scholarly journals Proteolytic interactions of factor IXa with human factor VIII and factor VIIIa

Blood ◽  
1992 ◽  
Vol 80 (12) ◽  
pp. 3120-3126 ◽  
Author(s):  
BJ Lamphear ◽  
PJ Fay
Keyword(s):  
Factor Viii ◽  
Heavy Chain ◽  
Light Chain ◽  
Prolonged Incubation ◽  
Heavy Chains ◽  
Factor Ixa ◽  
Factor Viiia ◽  
Human Factor Viii ◽  
A1 Domain ◽  
Proteolytic Cleavages

Abstract Factor IXa was shown to inactivate both factor VIII and factor VIIIa in a phospholipid-dependent reaction that could be blocked by an antifactor IX antibody. Factor IXa-catalyzed inactivation correlated with proteolytic cleavages within the A1 subunit of factor VIIIa and within the heavy chain (contiguous A1-A2-B domains) of factor VIII. Furthermore, a relatively slow conversion of factor VIII light chain to a 68-Kd fragment was observed after prolonged incubation. Sites of cleavage were identified within the A1 domain at Arg336-Met337 and within the factor VIII light chain at Arg1719-Asn1720. Factor IXa failed to cleave isolated factor VIII heavy chains, yet cleaved isolated factor VIII light chain. In addition, the purified A1/A3-C1-C2 dimer derived from factor VIIIa was a substrate for factor IXa; however, cleavage of the A1 subunit occurred at less than 30% the rate of cleavage of A1 in trimeric factor VIIIa. These data suggest that factor VIII light chain contributes to the binding site for factor IXa and also support a role for a heavy chain determinant located within the A2 subunit in the association of factor VIIIa with factor IXa. Furthermore, the capacity of factor IXa to proteolytically inactivate its cofactor, factor VIIIa, suggests a mode of regulation within the intrinsic tenase complex.

scholarly journals Proteolytic interactions of factor IXa with human factor VIII and factor VIIIa

Blood ◽  
1992 ◽  
Vol 80 (12) ◽  
pp. 3120-3126 ◽  
Author(s):  
BJ Lamphear ◽  
PJ Fay
Keyword(s):  
Factor Viii ◽  
Heavy Chain ◽  
Light Chain ◽  
Prolonged Incubation ◽  
Heavy Chains ◽  
Factor Ixa ◽  
Factor Viiia ◽  
Human Factor Viii ◽  
A1 Domain ◽  
Proteolytic Cleavages

Factor IXa was shown to inactivate both factor VIII and factor VIIIa in a phospholipid-dependent reaction that could be blocked by an antifactor IX antibody. Factor IXa-catalyzed inactivation correlated with proteolytic cleavages within the A1 subunit of factor VIIIa and within the heavy chain (contiguous A1-A2-B domains) of factor VIII. Furthermore, a relatively slow conversion of factor VIII light chain to a 68-Kd fragment was observed after prolonged incubation. Sites of cleavage were identified within the A1 domain at Arg336-Met337 and within the factor VIII light chain at Arg1719-Asn1720. Factor IXa failed to cleave isolated factor VIII heavy chains, yet cleaved isolated factor VIII light chain. In addition, the purified A1/A3-C1-C2 dimer derived from factor VIIIa was a substrate for factor IXa; however, cleavage of the A1 subunit occurred at less than 30% the rate of cleavage of A1 in trimeric factor VIIIa. These data suggest that factor VIII light chain contributes to the binding site for factor IXa and also support a role for a heavy chain determinant located within the A2 subunit in the association of factor VIIIa with factor IXa. Furthermore, the capacity of factor IXa to proteolytically inactivate its cofactor, factor VIIIa, suggests a mode of regulation within the intrinsic tenase complex.

FVIII Heavy Chain Enhances Tenase Activity Induced By FVIIIa Mimicking Bispesific Antibody, ACE910

Blood ◽  
2014 ◽  
Vol 124 (21) ◽  
pp. 1481-1481
Author(s):  
Hiroaki Minami ◽  
Keiji Nogami ◽  
Takehisa Kitazawa ◽  
Kunihiro Hattori ◽  
Midori Shima
Keyword(s):  
Factor Viii ◽  
Heavy Chain ◽  
Light Chain ◽  
Research Funding ◽  
Factor Xa ◽  
Factor X ◽  
Advisory Committees ◽  
Factor Ixa ◽  
Factor Viiia ◽  
Physiological Structure

Abstract Background: ACE910, asymmetric bispecific monoclonal antibodies to activated factor IX (IXa) and factor X, mimics the cofactor function of activated factor VIII (VIIIa) by modulating an optimal position on the tenase assembly. The estimated therapeutic range of ACE910 shows ~30% of thrombin generation in native tenase assembly, supporting that the structure on ACE910-mimicking tenase assembly is different from that on native tenase. Being close to physiological structure consisting from factor IXa, factor X, and factor VIIIa is important for potentiating the clotting function. We examined the effects of factor VIII subunits (light chain, heavy chain, A1 and A2, C2) on ACE910-tenase. Materials/Methods: The factor VIII light chain and heavy chain were isolated from EDTA-treated recombinant factor VIII following chromatography on SP- and Q- Sepharose columns. The A2 and A1 subunits were purified from thrombin-cleaved factor VIII heavy chain by Heparin-, SP- Sepharose columns. Purified factor Xa generation assays was examined with (i) factor VIII subunit (0-40 nM), ACE910 (10 µg/ml), phospholipid (PL) (40 µM), factor IXa (1 nM) and factor X (200 nM), (ii, iii) the A2 or heavy chain (40 nM), ACE910 (10 µg/ml), PL (40 µM), factor IXa and factor X (1 or 0-80 nM, and 0-300 or 200 nM, respectively). These mixtures were reacted for five minutes (i, ii) or one minute (iii). These assays were conducted at 37 °C. Results: (i) The factor Xa generation in ACE910-tenase complex in the absence of factor VIIIa was 10.1±2.2 nM. With the intact heavy chain and A2, amounts of factor Xa were increased dose-dependently, resulting in 1.3- and 1.2-fold increases, respectively. While, the light chain and A1 subunit failed to increase at all. (ii) Vmax for factor X in ACE910-tenase was 173.0±7.0 nM and Km was 31.2±3.9 nM. Vmax obtained with the heavy chain or A2 was 175.9±6.1 or 159.0±6.1 nM, whilst Km was 17.0±2.2 or 31.9±3.5 nM, respectively, indicating that the heavy chain enhanced the binding affinity for factor X in ACE910-tenase. (iii) Vmax for factor IXa in ACE910-tenase was 43.8±2.7 nM and Km was 36.9±4.8 nM. With the heavy chain or A2, Vmax was 46.8±3.0 or 45.0±3.1 nM, and Km was 36.4±3.0 or 32.1±4.9 nM, respectively, indicating that either the heavy chain or A2 did not enhance the catalytic activity and the binding affinity for factor IXa in ACE910-tenase. Conclusion: ACE910-tenase assembly seems to be close to physiological structure by the presence of intact heavy chain interacting with factor X. In addition, ACE910 may substitute the position such as the factor VIII(a) light chain associated with FIXa and FX on ACE910-tenase assembly defecting factor VIII. Disclosures Minami: Chugai Pharmaceutical Co., Ltd.: Research Funding. Nogami:Chugai Pharmaceutical Co., Ltd.: Membership on an entity's Board of Directors or advisory committees, Research Funding. Kitazawa:Chugai Pharmaceutical Co., Ltd.: Employment, Equity Ownership, Patents & Royalties. Hattori:Chugai Pharmaceutical Co., Ltd.: Employment, Equity Ownership, Patents & Royalties. Shima:Chugai Pharmaceutical Co., Ltd.: Consultancy, Honoraria, Membership on an entity's Board of Directors or advisory committees, Research Funding.

THROMBIN-ACTIVATED FACTOR VIIIa IS COMPOSED OF A NONCOVALENT 73/51 kD DIMER

1987 ◽  
Author(s):  
P J Fay
Keyword(s):  
Factor Viii ◽  
Heavy Chain ◽  
Light Chain ◽  
Polyacrylamide Gel Electrophoresis ◽  
Gel Filtration ◽  
Similar Extent ◽  
Factor Viiia ◽  
Polypeptide Patterns ◽  
Human Factor Viii ◽  
Sds Page

Human factor VIII purified from plasma concentrates consists of a series of heterodimers composed of a light chain of 83 kD noncovalently bound to a heavy chain which varies in size from 93 to 170 kD. Previously, we showed that each of the purified heterodimers wasactivated by thrombin to a similar extent. Activation to factor VIIIa was correlated with proteolysis of the light chain generating a73 kD polypeptide and cleavage of the heavy chain(s) generating polypeptides of 51 and 43 kD, whereas subsequent inactivation of factor VIIIa occurred in the absence of further proteolysis (Biochim Biophys Acta 871:268-278, 1986). SDS-polyacrylamide gel electrophoresis (SDS-PAGE) of reduced or nonreduced samples showed similar polypeptide patterns indicating that there were no covalent linkages between the 51 and 43 kD chains. However, prior data does not distinguish between a factor VIIIa complex of the 73, 51 and 43 kD polypeptides and a subset of these chains. To identify factor VIIIa, thrombin- treated factor VIII at peak activity was subjected to rapid gel filtration on Superose 12. Factor VIII activity eluted as a single peak representing about 30% of the applied activity after correction for spontaneous inactivation. SDS-PAGE followedby silver staining showed that activity was correlated to fractionscontaining the 73 and 51 kD polypeptides, which co-eluted and which were separated from both the 43 kD fragment and thrombin. Densitometric scans of the stained gel indicated the stoichiometry of the 73:51 kD polypeptides in eachactive fraction to be 1:1. Addition of EDTA(50 mM) to a similar thrombin-factor VIII mixture resulted in rapid inactivation of factor VIIIa. Gel filtration followed by SDS-PAGE analysis of this sample showed that the 73 and 51 kD polypeptides eluted separately and were more included, while the elution position of the 43 kD polypeptide was unchanged. These results suggest that factor VIIIa is represented by a noncovalent dimer consisting of a 73 kD polypeptide derivedfrom the light chain plus a 51 kD polypeptide derived from the heavy chain.

Identification of a Plasmin-Interactive Site in the Factor VIII Light Chain Responsible for Proteolytic Cleavage at Lys36 in the A1 Domain.

Blood ◽  
2006 ◽  
Vol 108 (11) ◽  
pp. 1709-1709
Author(s):  
Keiji Nogami ◽  
Midori Shima ◽  
Masahiro Takeyama ◽  
Kohei Tatsumi ◽  
Yuri Fujita ◽  
...  
Keyword(s):  
Factor Viii ◽  
Binding Site ◽  
Light Chain ◽  
Active Site ◽  
Dependent Manner ◽  
Factor Ixa ◽  
Lysine Residues ◽  
Extended Surface ◽  
A1 Domain ◽  
Proteolytic Cleavages

Abstract Factor VIII is inactivated by limited proteolytic cleavages at Arg336 and Lys36 by plasmin (Abst #1017 and 1018, BLOOD106, 2005). We recently identified a plasmin-interactive site in the A2 domain responsible for cleavage at Arg336, and also reported that the cleavage at Lys36 was selectively regulated by the light chain that might interact with its protease. In the current study, several approaches were employed to examine the localization of plasmin-interactive site(s) responsible for cleavage at Lys36. Rate constant of plasmin-catalyzed factor VIIIa inactivation by the addition of the 1649A3-C1-C2 (intact light chain) or 1690A3-C1-C2 fragment was reduced by ~2-folds in a dose-dependent manner using one-stage clotting assay, but not by the 1722A3-C1-C2. SDS-PAGE analysis revealed that Lys36 cleavage in A1/1722A3-C1-C2 dimer by plasmin was markedly slower than that in the A1/1649A3-C1-C2 or A1/1690A3-C1-C2 dimer. Surface plasmon resonance-based assay using anhydro-plasmin (Ah-plasmin), catalytically inactive derivative of plasmin in which the active-site serine was converted to dehydroalanine, showed that the 1722A3-C1-C2 bound to Ah-plasmin (Kd; 191 nM) with an ~3-fold lower affinity compared with 1649A3-C1-C2 or 1690A3-C1-C2 (68 or 83 nM, respectively). Recombinant A3 domain, expressed in E.Coli, also bound to Ah-plasmin with similar affinity (Kd; 44 nM), but the C2 failed to bind, suggesting the presence of a plasmin-binding site within N-terminus of the A3 domain. On the other hand, Glu-Gly-Arg-active-site modified factor IXa blocked the light chain binding to Ah-plasmin by ~60% (Ki: 10.5 nM), supporting that another plasmin-binding site juxtaposes or overlaps with a factor IXa-binding site (residues 1804–1818) in the A3 domain. The 6-aminohexanoic acid, a competitor of lysine-binding site of plasmin, blocked by >90% the light chain and Ah-plasmin interaction (Ki; 6.8 microM). Based on the major contribution of lysine-binding site(s) of plasmin for interaction with the light chain, evaluation of the A3 sequence revealed two regions involving clustered lysine residues of 1690–1705 and 1804–1818. Two synthetic peptides corresponding to these regions blocked the light chain binding to Ah-plasmin by ~50% (Ki: ~20 microM). We conclude that an extended surface centered the 1690–1705 and 1804–1818 regions within the A3 domain interacts with plasmin for the cleavage at Lys36 within the A1 domain.

Tissue Factor-Dependent Activation of Factor VIII by Factor VIIa in the Early Phase of Blood Coagulation

Blood ◽  
2008 ◽  
Vol 112 (11) ◽  
pp. 1036-1036
Author(s):  
Tetsuhiro Soeda ◽  
Keiji Nogami ◽  
Tomoko Matsumoto ◽  
Kenichi Ogiwara ◽  
Katsumi Nishiya ◽  
...  
Keyword(s):  
Factor Viii ◽  
Tissue Factor ◽  
Blood Coagulation ◽  
Heavy Chain ◽  
Light Chain ◽  
Early Phase ◽  
Factor Viia ◽  
Clotting Factors ◽  
Initiation Phase ◽  
A1 Domain

Abstract Factor VIIa (FVIIa), complexed with tissue factor (TF), is a trigger of blood coagulation through activation of factor X in the initiation phase. FVIIa can catalyze intrinsic clotting factors such as not only factor IX, but also factor VIII (FVIII). However the role and the mechanisms of the FVIIa-catalyzed FVIII are poorly understood. We first examined FVIIa-catalyzed FVIII activation in the presence of phospholipid (PL) using a one-stage clotting assay. The levels of FVIII activity elevated rapidly by ~4-fold within 30 sec after the addition of FVIIa (1 nM)-TF (1 nM)complex, and subsequently decreased to the initial level within 20 min. This time-dependent reaction was enhanced by the presence of TF and PL in dose-dependent manners, but was moderately inhibited (~50%) in the presence of von Willebrand factor at physiological concentration of 10 μg/mL. FVIII cleavage was evaluated using western blotting immediately after the addition of FVIIa-TF complex. The heavy chain of FVIII was proteolyzed more rapidly (at 15 sec) by cleavages at Arg740 (A2-B junction) and Arg372 (A1-A2 junction) by FVIIa-TF complex, whilst the cleavage at Arg336 in the A1 domain was appeared at ~2.5 min. However little cleavage of the light chain of FVIII was observed, supporting that cleavages at Arg740/Arg372 and Arg336 by FVIIa-TF complex contribute to the up- and down-regulation of FVIII(a) activity, respectively. Of interest, no proteolysis of isolated intact heavy chain was observed, indicating that the proteolysis of the heavy chain was governed by the presence of the light chain. Compared to FVIII activation by thrombin (0.1–1 nM), the activation by FVIIa (0.1–1 nM)-TF (1 nM) complex was observed more rapidly. The activation rate observed by the addition of FVIIa-TF complex was ~50-fold greater than that by thrombin. Kinetics by the chromogenic Xa generation assay showed the catalytic efficiency (kcat/Km; 8.9 min−1/32.8 nM) on FVIIa-TF complex-catalyzed FVIII activation showed ~4-fold greater than that on thrombin-catalyzed activation (kcat/Km; 7.5 min−1/86.4 nM). Furthermore, the catalytic efficiencies on cleavages at Arg740 and Arg372 of FVIII by FVIIa-TF complex were ~3- and ~20-fold greater compared to those by thrombin, respectively. These findings suggested that FVIIa-TF complex was a greater FVIII activator than thrombin in very early phase. In order to localize the binding region for FVIIa, we evaluated the interactions between FVIII subunit and Glu-Gly-Arg-active site modified FVIIa, lacking enzymatic activity, in a surface plasmon resonance-based assay. The heavy chain of FVIII bound to EGR-FVIIa with higher affinity than the light chain (Kd; 2.1 and 45 nM, respectively). Binding was particularly evident with the A2, A3, and C2 domains (Kd; 34, 37, and 44 nM, respectively), whilst the A1 domain failed to bind. In conclusion, we demonstrated that FVIIa-TF complex rapidly activated FVIII by proteolysis of the heavy chain and the activation was governed by the presence of the light chain. Furthermore, present results suggested the role of TF-dependent FVIII activation by FVIIa which is responsible for the initiation phase of blood coagulation.

Effect of Heterologous Factor V Heavy Chain Sequences on the Secretion of Recombinant Human Factor VIII

1996 ◽  
Vol 75 (01) ◽  
pp. 036-044 ◽  
Author(s):  
Thomas L Ortel ◽  
Karen D Moore ◽  
Mirella Ezban ◽  
William H Kane
Keyword(s):  
Factor Viii ◽  
Heavy Chain ◽  
Human Factor ◽  
Secreted Protein ◽  
Factor V ◽  
Minimal Effect ◽  
Repetitive Domain ◽  
Human Factor Viii ◽  
A1 Domain ◽  
A2 Domain

SummaryFactor VIII and factor V share a repetitive domain structure of A1-A2-B-A3-C1-C2. To define the region(s) within the factor VIII heavy chain that result in inefficient expression of the recombinant protein, we expressed a series of factor VIH/factor V chimeras that contained heterologous sequences from the A1 and/or A2 domains. Substitution of the factor VIIIA1 domain dramatically reduced secretion of factor V ~ 500-fold, whereas substitution of the factor VIII A2 domain had minimal effect on secretion. Conversely, substitution of the factor V A1 domain increased secretion of factor VIII ~3-fold, whereas substitution of the factor V A2 domain actually reduced secretion ~4-fold. Pulse chase experiments confirmed that reduced expression levels were due to decreased secretion rather than instability of secreted protein. Smaller substitutions did not further localize within the A1 domain the regions responsible for inefficient secretion.

Role of the Direct Interaction between Factor VIII C2 and Factor IXa Gla Domain in the Factor Xase Complex.

Blood ◽  
2007 ◽  
Vol 110 (11) ◽  
pp. 2687-2687
Author(s):  
Tetsuhiro Soeda ◽  
Keiji Nogami ◽  
Masahiro Takeyama ◽  
Kenichi Ogiwara ◽  
Kazuhiko Tomokiyo ◽  
...  
Keyword(s):  
Factor Viii ◽  
Light Chain ◽  
Dependent Manner ◽  
C2 Domain ◽  
Factor X ◽  
Calcium Dependent ◽  
Factor Ixa ◽  
Factor Viiia ◽  
Gla Domain

Abstract Factor VIII functions as a cofactor for factor IXa in the anionic phospholipid surface-dependent conversion of factor X to Xa. It is well-known that the A2 and A3 domains of factor VIII interact with the catalytic domain and EGF2 domain of factor IXa, respectively. Recently, Furie et al. have reported that the Gla domain of factor IXa (factor IXa-GD) interacts with the light chain of factor VIII. However, the factor IXa-GD-interactive site on the light chain remained to be investigated. In the current study, the recombinant C2 (rC2) domain of factor VIII was prepared using a yeast secretion system. ELISA-based assay in the absence of phospholipid showed the Glu-Gly-Arg-active site modified factor IXa (EGR-factor IXa) bound to the immobilized rC2 domain dose-dependently, and the binding ability was maximum under the condition of 150 mM NaCl/1 mM CaCl2. This binding was competitively inhibited by the addition of excess of factor VIII or rC2 domain, supporting the specificity of this interaction. Furthermore, the presence of high ionic strength and the metal-ion chelator EDTA blocked this binding by ∼95 and ∼75%, respectively. Surface plasmon resonance-based assay showed that the binding affinity (Kd) of rC2 domain for EGR-factor IXa was 108 ± 15.5 nM. GD less-factor IXa, deleting the GD completely, failed to bind to rC2 domain. A monoclonal antibody against factor IXa-GD specific for calcium-dependent conformation (mAbIXa-GD) also inhibited (∼ 95%) the rC2 domain binding to EGR-factor IXa in a dose-dependent manner (IC50; 758 nM), suggesting the authentic of the C2 domain and factor IXa-GD interaction. The addition of rC2 domain or mAbIXa-GD inhibited the factor IXa-catalyzed factor X activation with factor VIIIa in the absence of phospholipid (IC50; 15.7 μM or 43.2 nM, respectively), whilst both any little affected in the absence of factor VIIIa. In addition, the ∼8-kDa C2 fragment obtained by V8 protease digestion (residues 2182–2259) bound directly to EGR-factor IXa. Taken together, these results indicate that factor VIII C2 domain directly interacts with factor IXa-GD via both the electrostatic- and calcium-dependent interactions. Furthermore, our results provide the first evidence for an essential role of the C2 domain in the association between factor VIII and factor IXa in the factor Xase complex.

scholarly journals Characterization of des-(741–1668)-factor VIII, a single-chain factor VIII variant with a fusion site susceptible to proteolysis by thrombin and factor Xa

1995 ◽  
Vol 312 (1) ◽  
pp. 49-55 ◽  
Author(s):  
M J S H Donath ◽  
R T M de Laaf ◽  
P T M Biessels ◽  
P J Lenting ◽  
J W van de Loo ◽  
...  
Keyword(s):  
Factor Viii ◽  
Von Willebrand Factor ◽  
Heavy Chain ◽  
Light Chain ◽  
Factor Xa ◽  
Single Chain ◽  
Factor Ixa ◽  
Fusion Site ◽  
Von Willebrand ◽  
Willebrand Factor

A factor VIII variant has been characterized in which the heavy chain is directly fused to the light chain. Des-(741-1668)-factor VIII lacks the processing site at Arg1648, as Arg740 of the heavy chain is fused to Ser1669 of the light chain. The sequence of the fusion site is similar to that of other cleavage sites in factor VIII. The fusion site of des-(741-1668)-factor VIII was readily cleaved by both thrombin and factor Xa, and the same result was obtained for heavy chain cleavage. In contrast, des-(741-1668)-factor VIII cleavage by thrombin at position Arg1689 proceeded at a lower rate than the analogous cleavage by factor Xa, which presumably takes place at position Arg1721. The rate of cleavage at position Arg1689 by thrombin was also lower than that at the other processing sites. When des-(741-1668)-factor VIII was activated by thrombin, initial rates of factor Xa formation were similar to the rates obtained when plasma-derived factor VIII was activated by thrombin or factor Xa. Remarkably, activation of des-(741-1668)-factor VIII proceeded at a higher rate by factor Xa than by thrombin. These results indicate that factor VIII activation is strongly associated with cleavage at position Arg1689 or Arg1721. For the interaction between des-(741-1668)-factor VIII and von Willebrand factor, a Kd value of (0.8 +/- 0.3) x 10(-10) M was determined, which is similar to that of heterodimeric factor VIII. The affinity of single-chain des-(741-1668)-factor VIII for factor IXa was found to be 27 +/- 6 nM. The in vivo recovery and half-life of des-(741-1668)-factor VIII were assessed in guinea pigs. Upon infusion of des-(741-1668)-factor VIII at a dosage of 50 units/kg body weight, a rise of 1.0 +/- 0.3 unit/ml in factor VIII activity was obtained. The same recovery was determined for wild-type factor VIII. The half-life of des-(741-1668)-factor VIII was found to be 3 +/- 1 h, compared with 4 +/- 2 h for heterodimeric recombinant factor VIII. In conclusion, des-(741-1668)-factor VIII displays normal activity, is readily cleaved by thrombin and factor Xa at its fusion site, binds with high affinity to von Willebrand factor and factor IXa, and behaves like heterodimeric recombinant factor VIII in guinea pigs. By virtue of these properties, des-(741-1668)-factor VIII may prove useful for the treatment of bleeding episodes in patients with haemophilia A.

scholarly journals Cleavage of Factor VIII Heavy Chain Is Required for the Functional Interaction of A2 Subunit with Factor IXa

2001 ◽  
Vol 276 (15) ◽  
pp. 12434-12439 ◽  
Author(s):  
Philip J. Fay ◽  
Maria Mastri ◽  
Mary E. Koszelak ◽  
Hironao Wakabayashi
Keyword(s):  
Factor Viii ◽  
Heavy Chain ◽  
Fluorescence Anisotropy ◽  
Factor Xa ◽  
Factor X ◽  
Functional Interaction ◽  
Factor Ixa ◽  
Factor Viiia ◽  
A2 Domain ◽  
Stimulation Of

Factor VIII circulates as a noncovalent heterodimer consisting of a heavy chain (HC, contiguous A1-A2-B domains) and light chain (LC). Cleavage of HC at the A1-A2 and A2-B junctions generates the A1 and A2 subunits of factor VIIIa. Although the isolated A2 subunit stimulates factor IXa-catalyzed generation of factor Xa by ∼100-fold, the isolated HC, free from the LC, showed no effect in this assay. However, extended reaction of HC with factors IXa and X resulted in an increase in factor IXa activity because of conversion of the HC to A1 and A2 subunits by factor Xa. HC cleavage by thrombin or factor Xa yielded similar products, although factor Xa cleaved at a rate of ∼1% observed for thrombin. HC showed little inhibition of the A2 subunit-dependent stimulation of factor IXa activity, suggesting that factor IXa-interactive sites are masked in the A2 domain of HC. Furthermore, HC showed no effect on the fluorescence anisotropy of fluorescein-Phe-Phe-Arg-factor IXa in the presence of factor X, whereas thrombin-cleaved HC yielded a marked increase in this parameter. These results indicate that HC cleavage by either thrombin or factor Xa is essential to expose the factor IXa-interactive site(s) in the A2 subunit required to modulate protease activity.

Sign in / Sign up

Export Citation Format

Share Document