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PubMed2001

Factor XI assembly and activation on human umbilical vein endothelial cells in culture.

Biotin-FXI optimally bound to HUVEC in the presence of 40 nM high molecular weight kininogen (HK) and > or =7 microM Zn2+. There was little specific FXI binding in the absence of added HK and at concentrations of Zn2+ <15 microM. FXI and prekallikrein, but not prothrombin, blocked biotin-FXI binding to HUVEC in the presence of HK with an IC50 of 18 nM and 180 nM, respectively. Monoclonal antibody HKL16 and peptide SDD31 also inhibited biotin-XI binding in the presence of HK with an IC50 of 4.7 nM and 50 microM, respectively. Alternatively, peptide T249-F260 of FXI's apple domain 3 and heparin monosulfate were weak inhibitors of FXI binding to HUVEC. FXI bound to HUVEC with an apparent Kd of 6.9 +/- 3.0 nM and Bmax of 13 +/- 2.6 x 10(6) sites/cell. FXI bound to HK on HUVEC, but not prothrombin, became converted to FXIa. FXI activation on HUVEC resulted from tissue culture media bovine factor XIIa. HUVEC grown in human factor XI-deficient serum did not support FXI activation. FXI binding to HUVEC in culture was mostly mediated by HK and FXI activation on HUVEC is dependent on cell-associated factor XIIa.

Binding, Competitive↗
PubMed1978

Comparison of reagents for determining the activated partial thromboplastin time.

Six commercially available reagents for the determination of the activated partial thromboplastin time have been evaluated and compared with respect to their sensitivity to the coagulation factors VIII, IX and XI and to their response to heparin. Some variation was observed among the reagents regarding their sensitivity to factor XI and even greater differences were obtained with factors VIII and IX. It was also clear that none of the reagents was sensitive to the same extent to the factors tested. The sensitivity to heparin shows considerable variation, in terms of time as well as mode of response to increasing heparin levels. In four reagents this response is linear, it is logarithmic in one and the remaining one is yet again different. It seems unlikely that any standardization of the APTT determination is at present possible with the reagents studied.

Factor XI Deficiency↗
PubMed1988

Mechanism of transient adsorption of fibrinogen from plasma to solid surfaces: role of the contact and fibrinolytic systems.

The transient detection of fibrinogen on surfaces has been described (Vroman effect) and high-mol-wt kininogen (HK) has been shown to play a role in this reaction. In this study, we attempted to identify the form of HK responsible for preventing detection of the fibrinogen initially adsorbed from plasma to various artificial surfaces and to determine if other plasma components were involved. We compared 125I-fibrinogen adsorption in the presence of normal plasma to plasma deficient in specific proteins. On all surfaces tested, we found that fibrinogen was displaced from the surface. The extent of displacement was greatly reduced, however, but not eliminated in HK-deficient plasma. Factor XII-deficient plasma also showed reduced fibrinogen displacement. These data indicate that HK can actually displace fibrinogen; however, factor XII, or a factor XII-mediated reaction also appears to be necessary for this displacement to occur. Furthermore, when normal plasma was first subjected to extensive contact activation by dextran sulfate, during which the HK was extensively degraded to components smaller than the light chain (as assessed by Western blotting), we observed greatly reduced displacement of fibrinogen. Extensive contact activation of Factor XI-deficient plasma failed to show low-mol-wt derivatives, however, and displacement of fibrinogen was similar to normal plasma that had not undergone extensive activation. These data indicate that HKa (active cofactor produced during contact activation by factor XIIa or kallikrein) is primarily responsible for displacing fibrinogen, and that HKi (inactive cofactor generated by factor XIa) cannot displace fibrinogen. The fibrinogen from all plasma samples looked similar by Western blot analysis, suggesting that fibrinogenolysis was not a component of the Vroman effect. In addition, experiments performed with plasma prechromatographed on lysine agarose showed that a lysine-agarose adsorbable protein may be minimally involved in fibrinogen desorption and a synergism may exist between HK and that protein.

Adsorption↗
PubMed1982

Factor XII-induced fibrinolysis. Diminished proactivator activity and drastic reduction of activator activity in Fletcher factor-deficient plasma gamma globulin.

Fibrinolytic studies in gamma G fractions of three Fletcher factor-deficient plasmas (functionally deficient in prekallikrein) revealed weak or no factor XII-independent activator activity. Two of the three Fletcher trait patients showed no plasminogen activator activity in clot lysis, fibrin plate, and amidolytic assays. The third patient showed no activator activity as determined by clot lysis and amidolytic assays but gave 10% of the activator activity detected in normal undiluted gamma G fraction in absence of HFf when determined by fibrin plate assay. Normal plasma gamma G fractions showed detectable and significant plasminogen activator activity. These fractions did not contain kallikrein or activated factor XI activities, thus indicating that the activator activity could not be attributed to the presence in these fractions or trace of these activated factors. Furthermore, factor XI-deficient plasma gamma G fraction, which was shown to contain no activated prekallikrein, showed normal plasminogen activator activity. Finally, specific antibodies to prekallikrein were shown not to quench the activity of plasminogen activator present in normal plasma gamma fraction. A double genetic deficiency to explain the absence in Fletcher factor-deficient plasma gamma G fractions of both prekallikrein proactivator and activator activities is not likely. Thus plasma prekallikrein, besides being a known plasminogen proactivator, appears to be required for the expression of a plasminogen activator activity.

Electrophoresis, Polyacrylamide Gel↗