Self-association of bovine prothrombin fragment 1 in the presence of metal ions. Use of a covalent cross-linking reagent to study the reaction

Discovery and characterization of 2 novel subpopulations of aPS/PT antibodies in patients at high risk of thrombosis

Anti-phosphatidylserine/prothrombin (aPS/PT) antibodies are often detected in patients with antiphospholipid syndrome (APS), but how aPS/PT engage prothrombin at the molecular level remains unknown. Here, the antigenic determinants of immunoglobulin G aPS/PT were investigated in 24 triple-positive APS patients at high risk of thrombosis by using prothrombin mutants biochemically trapped in closed and open conformations, and relevant fragments spanning the entire length of prothrombin. Two novel unexpected findings emerged from these studies. First, we discovered that some aPS/PT are unique among other anti-prothrombin antibodies insofar as they efficiently recognize prothrombin in solution after a conformational change requiring exposure of fragment-1 to the solvent. Second, we identified and characterized 2 previously unknown subpopulations of aPS/PT, namely type I and type II, which engage fragment-1 of prothrombin at different epitopes and with different mechanisms. Type I target a discontinuous density-dependent epitope, whereas type II engage the C-terminal portion of the Gla-domain, which remains available for binding even when prothrombin is bound to the phospholipids. Based on these findings, APS patients positive for aPS/PT were classified into 2 groups, group A and group B, according to their autoantibody profile. Group A contains mostly type I antibodies whereas group B contains both type I and type II antibodies. In conclusion, this study offers a first encouraging step toward unveiling the heterogeneity of anti-prothrombin antibodies in correlation with thrombosis, shedding new light on the mechanisms of antigen-autoantibody recognition in APS.

Isolation and characterization of carinactivase, a novel prothrombin activator in Echis carinatus venom with a unique catalytic mechanism

The venom of the viper Echis carinatus contains a metalloprotease, ecarin, that is a potent prothrombin activator. We here show that the venom is also rich in another prothrombin activator, which does not belong to any known category of prothrombin activators. The novel enzyme, designated carinactivase-1 (CA-1), consists of two subunits held together non-covalently but very tightly. One subunit is a 62-kDa polypeptide that has metalloprotease activity and is homologous to the single-chain enzyme ecarin; the other subunit of 25 kDa consists of two disulfide-linked polypeptides of 17 and 14 kDa, and this subunit resembles the anticoagulant in the habu snake venom, IX/X-bp, that specifically binds the Gla domains of coagulation factors IX and X in a Ca2+-dependent fashion. The activation of prothrombin by CA-1 requires Ca2+ ions at millimolar concentrations and in the absence of Ca2+ ions this enzyme is virtually inactive. By contrast, activation by ecarin is completely independent of Ca2+ ions. CA-1, unlike ecarin, does not activate prothrombin derivatives, in which binding of Ca2+ ions has been perturbed, namely prethrombin-1 and acarboxyprothrombin. Furthermore, the isolated catalytic subunit, although its activity is greatly reduced as compared to that of the holoenzyme, no longer requires Ca2+ ions for the activation of prothrombin. Reconstitution with the non-catalytic 25-kDa subunit restores high level activity and the dependence on Ca2+ ions. Finally, prothrombin activation by CA-1 is inhibited by prothrombin fragment 1, and the isolated non-catalytic subunit is capable of binding fragment 1 in the presence of Ca2+ ions. From these observations, we postulate the following unique mechanism for the activation of prothrombin by CA-1. The enzyme primarily recognizes the Ca2+-bound conformation of the Gla domain in prothrombin via the 25-kDa regulatory subunit, and the subsequent conversion of prothrombin to active thrombin is catalyzed by the 62-kDa catalytic subunit.

Amino-terminal alanine functions in a calcium-specific process essential for membrane binding by prothrombin fragment 1

Two acetylation sites on prothrombin fragment 1 (amino-terminal 156 amino acid residues of bovine prothrombin) are essential for the tight calcium and membrane binding functions of the protein; calcium protects both of these sites from acetylation [Welsch, D. J., Pletcher, C. H., & Nelsestuen, G. L. (1988) Biochemistry (first of three papers in this issue)]. The epsilon-amino groups of the lysine residues (positions 3, 11, 44, 57, and 97) were not critical to protein function and were acetylated in the calcium-protected protein. The most reactive of the two essential acetylation sites was identified as amino-terminal alanine. To identify this site, fragment 1 was first acetylated in the presence of calcium to derivatize the nonessential sites. Removal of calcium and partial acetylation with radioactive reagent produced a single major radioactive peptide. Isolation and characterization of this peptide showed that the radioactivity was associated with amino-terminal alanine. In addition, sequence analysis of calcium-protected protein showed the presence of underivatized amino-terminal alanine. Surprisingly, covalent modification with a trinitrophenyl group did not alter membrane binding activity. Thus, the positive charge on the amino terminus did not appear critical to its function. Acetylation of amino-terminal alanine without acetylation of the second essential site produced a fragment 1 derivative which had a high requirement for calcium and which had lost most membrane binding function. However, this protein had only slightly altered affinity for magnesium ion. In agreement with this metal ion selectivity, protection of amino-terminal alanine was calcium specific, and magnesium ion did not protect this site from acetylation.(ABSTRACT TRUNCATED AT 250 WORDS)

Homo- and heterodimer formation with prothrombin and prothrombin fragment 1 in the presence of calcium ions

The purpose of the current study is to present further evidence for prothrombin self-association as assessed by chemical crosslinking. When the self-association (evaluated by covalent crosslinking with dithiobis(succinimidylpropionate) of prothrombin or fragment 1 was evaluated at the same molar concentration of protein, similar rates of dimer formation were observed for either protein. When prothrombin and fragment 1 were incubated together with the crosslinking reagent and calcium ions, a heterodimer consisting of prothrombin and fragment 1 was observed in addition to prothrombin dimer and fragment 1 dimer. Similar experiments with prethrombin 1 showed neither significant self-association nor effect on prothrombin self-association. Comparison of the formation of prothrombin fragment 1 heterodimer formation with the effect of fragment 1 on prothrombin activation by factor Xa suggests that the anticoagulant activity of fragment 1 is not solely a result of the formation of a heterodimer between prothrombin and fragment 1.

Purification of human prothrombin fragment 1 using hydrophobic interaction chromatography on phenyl-sepharose

Hydrophobic interaction chromatography on Phenyl-Sepharose for the purification of prothrombin fragment 1 is described. The results suggest that this method is both easier and more effective than the use of anion-exchange chromatography for the purification of fragment 1. In addition, the results presented here suggest that prothrombin has rather extensive hydrophobic properties.

Calcium-dependent changes in properties of human prothrombin: a study using high-performance size-exclusion chromatography and gel-permeation chromatography

High-performance size-exclusion chromatography using a TSK 3000 SW column and aqueous gel filtration with Sephacryl S-200 SF have been used to characterize the effects of calcium ions on the hydrodynamic properties of human prothrombin and prethrombin 1. The results suggest that the effective hydrodynamic radius of prothrombin is less in the presence than in the absence of calcium ions. In addition, when using the TSK-3000 SW column, Ca2+-dependent formation of a hydrophobic site in the fragment 1 region of prothrombin results in an apparent further decrease in hydrodynamic radius.