The Preparation and Activation of [sialyl-3H]Prothrombin

Interaction of an amphitropic protein (factor Xa) with membrane models in a complex system

Phosphatidylserine (PS) plays a crucial role, in the conversion of prothrombin into thrombin by the protease, factor Xa. Physiologically, the conversion occurs in the prothrombinase complex. The question of how water-soluble proteins that normally circulate in plasma bind remains to be unambiguously determined. We previously found that the amphitropic proteins (prothrombin, factors V and Va) penetrate into phospholipid layers. AC polarography has allowed the detection for the first time of insertion of factor Xa into condensed monolayers containing phosphatidylserine (PS) and phosphatidylcholine (PC) either 100% PS or 25% PS in the presence of Ca2+. This observation demonstrates that part of factor Xa can cross the phospholipid polar headgroup/hydrocarbon chain interface. In parallel experiments, radioactive surface measurements permitted measuring binding of tritium-labeled factor Xa onto a PS monolayer and calculate an association constant, 6x10(6) M(-1). Penetration of factor Xa into PS-containing vesicles was investigated also using photoactivable 5-[125I]iodonaphthalene-1-azide, which binds selectively to the lipid embedded domains of the protein. These experiments suggest that Factor Xa penetrates preferentially by its heavy chain, an alternative mode of binding to the commonly accepted binding via its Gla domain. Interaction of factor Xa with PS vesicles also changes its apparent K(m) for S 2222.

Highly sensitive optical chip immunoassays in human serum

Over the past decade the ability of refractometric optical sensors to quantitatively measure a wide range of biomolecules has been demonstrated. These include proteins, nucleic acids, microorganisms, and in competitive formats small molecules such as drugs and pesticides. Furthermore, by using high refractive index nanoparticles to amplify the biomolecular binding signal, sensitivities approaching those of well established diagnostic assays have been achieved. However, to date it has not been possible to show rapid detection of analytes in complex bodily fluids such as serum, in a one-step procedure, due to the interference resulting from non-specific binding (NSB) to the sensor surface. We have carried out preliminary work on the control of interference due to NSB using an optical chip based on the Hartman interferometer. This interferometer configuration employs a reference sensing region that can be functionalized separately from the specific sensing region. Optical chips were stored dry after surface functionalization, and rehydrated in serum. The observed level of background drift in serum was reduced by an order of magnitude when an exposed reference was used, compared to a reference which was blind to the sample. An additional 70% reduction in signal drift in serum was achieved by controlling the surface chemistry of the optical chip using a biotin-poly(ethylene glycol) (PEG) blocking agent. This functionalization procedure was combined with a sandwich assay using gold nanoparticles to develop a one-step assay for human chorionic gonadotropin (hCG) in human serum with a detection limit of 0.1 ng/ml for a 35 min assay.

Inhibition of calcium oxalate crystal growth and aggregation by prothrombin and its fragments in vitro: relationship between protein structure and inhibitory activity

During blood coagulation, prothrombin (PT) is ultimately degraded to three fragments, thrombin, fragment 1 (F1) and fragment 2 (F2), which, collectively, contain all of the structural features of PT. One of these fragments, F1, is excreted in human urine and is the principal protein occluded into calcium oxalate (CaOx) crystals precipitated from it. This urinary form of F1, which we have named urinary prothrombin fragment 1 is present in calcium stones and is a potent inhibitor of CaOx crystallization in urine in vitro. The aim of this study was to determine whether PT itself and its other activation products, namely, thrombin, F1 and F2 also inhibit CaOx crystallization, by comparing their effects in a seeded, inorganic crystallization system. A secondary objective was to assess the relationship between the structures of the proteins and their inhibitory activities. PT was isolated from a human blood concentrate rich in vitamin K-dependent proteins. Following initial cleavage by thrombin, the resulting fragments, F1 and F2, were purified by a combination of reversed phase HPLC and low pressure column chromatography. The purity of the proteins was confirmed by SDS/PAGE and their individual effects on CaOx crystallization were determined at the same concentration (16.13 nM) in a seeded, metastable solution of CaOx using a Coulter Counter. [14C]Oxalate was used to assess deposition of CaOx and crystals were visualized using scanning electron microscopy. The Coulter Counter data revealed that the proteins reduced the size of precipitated crystals in the order F1 > PT > F2 > thrombin. These findings were confirmed by scanning electron microscopy which showed that the reduction in particle size resulted from a decrease in the degree of crystal aggregation. [14C]Oxalate analysis demonstrated that all proteins inhibited mineral deposition, in the order F1 (44%) > PT (27.4%) > thrombin (10.2%) > F2 (6.5%). It was concluded that the gamma-carboxyglutamic acid domain of PT and F1, which is absent from thrombin and F2, is the region of the molecules which determines their potent inhibitory effects. The superior potency of F1, in comparison with PT, probably results from the molecule's greater charge to mass ratio.

Interaction of human gamma-carboxyglutamic acid domainless prothrombin with phospholipids

The conversion of prothrombin into thrombin occurs at the surface of stimulated platelets. In order to see the influence of gamma-carboxyglutamic acid in the interaction of prothrombin with phospholipid, we investigated the direct interaction of the peptide 1-41 and prothrombin (des 1-44) with phospholipid monolayers of various compositions. Adsorption of the labeled proteins was determined by surface radioactivity measurements. Penetration of the proteins into the lipid layers was inferred from capacitance variation of the monolayer, measured by a.c. polarography. Prothrombin (des 1-44) was found both to adsorb and to penetrate the lipid monolayers, in the presence and in the absence of Ca++. The effects are higher on 100% PS than on 25% PS. This protein was also found to increase the permeability of vesicles containing 25% PS to T1+ ions, in the presence and in the absence of Ca++. Comparison with prothrombin shows that Gla residues are clearly involved in the interaction at 25% PS; nevertheless, the peptide 1-41 does not penetrate. A model of interaction of prothrombin with phospholipid, including both adsorption of prothrombin by Gla residues and its penetration by another domain, is proposed.

Interaction of alpha-thrombin and prethrombin 2, with phosphatidylserine-containing monolayers

Prothrombin activation complex is located at a phospholipid surface on activated platelets. To see whether the thrombin domain of the molecule plays a role in the interaction with lipids, we investigated the direct interaction of human alpha-thrombin and its precursor prethrombin 2 with phospholipid monolayers of various compositions (PS/PC). Adsorption of the labeled proteins was determined by surface radioactivity measurements. Penetration of the proteins in the lipid layer was inferred from capacitance variation of the monolayer, measured by a.c. polarography. Disulfide bridges reduced at the electrode were determined by cyclic voltametry. In all the cases studied, although in different manners thrombin was found both to adsorb and penetrate the lipid layer, whereas prethrombin 2 did not penetrate pure phosphatidylcholine (PC). In the case of thrombin, but not of prethrombin 2, penetration is accompanied by S-S reduction which is maximum at 10 per cent of phosphatidylserine (PS). This indicates a different orientation for prethrombin 2 and thrombin in the lipid layer. This observation might be of importance for the comprehension of the architecture of the prothrombin activation complex and for the regulation of thrombin formation within the complex.

Binding of coagulation factors IX and X to the endothelial cell surface

Bovine coagulation factors IX and X bind to independent sites on bovine aortic endothelial cells. Binding studies with cells maintained serum-free showed that there are at least two classes of binding sites for factor IX and factor X with a dissociation constant of 4.9 x 10(-9) M and 2.1 x 10(-8) M for the respective high affinity sites. Ca+2 was required for specific binding and was reversed by addition of EDTA or EGTA. Competition experiments showed that factor IX and factor IXa bind to the same sites, which are different from the factor X binding sites. Neither binding of factor IX or factor X is inhibited by addition of prothrombin or protein C. Indirect immunofluorescence of factor IX indicated that binding was diffuse on the cell surface.

Effect of polylysine on the activation of prothrombin. Polylysine substitutes for calcium ions and factor V in the factor Xa catalyzed activation of prothrombin

Polylysine has been demonstrated to dramatically accelerate the rate of the factor Xa catalyzed activation of both prothrombin and prethrombin 1. Under the present experimental conditions (pH 8.0, 23 C), no detectable activation of prothrombin or prethrombin 1 occurs with either factor Xa or polylysine alone. The activation of prethrombin 2, the direct precursor of alpha-thrombin, by factor Xa is not stimulated by polylysine. The activation of either prothrombin or prethrombin 1 by factor Xa in the presence of polylysine is partially inhibited by the presence of 5 mM CaCl2. Electrophoretic analysis in sodium dodecyl sulfate showed that the products that were formed in the above activation system comigrated with the reaction products derived from prothrombin activated by factor Xa in the presence of calcium ions and phospholipid. It is suggested that polylysine stimulates the factor Xa-catalyzes activation of prothrombin by replacing the combination of calcium ions and factor V.