagulation therapy greatly enhances the plasmin-mediated cleavage of the Lys(317)-Thr(318) site in beta2-GPI. Because the cleaved form cannot bind to phospholipids effectively, the combined actions of heparin and plasmin result in a diminished ability of beta2-GPI to recognize phospholipids. This, in turn, decreases the prothrombotic activity of the endogenous circulating anti-beta2-GPI antibodies in the patients. Thus, heparin exerts its beneficial effects in APS patients by at least two distinct mechanisms.

Flexible loop of beta 2-glycoprotein I domain V specifically interacts with hydrophobic ligands

Beta2-glycoprotein I (beta2-GPI), which consists of four complement control protein modules and a distinctly folded fifth C-terminal domain, is an essential cofactor for the binding to phospholipids of anti-cardiolipin antibodies, isolated from patients with anti-phospholipid antibody syndrome, and its fifth domain has attracted attention as a specific phospholipid-binding site. We focused on the fifth domain of beta2-GPI (Domain V) and examined the interaction of intact Domain V, Domains IV-V, and nicked Domain V with various hydrophobic ligands, as a model molecule of phospholipid. We found that electrostatic and hydrophobic interactions are important for Domain V binding to the ligand molecules. We also found that, while Domain IV has no significant effect on the interactions with ligands, the nicked Domain V with cleavage in the flexible loop decreases the affinity, indicating that the flexible loop region is the binding site of the hydrophobic ligands. The synthetic peptide corresponding to the loop region was disordered and interacted with bis-ANS, confirming the critical role of the loop region. To clarify the nature of the interaction between the loop region and hydrophobic compounds, we prepared the reduced and alkylated Domain V, which was denatured but was assumed to be a collapsed state. Alkylation by iodoacetic acid decreased the interaction of Domain V with bis-ANS, probably because the protein net charge was decreased by the six introduced carboxyl groups and consequently the electrostatic interactions were decreased. In contrast, Domain V alkylated by iodoacetamide, therefore retaining a high positive net charge, bound bis-ANS more strongly than the intact Domain V. These results suggested that the interaction of Domain V with hydrophobic compounds through the flexible loop is similar to the binding of hydrophobic compounds to the protein folding intermediate.

Identification of the phospholipid-binding site of human beta(2)-glycoprotein I domain V by heteronuclear magnetic resonance

To understand the mechanism of the interaction between human beta(2)-glycoprotein I (beta(2)-GPI) and negatively charged phospholipids, we determined the three-dimensional solution structure of the fifth domain of beta(2)-GPI by heteronuclear multidimensional NMR. The results showed that the molecule is composed of well-defined four anti-parallel beta-strands and two short alpha-helices, as well as a long highly flexible loop. Backbone dynamic analysis demonstrated significant mobility of the flexible loop on a subnanosecond time scale. Structural modeling of the nicked fifth domain, in which the Lys317-Thr318 peptide bond was specifically cleaved, revealed the importance of this long C-terminal loop for the interaction between beta(2)-GPI and negatively charged phospholipids. A titration experiment with the anionic surfactant SDS showed that this highly mobile loop, as well as the short beta-hairpin between betaC and betaD strands, which is rich in positively charged residues, specifically interact with the surfactant. The mobile loop, together with the surrounding positively charged residues, probably construct the binding site for negatively charged phospholipids such as cardiolipin.

Intrinsic fluorescence study of the interaction of human apolipoprotein H with phospholipid vesicles

Apolipoprotein H (ApoH) is a plasma glycoprotein with its in vivo physiological and pathogenic roles being closely related to its interaction with negatively charged membranes. In this paper, the interaction of ApoH with phospholipid vesicles was characterized by (i) detecting the wavelength shift of the fluorescence spectrum of ApoH and (ii) measuring the fluorescence quenching extent of ApoH by the membrane resident quencher 1-palmitoyl-2-stearoyl-(5-doxyl)-sn-glycero-3-phosphocholine (DPC). The observed blue shift upon addition of DMPG vesicles indicated that the tryptophan residues of ApoH moved from a polar to a nonpolar environment. The insertion ability of ApoH into PG-containing vesicles did not depend on the PG content in a stoichiometric way as did the blue shift, indicating that the negatively charged DMPG does not serve as a specific binding site but rather provides a suitable microenvironment for ApoH interaction. The finding that the detachment effect of cations on the blue shift is remarkably different from that on the quenching extent suggests that ApoH is capable of existing in two different conformations when membrane-bound.

Beta2-glycoprotein I is necessary to inhibit protein C activity by monoclonal anticardiolipin antibodies

OBJECTIVE

To clarify mechanisms of the thrombosis associated with anticardiolipin antibodies (aCL), we examined the effects on activated protein C (APC) of monoclonal aCL and beta2-glycoprotein I (beta2GPI), which is required for formation of the epitopes of aCL.

METHODS

We developed the chromogenic assay, in which the degradation of coagulation factor Va by APC is reflected in the reduced generation of thrombin from prothrombin, using soybean trypsin inhibitor to inhibit APC. APC activities were measured in the presence and absence of 3.4 microM beta2GPI and/or 2.5 microg/ml of IgM monoclonal aCL (EY2C9 and EY1C8) established from peripheral blood lymphocytes obtained from a patient with aCL.

RESULTS

Without APC, the formed thrombin activity decreased by the addition of 3.4 microM beta2GPI. When 12.8 nM APC was added, beta2GPI partially reversed the APC-induced inhibition of thrombin generation in a concentration-dependent manner. With 3.4 microM beta2GPI, the thrombin generation in monoclonal aCL (2.5 microg/ml) decreased to 77.1-80.2% by the addition of 12.8 nM APC, but the values were above that in the control IgM (72.7%). Without beta2GPI, the APC activity was unaffected by the addition of monoclonal aCL.

CONCLUSION

Beta2-glycoprotein I exhibits procoagulant activity by inhibiting APC activity and anticoagulant activity by inhibiting thrombin generation. Any further inhibition of APC activity was caused by monoclonal aCL and only in the presence of beta2GPI.

The insertion of human apolipoprotein H into phospholipid membranes: a monolayer study

Apolipoprotein H (ApoH) is a plasma glycoprotein isolated from human serum. The interactions of ApoH with lipid membrane were reported to be essential for its physiological and pathogenic roles. In this paper we studied the ability of ApoH to insert into phospholipid membranes using the monolayer approach. The results show that ApoH is surface active and can insert into the lipid monolayers. The insertion ability of ApoH is stronger when a higher content of negatively charged lipids is present in the membrane. The acidic-pH and low-ionic-strength conditions will also enhance ApoH insertion, but these factors may not have much influence on the final insertion ability of ApoH, suggesting that, in the mechanism of ApoH insertion, not only electrostatic forces, but also hydrophobic interactions, are evidently involved. Modification by heat inactivation and reduction/alkylation does not change the critical insertion pressure (pic) of ApoH, suggesting a stable domain, maybe a linear sequence motif, but not the native three-dimensional structure of ApoH, is responsible for its insertion. The extent to which insertion of ApoH into phospholipid membranes may facilitate the 'immune cleaning' of plasma liposomes is discussed.

Current insights into the "antiphospholipid" syndrome: clinical, immunological, and molecular aspects

Advances in defining the target antigen(s) for the autoantibodies in the APS highlight the inadequacies of the current classification of these autoantibodies into anticardiolipin and LA antibodies. The discovery that beta 2GPI is the target antigen for the autoantibodies detected in solid-phase immunoassays has opened a number of areas of research linking these autoantibodies to atherogenesis and thrombus formation. Although the role of beta 2GPI in the regulation of blood coagulation in unclear, current evidence suggests that anti-beta 2GPI antibodies interfere with its "normal" role and appear to promote a procoagulant tendency. The expansion of research in this area and the diversity of the clinical manifestations of patients with APS have resulted in the inclusion of molecular biologists and pharmaceutical companies joining immunologists, hematologists, rheumatologists, obstetricians, neurologists, vascular surgeons, and protein and lipid biochemists in attempting to understand the pathophysiology of this condition. Although the published literature may result in conflicting results and introduce new controversies, developing standardized laboratory methods and extrapolation of in vitro experimental results to the vivo situation will advance our understanding of the regulation of the immune system and its interaction with normal hemostatic mechanisms. Since the authors' last review in 1991, the study and understanding of the pathophysiology of APS have evolved from lipid biochemistry to molecular techniques that may eventually provide specific therapies for the clinical manifestations of this condition. Although current treatment has improved the morbidity associated with this condition, especially in improving pregnancy outcomes, future therapies, as outlined in this review, may specifically address the biological abnormalities and have fewer side effects. Better diagnostic tools, such as magnetic resonance imaging with perfusion studies, will allow the study of the true incidence and prevalence of vascular flow changes/tissue ischemia and infarction associated with aPL antibodies and help determine treatment and prophylaxis for APS patients. APS is still the only hypercoagulable condition where both arterial and venous beds can be affected independently or in the same individual.

Plasmin Can Reduce the Function of Human β2Glycoprotein I by Cleaving Domain V Into a Nicked Form

β2-Glycoprotein I (β2GPI) is a highly glycosylated plasma protein with the ability to bind negatively charged substances such as DNA, heparin, dextran sulfate, and negatively charged phospholipids. The most relevant physiological role of β2GPI is supposed to be the regulation of the function of anionic phospholipids like cardiolipin (CL). β2GPI consists of a single polypeptide chain (326 amino acid residues) with a molecular mass of about 50 kD and with five tandem repeated domains (I, II, III, IV, and V). In the previous study, we found that factor Xa can produce the nicked form by cleaving Lys 317-Thr 318, using recombinant human domain V (r-Domain V). However, the reaction was extremely slow. In the present paper, we found that plasmin can produce the nicked form of domain V, using recombinant domain V (r-Domain V) and β2GPI from human plasma. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, r-Domain V was rapidly cleaved into a nicked form by plasmin, very slowly by factor Xa, but not by thrombin, tissue-type plasminogen activator, urokinase, and tissue factor/factor VIIa. The cleavage site of r-Domain V and β2GPI by plasmin was proved to be Lys 317-Thr 318 by amino acid sequence analysis of the digest and of the C-terminal peptide isolated by high-performance liquid chromatography. The cleavage was completely inhibited by plasmin inhibitor (α2PI). The nicked form was demonstrated to show reduced affinity for CL with a dissociation constant of one order of magnitude larger than that of the intact β2GPI. To determine whether the specific cleavage of β2GPI by plasmin can occur also in plasma, human plasma was first acid-treated to inactivate α2PI and then incubated with urokinase. About 12% of β2GPI in plasma was nicked when α2PI activity decreased to 80%. The nicked form was not generated in plasminogen-depleted plasma. These results suggest that plasmin can produce the nicked form of β2GPI with the reduced ability to bind phospholipids in vivo.

Plasmin Can Reduce the Function of Human β2Glycoprotein I by Cleaving Domain V Into a Nicked Form

β2-Glycoprotein I (β2GPI) is a highly glycosylated plasma protein with the ability to bind negatively charged substances such as DNA, heparin, dextran sulfate, and negatively charged phospholipids. The most relevant physiological role of β2GPI is supposed to be the regulation of the function of anionic phospholipids like cardiolipin (CL). β2GPI consists of a single polypeptide chain (326 amino acid residues) with a molecular mass of about 50 kD and with five tandem repeated domains (I, II, III, IV, and V). In the previous study, we found that factor Xa can produce the nicked form by cleaving Lys 317-Thr 318, using recombinant human domain V (r-Domain V). However, the reaction was extremely slow. In the present paper, we found that plasmin can produce the nicked form of domain V, using recombinant domain V (r-Domain V) and β2GPI from human plasma. On sodium dodecyl sulfate-polyacrylamide gel electrophoresis, r-Domain V was rapidly cleaved into a nicked form by plasmin, very slowly by factor Xa, but not by thrombin, tissue-type plasminogen activator, urokinase, and tissue factor/factor VIIa. The cleavage site of r-Domain V and β2GPI by plasmin was proved to be Lys 317-Thr 318 by amino acid sequence analysis of the digest and of the C-terminal peptide isolated by high-performance liquid chromatography. The cleavage was completely inhibited by plasmin inhibitor (α2PI). The nicked form was demonstrated to show reduced affinity for CL with a dissociation constant of one order of magnitude larger than that of the intact β2GPI. To determine whether the specific cleavage of β2GPI by plasmin can occur also in plasma, human plasma was first acid-treated to inactivate α2PI and then incubated with urokinase. About 12% of β2GPI in plasma was nicked when α2PI activity decreased to 80%. The nicked form was not generated in plasminogen-depleted plasma. These results suggest that plasmin can produce the nicked form of β2GPI with the reduced ability to bind phospholipids in vivo.

Human plasmatic apolipoprotein H binds human immunodeficiency virus type 1 and type 2 proteins

Apolipoprotein H (apo H), isolated from human plasma albumin solution, was shown to capture HIV-1-related antigens from antigen-positive sera (HIV-1 AG+) of AIDS patients, by using HIV-1-specific polyclonal antibodies. In an enzyme-linked immunosorbent assay and ligand blot and dot assays, apo H was able to bind recombinant retroviral HIV antigens, especially Gag proteins p18 of HIV-1, p26 of HIV-2, and Env gp160 of HIV-1. Binding was shown to be pH and NaCl dependent, with an optimum at acidic pH and low ionic strength. Specificity was demonstrated by saturation of this binding and inhibition either by homologous competition or by specific antisera. Binding was also observed in cell line-harvested viral proteins. The mechanism of this apo H-polyspecific binding is discussed in relation to conformational changes due to the influence of lipids or detergents.

Characterization of binding of human beta 2-glycoprotein I to cardiolipin

beta 2-Glycoprotein I-cardiolipin complexes are reported to be a target antigen for the binding of a subset of anti-phospholipid antibodies. The characteristics of binding of beta 2-glycoprotein I to cardiolipin are reported in this paper. Binding at neutral pH is specific, saturable, dependent on ionic strength and independent of bivalent cation. Binding at low pH is qualitatively different from that at neutral pH, and is not dependent on ionic strength. Denaturation of beta 2-glycoprotein I by heat inactivation and reduction/alkylation indicates that beta 2-glycoprotein I-cardiolipin interaction does not require the native three-dimensional structure of beta 2-glycoprotein I, implying that a linear sequence motif may be responsible. Modification of amino acid residues by KCNO treatment completely destroys binding capacity, indicating crucial involvement of lysine residues in binding of beta 2-glycoprotein I to cardiolipin. Complement factor H, which has some similar highly charged linear sequence motifs to beta 2-glycoprotein I and is composed of the same type of protein module, was found to bind to cardiolipin and inhibit the binding of beta 2-glycoprotein I to cardiolipin. Three different lysine-rich segments of the fifth domain of beta 2-glycoprotein I may be involved in binding to cardiolipin.

beta 2-Glycoprotein I reactivity of monoclonal anticardiolipin antibodies from patients with the antiphospholipid syndrome

OBJECTIVE

To elucidate the specificity of anticardiolipin antibodies (aCL) from patients with the antiphospholipid syndrome (APS) to various phospholipids (PLs), DNA, and beta 2-glycoprotein I (beta 2-GPI).

METHODS

Five monoclonal aCL were established from peripheral blood lymphocytes of 3 patients with the APS. The reactivity of monoclonal aCL with various PLs, with DNA, and with beta 2-GPI was examined by enzyme-linked immunosorbent assay (ELISA).

RESULTS

All of the monoclonal aCL bound to anionic PLs, only in the presence of beta 2-GPI. Neither monoclonal aCL nor beta 2-GPI bound to DNA. Monoclonal aCL bound to solid-phase beta 2-GPI on polystyrene ELISA plates that had carboxyl groups on their surface, but did not react with solid-phase beta 2-GPI on ordinary polystyrene plates. A mixture of beta 2-GPI and CL inhibited the binding of monoclonal aCL to beta 2-GPI, but CL or beta 2-GPI alone did not.

CONCLUSION

Monoclonal aCL may recognize a cryptic epitope, which appears as a result of beta 2-GPI binding to anionic PLs or to polystyrene with carboxyl groups.

Elution of anticardiolipin antibodies and their cofactor beta 2-glycoprotein 1 from the placentae of patients with a poor obstetric history

Anticardiolipin antibodies (aCL) were eluted from the placentae of four women with elevated serum levels of aCL, demonstrating that these antibodies are bound to affected placentae. Anticardiolipin antibodies bound to affected placentae were only of the IgG isotype and the level of aCL in placental eluates did not reflect serum levels. Anticardiolipin antibodies were not isolated from placental eluates of control normal pregnancies. beta 2-Glycoprotein 1, the anticardiolipin antibody cofactor, was present in the placental eluates from both control and antiphospholipid antibody (aPL) affected pregnancies and was localised in the syncytiotrophoblast by immunohistochemical analysis. Antinuclear antibodies were present in the placental eluates of 3 of the 4 patients with antiphospholipid antibody syndrome and were absent from the placental eluates of control pregnancies. The authors propose that anticardiolipin antibody binds directly to placental tissue, disrupting uteroplacental blood flow and/or transport through the villi.

Activity, disulphide mapping and structural modelling of the fifth domain of human beta 2-glycoprotein I

Complexes formed by the interaction of negatively charged phospholipids and beta 2-glycoprotein I (beta 2-I) are the target of autoantibodies in systemic lupus erythematosus. The highly positively charged fifth (C-terminal) domain of human beta 2-I was produced as a fusion protein in an Escherichia coli expression system and was shown to bind to the negatively charged phospholipid, cardiolipin, almost as well as the intact protein. In an attempt to define the 3D structure of this domain, the disulphide linkage pattern was determined and shown to be Cys 1-4, Cys 2-5 and Cys 3-6 in contradiction to an earlier report. In the light of this information, the sequence of the fifth domain of beta 2 I (beta 2-I-5) is readily aligned with that of the 16th repeat of factor H, of which the 3D structure is known, and a model of beta 2I-5 has been built by homology. On the basis of the model we suggest residues that might be the target of profitable site-directed mutagenesis in structure-function studies.

Subclasses of IgG anticardiolipin antibodies in patients with systemic lupus erythematosus

Antibodies directed to a co-factor associated with negatively charged phospholipids, such as cardiolipin, occur in patients with systemic lupus erythematosus (SLE), and possibly more often in those with venous or arterial thrombosis, thrombocytopenia or recurrent fetal loss. They are also found in patients without any of these manifestations and their biological effect, if any, might thus be related to their IgG subclass. To investigate this possibility, we determined anticardiolipin antibodies (ACA) by enzyme immunoassay (EIA) using monoclonal antibodies (MoAb) against human IgG subclasses. A net absorbance of x +3 SD of the value of 30 blood donors was taken as the cut-off point. The specificity of the assay was verified through inhibition experiments using cardiolipin micelles. Thirty-three patients with SLE were studied, all of whom had been shown to have ACA by a point dilution screening assay. IgG1 ACA were found in 85% of the patients, and ACA of the IgG2, IgG3 and IgG4 subclasses in 42%, 39% and 15%. There was a significant correlation between the presence of IgG3 ACA and of anti-DNA antibodies but none between subclass distribution and major clinical manifestations of SLE.

Complete nucleotide and deduced amino acid sequence of human beta 2-glycoprotein I

The nucleotide and complete amino acid sequence for the human beta 2-glycoprotein I (beta 2I) was derived by sequencing the cDNA clone pB2I-1. In addition to the 326 amino acid residues of the mature protein this clone codes for a putative leader peptide and contains sequence representing 5' and 3' untranslated regions. When this amino acid sequence was compared with the previously published primary sequence, three major amino acid substitutions were found, two involving cysteine residues. These substitutions lead to a new alignment of the complement control protein (CCP) repeats present in beta 2I and a prediction of the complete disulphide bond organization. Northern-blot analysis indicates that hepatocytes are a major site of biosynthesis for this protein. A transcription signal of about 1.5 kb was detected by using RNA from HepG2 cells.

Anti-phospholipid antibodies are directed against a complex antigen that includes a lipid-binding inhibitor of coagulation: beta 2-glycoprotein I (apolipoprotein H)

Anti-phospholipid (aPL) antibodies that exhibit binding in cardiolipin (CL) ELISA can be purified to greater than 95% purity by sequential phospholipid affinity and ion-exchange chromatography. However, these highly purified aPL antibodies do not bind to the CL antigen when assayed by a modified CL ELISA in which the blocking agent does not contain bovine serum, nor do they bind to phospholipid affinity columns. Binding to the phospholipid antigen will only occur if normal human plasma, human serum, or bovine serum is present, suggesting that the binding of aPL antibodies to CL requires the presence of a plasma/serum cofactor. Using sequential phospholipid affinity, gel-filtration, and ion-exchange chromatography, we have purified this cofactor to homogeneity and shown that the binding of aPL antibodies to CL requires the presence of this cofactor in a dose-dependent manner. N-terminal region sequence analysis of the molecule has identified the cofactor as beta 2-glycoprotein I (beta 2GPI) (apolipoprotein H), a plasma protein known to bind to anionic phospholipids. These findings indicate that the presence of beta 2GPI is an absolute requirement for antibody-phospholipid interaction, suggesting that bound beta 2GPI forms the antigen to which aPL antibodies are directed. Recent evidence indicates that beta 2GPI exerts multiple inhibitory effects on the coagulation pathway and platelet aggregation. Interference with the function of beta 2GPI by aPL antibodies could explain the thrombotic diathesis seen in association with these antibodies.

The subunit composition of factor XIII proteins in normal and factor XIII deficient plasma and serum analysed by line immunoelectrophoresis

Line immunoelectrophoresis was used for the characterization of human coagulation factor XIII in normal and factor XIII-deficient plasma and serum. In normal plasma, two immunoreactive factor XIII proteins termed XIII-p (Mr 330,000 Da) and XIII-s (Mr 170,000 Da) were identified within the beta-globulin fraction. Their reactions with monospecific antisera against known factor XIII subunits, suggests that the factor XIII-p and -s proteins recognized represent the factor XIII a2b2 tetramer and -b2 dimer, respectively. The concentration of the XIII-p protein was, in normal plasma, approximately 6 mg/l, in plasma from a case of congenital factor XIII, approximately 1 mg/l and in normal serum approximately 3 mg/l. The concentration of the XIII-s protein (approximately 18 mg/l) was unaffected by the coagulation process and normal in plasma and serum from the case of congenital factor XIII deficiency. Thus, the present investigation indicates the presence of an excess of the anticatalytic, DNA-binding factor XIII carrier protein in normal as well as in factor XIII-deficient plasma and serum.

The effect of beta 2-glycoprotein I on the dextran sulfate and sulfatide activation of the contact system (Hageman factor system) in the blood coagulation

1. beta 2-Glycoprotein I, inhibits the initiation of the contact system in plasma accomplished by dextran sulfate. 2. The dextran sulfate induced activation could be inhibited both when dextran sulfate was preincubated with beta 2-glycoprotein I and when the amount of beta 2-glycoprotein I in plasma was increased. 3. The concentration of beta 2-glycoprotein I at which an inhibitory effect could be registered was dependent upon the concentration of negatively charged groups on the surface. Calculation of the molar ratios between beta 2-glycoprotein I and sulfate residues in dextran sulfate showed that beta 2-glycoprotein I had to be present in excess of a 1:1 stoichiometric ratio of the sulfate group in order to inhibit the activation. 4. beta 2-Glycoprotein I does not inhibit the initiation of the contact system in plasma accomplished by sulfatide, unless the sulfatide has been preincubated with beta 2-glycoprotein I.

The application of DNA-cellulose chromatography in the isolation of immunoglobulin M and complement component C4b-binding protein from human serum

By utilising the ability of some anionic serum proteins to bind to DNA-cellulose, at physiological pH and ionic strength, two such proteins have been purified and identified as immunoglobulin M (IgM) and complement component C4b-binding protein (C4BP). The method entails ammonium sulphate precipitation followed by QAE-Sephadex, DNA-cellulose and Sephacryl S-300 chromatographic steps. Fractionation of the anionic proteins eluting from QAE-Sephadex over a selected range of ionic strength has afforded IgM which was characterised electrophoretically and immunochemically. A mixture containing IgM complexed with C4BP has also been obtained by utilizing fractions eluted from QAE-Sephadex at higher ionic strength. C4BP can be isolated from this mixture by using immunoaffinity chromatography to remove IgM.

Amino acid sequence of the UP1 calf thymus helix-destabilizing protein and its homology to an analogous protein from mouse myeloma

A complete amino acid sequence has been determined for the UP1 single-stranded DNA binding protein from calf thymus that was first described by G. Herrick and B. M. Alberts [(1976) J. Biol. Chem. 251, 2124-2132]. Peptides required to establish the UP1 sequence were isolated by reversed-phase HPLC of digests produced by endoproteinase Lys-C, trypsin, chymotrypsin, Staphylococcus aureus V8 protease, and cyanogen bromide cleavage of UP1. The purified peptides were coupled to aminopolystyrene prior to solid-phase sequencing. UP1 contains 195 amino acids and has a molecular weight of 22,162. UP1 has a blocked NH2 terminus and contains a single NG,NG-dimethylarginine residue near its COOH terminus. Gas-phase sequencing of tryptic peptides derived from an analogous protein from mouse myeloma cells [Planck, S. R. & Wilson, S. H. (1980) J. Biol. Chem. 255, 11547-11556] revealed that this mouse helix-destabilizing protein shares a high degree of sequence homology with UP1. Of the 59 amino acids in the mouse protein that have so far been found to be homologous with UP1, 48 correspond exactly to sequences found in UP1. Most of the 11 differences that have been found between the sequences of these two proteins are conservative in nature, involving primarily the interchange of chemically similar amino acids. One 9-residue mouse sequence that is not obviously homologous to UP1 may be a result of the larger size of the mouse myeloma protein as compared to UP1. Since none of the UP1 or mouse myeloma helix-destabilizing protein sequence appears to be homologous to that of any previously sequenced protein, we presume that these two proteins represent a distinct class of single-stranded nucleic acid binding proteins that probably play a role in metabolism of single-stranded RNA or DNA in vivo.

Tumor-induced growth-inhibitory activity isolated from the Yoshida ascites fluid by extraction with methanol

Growth-inhibitory activity was isolated from the Yoshida ascites fluid by sequential precipitation with polyethylene glycol, extraction with methanol, LH 20 Sephadex chromatography and preparative agarose gel electrophoresis. The cell non-specific activity was tumor-related as far as analogous fractions prepared from normal sera were inactive. Flow cytometric analysis indicates that the inhibition of cell growth was caused by blockage of the G1-S and possibly the G2-M phase transitions. The active component migrates electrophoretically associated to an unidentified alpha 1-antigen. In aqueous solution the molecular size of the inhibitor is ill defined, due to a tendency to autoaggregation and hydrophobic interaction with the chromatographic media. The molecular weight of the inhibitor as estimated by LH 20 Sephadex chromatography in methanol is approximately 350 daltons. This chromatographic fraction contains prostaglandin-E2 cross-reactive material in amounts suggesting the participation of a prostaglandin derivative in the observed growth-inhibitory activity.

Crossed immunoelectrophoresis of rat serum

A crossed immunoelectrophoresis (X-IEP) reference profile of normal (Sprague-Dawley) rat serum antigens was developed. Twenty-three antigens were detected; 13 were identified. These included inter-alpha-trypsin inhibitor, alpha 1-antichymotrypsin, and fibronectin, which had not previously been identified in rat serum. Many of the antigens were similar electrophoretically to their analogs in human and mouse serum, but some were substantially different. Inter-alpha-trypsin inhibitor, fibronectin, and C3 globulin were more anodic, while transferrin was more cathodic. Rat haptoglobin's mobility was between those of human and mouse haptoglobins. As in the mouse, but more so, alpha-macroglobulin in the rat had alpha 1 mobility rather than alpha 2 seen for this protein in human serum. Only rat alpha 1-antichymotrypsin exhibited a double peak. There were variations in haptoglobin and alpha 1-lipoprotein mobilities among different strains of rat. This establishment of a standard X-IEP map of rat serum will simultaneously permit multiple quantitative and qualitative analyses of rat serum antigens for various experimental and clinical purposes.

Effect of beta 2-glycoprotein I on the activity of adenylate cyclase in platelet membranes

Washed human platelets were preincubated with 0-300 micrograms/ml human beta 2-glycoprotein I and the effect of this on the adenylate cyclase activity (EC.4.6.1.1.) was studied. Adenylate cyclase activity could be increased 2-3 fold. The same degree of activation was seen when low concentrations of prostaglandin E1 (1 microM) had been present concomitant with beta 2-glycoprotein I during preincubation. The dose-response curves of the adenylate cyclase activity measured as a function of the beta 2-glycoprotein I concentration were S-shaped in the absence of prostaglandin E1 and hyperbolic in its presence. The results suggest a biological function of beta 2-glycoprotein I as a compound conserving and activating the membrane-bound adenylate cyclase.

The purification of alpha 1-antichymotrypsin from human serum using DNA-cellulose chromatography

By exploiting its capacity for binding to DNA, the protease inhibitor alpha 1-antichymotrypsin has been isolated from human serum by ammonium sulfate fractionation and successive chromatography on QAE-Sephadex, DNA-cellulose, and Sephacryl S-300. This experimental procedure compares favorably with existing methods for preparing alpha 1-antichymotrypsin in terms of overall yield and practical convenience. The purified alpha 1-antichymotrypsin was homogeneous as judged by electrophoretic and immunoelectrophoretic criteria. From its inhibition of the fluorimetric titration of chymotrypsin with 4-methylumbelliferyl-p-trimethylammonium cinnamate it was shown to combine with chymotrypsin in a 1:1 molar ratio and thus to retain its biological activity.

Characterization of the interaction between beta 2-glycoprotein I and mitochondria, platelets, liposomes and bile acids

Several different binding mechanisms appear to be involved in the binding of beta 2-glycoprotein I to biological membranes. One of these mechanisms is a hydrophilic interaction between negatively-charged phospholipids in the membrane and histidine residues in beta 2-glycoprotein I. This mechanism seems to be involved in binding of the protein to mitochondria but not to platelets. Another mechanism may involve a site on beta 2-glycoprotein I, which binds to the steroid ring system particularly to such steroids not having a 7-hydroxy group. This type of binding may be involved in the interaction between beta 2-glycoprotein I and platelets as well as mitochondria.

Addition of deoxycholate in electroimmunoassay and crossed immunofocusing for quantification of beta 2-glycoprotein I and its subfractions

Beta 2-Glycoprotein I was shown to be a hydrophilic protein exhibiting no charge shift in the presence of a cationic detergent, but a charge shift in the presence of an anionic detergent. The latter was suggested to be caused by a binding of beta 2-glycoprotein I to deoxycholate in the Triton X-100/deoxycholate micelles. Quantification by electroimmunoassay of asialo-beta 2-glycoprotein I and subfractions of beta 2-glycoprotein I gave different results although identical results were obtained in single radial immunodiffusion. Addition of 0.2% (w/v) of deoxycholate to the agarose gels containing Triton X-100 prior to electrophoresis, however, eliminated these differences. The effect of deoxycholate on the rate of migration of beta 2-glycoprotein I was found applicable for electroimmunoassay of the protein. Crossed immunofocusing of plasma from individual donors, electrophoresed in an agarose containing Triton X-100/deoxycholate micelles confirmed a postulated variation in the relative composition of subfractions of beta 2-glycoprotein I in plasma earlier suggested by Finlayson and Mushinski (Finlayson, J.S. and Mushinski, J.F. (1967) Biochim. Biophys. Acta 147, 413-420).

Crossed immunoelectrophoresis of mouse serum

Because crossed immunoelectrophoresis (X-IEP) is quantitative and offers more information than immunoelectrophoresis (IEP), we selected X-IEP to study various physiologic and pathologic effects on the antigen composition of mouse serum. We therefore needed and X-IEP map of normal mouse serum. This paper presents one that we have developed, along with brief descriptions of how the mouse serum antigens were identified. Cross-reactivity between several mouse and human serum antigens was especially helpful. Some data from using ratios of the areas of precipitation of detected antigens to that of an internal standard antigen, alpha-macroglobulin, are presented. They show that X-IEP readily detects quantitative abnormalities among any of the several serum antigens detected by this technique, and that 'normal' values for these antigens in one population of mice can be established in the same way of using ratios with just a few analyses.

Purification, characterization and identification of an agglutinin in human serum

A serum protein named agglutinin is able to induce mitochondria to agglutinate. The protein has been purified from human serum by chromatography on DE-52. Sephadex G-200 and immunoglobulin-Sepharose 4B columns. Agglutinin is a glycoprotein that migrates electrophoretically as a gamma-globulin. Its molecular weight was determined to be 50,000 on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. Monospecific antiserum prepared against the agglutinin was found to be identical with anti-beta 2-glycoprotein I and agglutinating activity could be adsorbed on anti-beta 2-glycoprotein I-Sepharose 4B columns. Thus, the agglutinin has been identified as beta 2-glycoprotein I. The reaction between mitochondria and agglutinin shows positive cooperativity, which is independent on the stage of purification of agglutinin. The agglutinating activity could be diminished (inhibited) by acidic non-soluble lipids such as oleic acid, phosphatidic acid, phosphatidyl serine and phosphatidyl inositol.