Production of autoantibodies by human-human hybridomas

Peripheral blood lymphocytes and splenocytes of patients with autoimmune disease were used to prepare human-human hybridomas that produce autoantibodies. Because exogenous immunization was not used, the hybridoma antibodies were derived from B cells that spontaneously produced autoantibodies. 108 hybrids grew from 4,254 wells (2.5%). Optimal conditions for obtaining hybridomas with the GM 4672 myeloma line included initial growth in 2-ml wells, the use of 44% polyethylene glycol, a mononuclear cell/GM 4672 cell ratio 5:1, and prior stimulation of the B lymphocytes with pokeweed mitogen. Hybridoma supernatants had activity against ssDNA, platelets, and erythrocytes. The results demonstrate the feasibility of producing human-human hybridomas from lymphocytes of patients with various autoimmune diseases.

Studies on a family with combined functional deficiencies of vitamin K-dependent coagulation factors

Two siblings with m ild hemorrhagic symptoms had combined functional deficiencies of vitamin K-dependent clotting factors. Prothrombin (0.18-0.20 U/ml) and Stuart factor (Factor X, 0.18-0.20 U/ml) and Stuart factor (Factor X, 0.18-0.20 U/ml) were most severely affected. Antigenic amounts of affected coagulation factors were normal and normal generation of thrombin activity occurred in the patients' plasmas after treatment with nonophysiologic activators that do not require calcium for prothrombin activation. Hepatobilary disease, malabsorptive disorders, and plasma warfarin were not present. Both parents had normal levels of all coagulation factors. The patients' plasmas contained prothrombin that reacted both with antibody directed against des-gamma-carboxyprothrombin and native prothrombin. Crossed immunoelectrophoresis of patients' plasmas and studies of partially purified patient prothrombin suggested the presence of a relatively homogeneous species of dysfunctional prothrombin, distinct from the heterologous species found in the plasma of warfarin-treated persons. These studies are most consistent with a posttranslational defect in hepatic carboxylation of vitamin K-dependent factors. This kindred uniquely possesses an autosomal recessive disorder of vitamin K-dependent factor formation that causes production of an apparently homogeneous species of dysfunctional prothrombin; the functional deficiencies in clotting factors are totally corrected by oral or parenteral administration of vitamin K1.

Computer-generated models of blood coagulation factor Xa, factor IXa, and thrombin based upon structural homology with other serine proteases

Computer-generated molecular models of the trypsin-like domains of blood coagulation factor IXa, Factor Xa, and thrombin have been prepared. These hypothetical models are based upon the sequence homology of the blood coagulation enzymes with the pancreatic serine proteases and the known three-dimensional structure of the pancreatic serine proteases. The internal structures and active sites of these enzymes are highly conserved. The high degree of substrate specificity which characterizes the blood coagulation enzymes appears to be defined not entirely by the active site, but by the unique molecular surface surrounding the active site of each enzyme. Several regions which demonstrate high sequence variability among these enzymes likely participate in forming the putative extended substrate binding sites.

Gamma-carboxyglutamic acid

Gamma-carboxyglutamic acid is an amino acid with a dicarboxylic acid side chain. This amino acid, with unique metal binding properties, confers metal binding character to the proteins into which it is incorporated. This amino acid has been discovered in blood coagulation proteins (prothrombin, Factor X, Factor IX, and Factor VII), plasma proteins of unknown function (Protein C, Protein S, and Protein Z), and proteins from calcified tissue (osteocalcin and bone-Gla protein). It has also been observed in renal calculi, atherosclerotic plaque, and the egg chorioallantoic membrane, among other tissues. Gamma-carboxyglutamic acid is synthesized by the post-translational modification of glutamic acid residues. This reaction, catalyzed by a hepatic carboxylase, requires reduced vitamin K, oxygen, and carbon dioxide. The function of gamma-carboxyglutamic acid is uncertain. In prothrombin gamma-carboxyglutamic acid residues bound to metal ions participate as an intramolecular non-covalent bridge to maintain protein conformation. Additionally, these amino acids participate in the calcium-dependent molecular assembly of proteins on membrane surfaces through intermolecular bridges involving gamma-carboxyglutamic acid and metal ions.

Acquired vitamin K-dependent carboxylation deficiency in liver disease

gamma-Carboxyglutamic acid residues on prothrombin are synthesized from glutamic acid on a prothrombin precursor in the liver through a vitamin K-dependent carboxylase. In the absence of vitamin K or in the presence of vitamin K antagonists, an inert form of prothrombin - abnormal prothrombin - circulates in the blood. We have developed specific immunoassays for native and abnormal human prothrombin. The prothrombin concentration in our normal subjects was 108 +/- 19 microgram per milliliter. The abnormal-prothrombin concentration varied over four orders of magnitude between the limits of detection in normal plasma and the level in patients with cirrhosis (0 to 5 microgram per milliliter), acute hepatitis (0 to 33 microgram per milliliter), or vitamin K deficiency (32 to 100 microgram per milliliter) and in those treated with sodium warfarin (12 to 65 microgram per milliliter). These studies indicate that abnormal prothrombin is not a component of normal plasma but appears in a variety of hepatic and nutritional disorders characterized by impaired hepatic vitamin-K-dependent carboxylation.

A computer-generated three-dimensional model of the B chain of bovine alpha-thrombin

A computer graphic molecular display system has been used to construct a three-dimensional model of the B chain of bovine thrombin. The model is derived from the bovine alpha-chymotrypsin structure as determined by X-ray crystallographic studies. The amino acid sequence of bovine thrombin has been substituted for that of alpha-chymotrypsin, preserving the beta-barrel structure and maximizing homology of the amino acid sequence of the two proteins. With the exception of an area in the vicinity of the specificity binding pocket, most of the changes observed in thrombin occur on the surface of the molecule. The most notable changes observed in the model are the increases on the surface of positively charged (arginine and lysine) and negatively charged (glutamate and aspartate) residues. A glutamate replaces methionine 192 near the entrance to the specificity binding pocket. The nature of this site was further altered by the substitution of an aspartate for serine 189 and an alanine for serine 190. The structure of the resulting specificity binding pocket is consistent with that of serine proteases, which have trypsin-like substrate specificity. The computer graphics molecular display system has been used to insert models of synthetic thrombin inhibitors into the active site of the thrombin B chain model. With the model, it has been possible to correlate the interaction of thrombin with the observed binding constants of two inhibitors of trypsin-like serine proteases, p-amidinophenylmethylsulfonylfluoride (Ki = 1.27 x 10(-6) M) and m-[m-(trifluoromethyl)phenoxypropoxy]benzamidine (KD = 2.9 x 10(-6) M).

Polyspecific monoclonal lupus autoantibodies reactive with both polynucleotides and phospholipids

Hybridomas the produce anti-DNA autoantibodies were prepared from spleen cells of unimmunized MRL/1 mice, a strain that spontaneously develops severe systemic lupus erythematous (SLE). Reactivities of these monoclonal antibodies with a wide range of polynucleotides prompted tests of their reactions with phospholipids which, like polynucleotides, contain diester-linked phosphate groups in their backbones. In competitive radioimmunoassays, cardiolipin, phosphatidic acid, and phosphatidyl glycerol blocked the binding of these hybridoma antibodies to denatured DNA. These phospholipids also specifically inhibited the reaction between a hybridoma antibody and a site-specific anti-idiotypic antibody. The antinuclear reaction of one of these antibodies was specifically inhibited by cardiolipin. This same antibody prolonged the activated partial thromboplastin time in a manner characteristic of a lupus anticoagulant, presumably by binding to phospholipid in the test system. The polyspecific reactivity of a single molecular species of lupus autoantibody suggests that some of the diverse serological abnormalities of SLE may be a result of the binding of certain autoantibodies to a phosphodiester-containing epitope that is present in diverse biological molecules.

Thrombotic stroke associated with elevated plasma factor VIII

A previously healty 43-year-old woman developed bilateral middle cerebral artery occlusion, thrombocytopenia, and multiple thrombi of arteries in the spleen. No underlying cause of her thrombotic disease was detected except for a factor VIII level that was elevated fivefold. An etiological relationship between the cerebral occlusive disease and elevated factor VIII level is suggested. We recommend that factor VIII levels be assayed in patients with stroke when an underlying disorder is not identified.

Zymogens and cofactors of blood coagulation

Blood coagulation is a system in which a series of zymogens of serine proteases are sequentially activated. In this regard, there is little fundamental difference between coagulation and the activation of the homologous pancreatic zymogens. There are, however, several aspects unique to coagulation which are discussed in detail. These are (1) the requirement for a high-molecular-weight protein or lipoprotein cofactor for optimal reaction rates, (2) the requirement for membranes or a membrane-like surface which further distinguishes this system; (3) a metal ion requirement for most reactions (in contrast to the pancreatic serine proteases) relating to the content of the newly described amino acid gamma-carboxyglutamic acid in the four vitamin K-dependent proteins, regarding which recent data relating to the metal binding sites on prothrombin are discussed in detail; and (4) the uniqueness of the initiating reactions in comparison to those which activate the pancreatic zymogens, insofar as no enzyme corresponding to enterokinase has been identified. The implications of this phenomenon are analyzed with particular attention to the potential role of the endogenous activity of certain zymogens in initiating coagulation. The article deals finally with the specific problems attendant on analyzing a system in which many serine proteases lacking absolute specificity are generated and regulated.

Metal binding sites of a gamma-carboxyglutamic acid-rich fragment of bovine prothrombin

The metal binding sites of a gamma-carboxyglutamic acid-rich fragment derived from bovine prothrombin were examined using paramagnetic lanthanide ions to evaluate the role of gamma-carboxyglutamic acid resideus in metal binding. A gamma-carboxyglutamic acid-rich peptide, fragment 12-44, was isolated from a tryptic digest of prothrombin. Using 153Gd(III), fragment 12-44 was found to contain one high affinity metal binding site (KD = 0.55 microM) and four to six lower affinity metal binding sites (KD approximately 4 to 8 microM). The S-carboxymethyl derivative of fragment 12-44, in which the disulfide bond in fragment 12-44 was reduced and alkylated, contained no high affinity metal binding site and four or five lower affinity sites (KD = 8 microM). The effects of paramagnetic lanthanide ions on fragment 12-44 and its S-carboxymethyl derivative were studied by natural abundance 13C NMR spectroscopy. The 13C NMR spectrum of fragment 12-44 was recorded at 67.88 MHz and the resonances were assigned by comparison to the chemical shift of carbon resonances of amino acids and peptides previously studied. The proximity between bound metal ions and carbon atoms in fragment 12-44 was estimated using Gd(III), based upon the strategy that the magnitude of the change in the transverse relaxation rate of resonances of carbon nuclei induced by bound metal ions is related in part to the interatomic distances between bound metal and carbon nuclei. Titration of fragment 12-44 with Gd(III) resulted in the selective broadening of the gamma-carboxyl carbon, C gamma, C beta, and C alpha resonances of gamma-carboxyglutamic acid, and the C epsilon of the arginines. S-Carboxymethyl fragment 12-44, which lacked the high affinity metal binding site, showed markedly decreased perturbation of the C epsilon of the arginine residues upon titration with Gd(III). These studies indicate that gamma-carboxyglutamic acid residues in prothrombin fragment 12-44 participate in metal liganding. A high affinity metal binding site in fragment 12-44 is in close proximity of Arg 16 and Arg 25 and is stabilized by the disulfide bond. On the basis of these data, a model of the metal binding sites is proposed in which the high affinity site is composed of two gamma-carboxyglutamic acid residues which participate in intramolecular metal-dependent bridging of two regions of the polypeptide chain. The lower affinity metal binding sites, formed by single or paired adjacent gamma-carboxyglutamic acid residues, then may participate in intermolecular metal-dependent protein . protein or protein . membrane complex formation.

Conformation of the γ-Carboxyglutamic Acid-Rich Region of Prothrombin: Conformation-Specific Antibodies as Structural Probes

Stabilization of the tertiary structure of bovine prothrombin (PT) by Ca(II) has been evaluated immunochemically using conformation-specific antibodies directed against limited regions of the protein. Antisera raised against native PT or des-γ-carboxy PT (AbPT) were fractionated by sequential immunabsorption using affinity chromatography. Three antibody subpopulations, specific for different determinants on the protein, were isolated and evaluated using RIA: (1) Anti-(12-44), directed against the Gla-rich region 12-44, requires Ca(II) or other metals to interact strongly with PT. Anti-(12-44) cross-reacts poorly with AbPT, factor IX, factor X, and human PT; (2) Anti-PT-Ca(II) is specific for the binary PT-Ca(II) complex and does not interact with PT. Anti-PT-Ca(II) is directed against multiple antigenic determinants in the fragment 1 region; (3) Anti-AbPT is specific for AbPT and does not cross-react with PT. We conclude that occupancy of the high affinity metal binding site by metal ions is associated with a significant change in the conformation of the 12-44 region and other aspects of the N-terminal portion of PT. These antigenic surfaces are not present in AbPT. Conversely, some determinants on AbPT are not observed on PT. These results are consistent with our current model of the role of Gla in metal-dependent intramolecular and intermolecular bridging in the vitamin-K-dependent proteins.