The blood-clotting protein prothrombin can be converted to thrombin in free solution by the proteolytic enzyme, activated factor X. When prothrombin is bound to the surface of phospholipid vesicles, the rate of thrombin generation is increased more than 30-fold over the rate of unbound prothrombin. If the prothrombin activation process is terminated after a time interval in which less than 10% of the expected thrombin has been produced, two major products are found in the activation mixture. These products have been termed intermediate 1, a precursor of thrombin, and fragment 1. The conversion of intermediate 1 to thrombin is not accelerated by phospholipid nor can binding of this intermediate to phospholipid particles be demonstrated. In contrast, fragment 1, the other activation product derived from prothrombin, binds to phospholipid particles under the same conditions as prothrombin. On the basis of these observations, we propose that the prothrombin molecule contains a specific region of polypeptide chain for binding to phospholipid particles. This specific polypeptide region or lipid interaction site is part of the nonthrombin-forming activation fragment.

Animals↗

PubMedDate not supplied

Biochemical and physiological aspects of protein c.

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Animals↗

PubMedDate not supplied

Concordant expression of tissue factor and class II MHC antigens in human placental endothelium.

Villitis of unestablished etiology is a placental lesion frequently associated with high risk pregnancies: it is also found in placentae from normal term pregnancies. The etiology of the lesion is unknown. Vasculitis and thrombosis have been described in villitis areas of placentae from normal and high risk pregnancies. We asked if fetal stem vessel endothelium in villitis lesions expresses MHC class II antigens, and if this is associated with a thrombogenic activity of these vessels. We found that endothelium of fetal stem vessels in villitis areas was usually MHC class II (HLA-DR, DP and DQ) reactive. Reactivity of fetal stem vessel endothelium for MHC class II antigens was associated with the presence of tissue factor reactivity and the absence of thrombomodulin reactivity. These changes on endothelial plasma membranes can promote intravascular coagulation, ischemic necrosis, vasculitis and other histological changes characteristic of villitis.

Chorionic Villi↗

PubMedDate not supplied

Inflammation and thrombosis: the impact of inflammation on the protein C anticoagulant pathway.

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Animals↗

PubMedDate not supplied

Lupus anticoagulants and thrombosis: the role of phospholipids.

BACKGROUND AND OBJECTIVE: Lupus anticoagulants (LAs) are loosely defined as immunoglobulins that inhibit phospholipid dependent coagulation assays. Antiphospholipid antibodies (APAs) are those immunoglobulins that are observed to bind to phospholipids, usually cardiolipin, in ELISA type assays. Interest in these antibody populations derives from the observation that rather than being associated with bleeding disorders as would be expected, they correlate with an increased risk of thrombosis. Many mechanisms have been proposed to account for the prothrombotic activity of some LAs and APAs. These mechanisms are as diverse as inhibition of the production of endothelial prostacyclin synthesis or impaired fibrinolysis to interaction with beta 2-glycoprotein 1 or prothrombin bound to phospholipids. For the purposes of this review, we would like to focus on a potential mechanism that has been proposed by several labs in addition to our own, namely inhibition of the protein C anticoagulant pathway. INFORMATION SOURCES: The authors have been working in this field and contributing original papers. In addition, the material examined in the present paper includes articles published in journals covered by the Science Citation Index and Medline. STATE OF ART AND PERSPECTIVES: In general, correlation of phospholipid specificity and thrombosis has not been performed on a large scale. We were therefore led to ask two questions. Are the membrane requirements of the protein C anticoagulant pathway really the same as those for the procoagulant complexes? Secondly, if they are not, do the membrane requirements of the anticoagulant complexes mimic those of the thrombotic LAs? The membrane requirements for the activated protein C anticoagulant complex differ from those of the prothrombinase complex. These requirements, i.e. the need for phosphatidylethanolamine for optimal activity, mimic the lipid requirements for at least a population of lupus anticoagulants associated with thrombosis. These observations may provide both the specificity and the link between the activated protein C pathway, lupus anticoagulants and thrombosis. Of course, no conclusion is ever that black and white. Only future studies into the fine specificity of lupus anticoagulants and anti-phospholipid antibodies associated with thrombosis will bear out the hypothesis that those directed towards the activated protein C pathway will be predictive of thrombotic risk.

Humans↗