Gene/Protein Disease Symptom Drug Enzyme Compound
Pivot Concepts:   Target Concepts:
Query: EC:3.4.21.5 (thrombin)
 33,306 document(s) hit in 31,850,051 MEDLINE articles (0.00 seconds)

It is known that the antibiotic ristocetin exposes the platelet membrane receptor for factor VIII/von Willebrand glycoprotein (FVIII/vWF). Recent reports suggest that low concentrations of thrombin also cause platelet membrane receptors to become available for FVIII/vWF. As a consequence, the suspicion has been raised that thrombin provides similar or equivalent activity in vivo to that observed for ristocetin under in vitro conditions. In this study, we quantitated the extent to which thrombin promotes the binding of FVII/vWF to platelets and determined whether or not this interaction initiates or complements platelet aggregation. With ristocetin present, the amount of 125I-FVIII/vWF that became platelet-bound correlated closely with the onset, rate, and extent of platelet aggregation. In contrast, at every thrombin concentration tested, the amount of 125I-FVIII/vWF that specifically bound to platelets was about 6% of that observed with ristocetin. Significantly, FVIII/vWF did not augment the rate of aggregation of platelets in response to thrombin or initiate platelet aggregation when subaggregating doses of thrombin were used. These observations indicate that the minimal association that occurs between FVIII/vWF and the platelet membrane in the presence of thrombin does not correlate with platelet aggregation and therefore is not analogous to the effects of ristocetin. Whether the low level of binding relates to another process, such as platelet-endothelial interactions, remains unknown.
 ...
PMID:Comparison of thrombin and ristocetin in the interaction between von Willebrand factor and platelets. 630 33

Specific antibodies against anti-human FVIII/vW protein were isolated by affinity chromatography on glutaraldehyde-activated gel (Ultrogel AcA22). They were coupled directly with peroxidase or visualized with anti-rabbit IgG (sheep)-peroxidase (Institut Pasteur). Fab fragments of the same specific antibodies were prepared to enhance the intracellular penetration and coupled to peroxidase. In washed human platelets, staining was observed on the plasma membrane and in the canalicular system, whereas in previous studies whole specific antibodies incubated with fixed platelets showed the labeling only on the plasma membrane. After thrombin activation, the release of granules containing FVIII/vW protein was better visualized in the surface canalicular system. This localization was discussed in regard to the exocytosis process: membrane fusion, granule labeling.
 ...
PMID:Immunocytochemical localization of factor VIII/von Willebrand factor antigen in human platelets. 637 Mar 54

The release of platelet-activating factor (PAF) from stimulated human endothelial cells (HEC) cultured from normal term, umbilical cord veins is described. HEC in primary cultures released PAF after challenge with A23187, rabbit anti-human factor VIII (RaHu/FVIII), angiotensin II, and vasopressin. HEC subcultures maintained the ability to release PAF in the presence of A23187 and RaHu/FVIII, whereas the release of PAF in response to angiotensin II and vasopressin was not constant and was reduced. Control cultured, smooth muscle cells derived from umbilical cord veins, previously depleted of endothelial cells, did not release PAF under the above-mentioned stimulation. Plastic-adherent or cultured monocytes released PAF with A23187, but not with RaHu/FVIII, angiotensin II, and vasopressin. The release of PAF from HEC in primary cultures required the presence of extracellular cations and the activation of membrane phospholipase A2. PAF release induced by A23187, RaHu/FVIII, angiotensin II, and vasopressin was unaffected by indomethacin, an inhibitor of cyclooxygenase, which, however, favored the release of PAF from HEC stimulated with thrombin, a stimulus that did not affect HEC in the absence of indomethacin. PGI2 inhibited PAF release from stimulated HEC. The relevance of an acetylation process in the biosynthesis of PAF and HEC was supported by the following evidence: 1) the increase in PAF yield in the presence of sodium acetate and, particularly, of acetyl-CoA; 2) the incorporation of [14C]acetate into PAF molecules; 3) the loss of radioactivity and of biologic activity after treatment with phospholipase A2. These results indicate that HEC in culture are able to release PAF and that metabolic pathways similar to those described for leukocytes are involved.
 ...
PMID:The release of platelet-activating factor from human endothelial cells in culture. 641 66

Activation of washed platelets in the presence of EDTA with either 1 U/ml of alpha-thrombin or 2 microM calcium ionophore (A23187) caused the release of one-third to one-half of the platelet factor VIII/von Willebrand factor (FVIII/vWF) into the supernatant. When calcium was present in excess, only 10% of the platelet FVIII/vWF was detected free in the supernatant, regardless of whether calcium was present before stimulation or added to the platelets after thrombin activation. Release of [14C]serotonin and beta-thromboglobulin were not affected by divalent cations indicating that reduced supernatant levels of FVIII/vWF in the presence of calcium were not due to differential release, but were probably due to a calcium-dependent association of released FVIII/vWF with the platelet surface. The presence or absence of intact glycoprotein Ib on the platelet surface made no significant difference to the observed FVIII/vWF partition. Platelets from a patient with Glanzmann's thrombasthenia, however, failed to show a calcium effect with respect to released FVIII/vWF. The combined results suggest that as well as the ristocetin-dependent, divalent cation-independent binding of FVIII/vWF to glycoprotein Ib, there is a divalent cation-dependent binding of FVIII/vWF to the activated platelet surface which is mediated via the glycoprotein IIb/IIIa complex.
 ...
PMID:Characterization of calcium-dependent binding of endogenous factor VIII/von Willebrand factor to surface activated platelets. 643 80

The plasma values for factors (F)VII, FVIII:C, FVIIIR:Ag, FIX, FX, and FXI and the thrombin clotting time (TCT) were determined for 28 dogs with naturally occurring hepatic disease. The major morphologic type of hepatic disease present in a given dog, as determined by hepatic biopsy and histopathologic examination, was degeneration (12 dogs), inflammation (9 dogs), cirrhosis (3 dogs), or neoplasia (4 dogs). A specific morphologic diagnosis also was made for each dog in the study. Plasma coagulation factor values and screening tests were consistently abnormal in greater than 50% of the dogs with each type of hepatic disease as follows: degeneration--decreased FXI; inflammation--increased FVIIIR:Ag; cirrhosis--shortened TCT, decreased FIX, FX, and FXI, and increased FVIIIR:Ag; and neoplasia--shortened TCT, decreased FVIII:C, and increased FVIIIR:Ag. The plasma coagulation factor values were compared with serum alanine aminotransferase (ALT) and alkaline phosphatase (ALP) activities, fibrinogen-fibrin degradation product (FDP) concentration, and the prothrombin time (PT) and activated partial thromboplastin time (APTT) to determine the sensitivity and specificity of each test in detection of hepatic disease. Of all dogs with hepatic disease, 93% had at least 1 abnormal coagulation test value. The PT and APTT were abnormal in 50% and 75%, respectively, of these same dogs. Increased serum ALT and ALP activities were present in 61% and 50%, respectively, and FDP concentrations were increased in 14% of dogs with hepatic disease.(ABSTRACT TRUNCATED AT 250 WORDS)
 ...
PMID:Plasma coagulation factor abnormalities in dogs with naturally occurring hepatic disease. 666 Jun 23

Fibrin deposition and platelet thrombus dimensions on subendothelium were studied in four groups of patients with coagulation factor deficiencies. Five patients with factor VIII deficiency (APTT 120 +/- 8 sec) and three patients with factor IX deficiency (APTT 125 +/- 11 sec) were severe bleeders, whereas four patients with factor XII deficiency and seven with factor XI deficiency were either asymptomatic or only mild bleeders despite APTT values of 439 +/- 49 and 153 +/- 13 sec, respectively. Everted segments of deendothelialized rabbit aorta were exposed at a shear rate of 650 sec(-1) for 5 and 10 min to directly sampled venous blood in an annular chamber. Blood coagulation was evaluated by measuring fibrin deposition (percent surface coverage) on the subendothelium and post-chamber fibrinopeptide A levels; platelet thrombus dimensions on the subendothelium were evaluated by determining the total thrombus volume per surface area (using an optical scanning technique) and the average height of the three tallest thrombi. Consistent differences were observed among the patient groups for both the 5-min and 10-min exposure times. The larger of the 5- and 10-min exposure-time values was used to calculate group averages. Fibrin deposition in normal subjects was 81% +/- 5% surface coverage, and post-chamber fibrinopeptide A values were 712 +/- 64 ng/ml. Markedly decreased fibrin deposition and fibrinopeptide A levels were observed in factor VIII deficiency (2% +/- 1% and 102 +/- 19 ng/ml) and factor IX deficiency (11% +/- 7% and 69 +/- 11 ng/ml). In contrast, significantly higher values were obtained in patients deficient in factor XI (33% +/- 5% and 201 +/- 57 ng/ml) and factor XII (66% +/- 12% and 306 +/- 72 ng/ml). Differences in thrombus dimensions were also observed. In normal subjects, the value for thrombus volume and average height of the tallest thrombi were 8.3 +/- 1.3 cu micron/sq micron and 145 +/- 11 micron, respectively, and in patients were as follows: FVIII, 2.7 +/- 0.6 and 71 +/- 7; FIX, 4.5 +/- 1.8 and 88 +/- 14; FXI, 11.8 +/- 1.9 and 125 +/- 10; and FXII, 7.9 +/- 3.1 and 130 +/- 25. Platelet thrombus dimensions were normal in a patient with fibrinogen deficiency, indicating that the smaller thrombi in factor VIII and factor IX deficiencies were probably due to impaired evolution of thrombin rather than diminished fibrin formation.(ABSTRACT TRUNCATED AT 400 WORDS)
 ...
PMID:Fibrin formation, fibrinopeptide A release, and platelet thrombus dimensions on subendothelium exposed to flowing native blood: greater in factor XII and XI than in factor VIII and IX deficiency. 671 90

The accumulated evidence suggests that synthetic substrates measure a similar biological expression as their natural counterpart with, in some cases, a better precision and simplicity of operation. Proteolytic enzymes are involved in a variety of physiological and pathological processes extending from blood coagulation to the release of specific enzymes from certain cells and bacteria. The variation in the level of these proteases, and their inhibitors in plasma or in cells, will be an excellent indicator of any malfunction in the particular system. Excellent agreement between these new generation tests and other conventional methods particularly in measuring fibrinolytic components is seen, and the assay time is cut from hours to a matter of minutes using a reaction rate analyser. Some discrepancy in the estimated potency was observed between methods used for the assay of thrombin, FVIII and several other clotting factors and their inhibitors. There has been methodological improvement and, in some cases, the cause of discrepancy has been revealed. Nevertheless, greater emphasis should still be given to standardization problems. It is, therefore, suggested that until the problems of standardization of various assay methods are resolved and the complications due to non-specificity in chromogenic methods and non-selectivity in clotting tests are overcome, the ratio of amidolytic to clotting activities should be included in the request for potency estimations and in the diagnostic assessment. Whether or not the use of substrates should be complementary or supplementary is left to individual centres according to their particular circumstances.
 ...
PMID:The values of synthetic substrates in the improvement of diagnostic tests for haemostatic function. 678 30

We have examined the effects of purified human alpha-thrombin on factor VIII antigen (FVIII-Ag) release by human umbilical vein endothelial cells in culture. Alpha-thrombin induced a time and dose-dependent release of FVIII-Ag into supernatant medium. Alpha-thrombin-mediated FVIII-Ag release was not dependent on protein synthesis and was observed in both serum-free and serum-containing media. FVIII-Ag release, however, was prevented when the serine esterase activity of thrombin was inhibited. Pretreatment of human endothelial cells with alpha-thrombin, but not diisofluorophosphate-thrombin, prevented subsequent FVIII-Ag release by alpha-thrombin. Thrombin-mediated FVII-Ag release was not associated with significant 51Cr release from prelabeled endothelial monolayers. We conclude that alpha-thrombin induces release of preformed FVIII-Ag from human umbilical vein endothelial cells by a receptor-independent, nonlytic mechanism requiring serine esterase activity.
 ...
PMID:Thrombin-mediated release of factor VIII antigen from human umbilical vein endothelial cells in culture. 680 73

The multimeric structure of platelet factor VIII/von Willebrand factor (FVIII/vWF) in cell extracts and in collagen and thrombin releasates has been analyzed by SDS polyacrylamide gel electrophoresis followed by detection with 125I-anti-FVIII/vWF. Platelets contained larger multimers than those normally present in plasma. When secreted FVIII/vWF was analyzed, all platelets. In contrast, in thrombin releasates the larger multimers were lost in a manner dependent on divalent cations, time, and thrombin dose. This loss could not be accounted for by modification of FVIII/vWF by thrombin or platelet enzymes since no effect of thrombin on the multimeric structure of FVIII/vWF in the absence of platelets or in the presence of platelet lysates was observed. Large multimers of 125I-labeled purified FVIII/vWF underwent divalent cation-dependent association with platelets in the presence of thrombin, indicating that the loss of FVIII/vWF from thrombin releasates was due to reassociation with the platelet. These studies show a structural difference between platelet and plasma FVIII/vWF that suggests a specific role for platelet FVIII/vWF in hemostasis.
 ...
PMID:Multimeric structure of platelet factor VIII/von Willebrand factor: the presence of larger multimers and their reassociation with thrombin-stimulated platelets. 698 84

The reactions of covalently immobilized heparin, abbreviated as I-Hep, with thrombin or Factor Xa were investigated both in the presence and absence of antithrombin III, AT III. Although I-Hep was able to bind to thrombin, the complex formation of thrombin and I-Hep did not affect the thrombin activity when measured by using a small artificial substrate, a peptide-MCA. Similarly, Factor Xa bound to I-Hep, but the activity of Factor Xa was not decreased in the absence of AT III, when a peptide-MCA was used for Factor Xa assay. Thrombin bound to I-Hep in much larger amounts than Factor Xa. Thrombin and Factor Xa were instantaneously inhibited by AT III in the presence of soluble heparin. However, when I-Hep was used instead of soluble heparin, instantaneous inhibition was not observed. When a natural, high-molecular-weight substrate was used for assay, the results were dependent on the structure of the immobilization carrier. Heparin immobilized on Sepharose 4B or Poly HEMA showed considerable prolongation of plasma recalcification time. However, heparin immobilized on the surface of PVA fiber did not prolong plasma recalcification time.
 ...
PMID:The characteristics of anticoagulation by covalently immobilized heparin. 734 68


<< Previous 1 2 3 4 5 6 7 8 9 10 Next >>