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anesthesiologists blood conservation clinical practice guidelines. Ann Thorac Surg 2011; 91: 944-82

Background Microvesicles (MVs) express antigens from their parental cells and have a highly procoagulant surface. Animal studies suggest that P-selectin glycoprotein ligand-1-positive (PSGL-1 ) MVs play a role in the pathogenesis of venous thromboembolism (VTE). Objective The aim of this study was to determine plasma levels, the cellular origin and the morphological characteristics of PSGL-1 MVs in patients with unprovoked VTE. Methods We conducted a population-based case-control study in 20 patients with a history of unprovoked VTE and 20 age- and sex-matched healthy controls recruited from the general population. Plasma levels, the cellular origin and the morphological characteristics of PSGL-1 MVs were evaluated using flow cytometry, electron microscopy and confocal microscopy. Results Plasma levels of PSGL-1 MVs were associated with increased risk of VTE. The odds ratio per one standard deviation increase in PSGL-1 MVs was 3.11 (95% confidence interval [CI], 1.41-6.88) after adjustment for age and sex, and 2.88 (95% CI, 1.29-6.41) after further adjustment for body mass index. The PSGL-1 MVs originated mainly from monocytes and endothelial cells determined by double staining with markers of parental cells using flow cytometry and transmission electron microscopy. Scanning electron microscopy of PSGL-1-labeled plasma-derived MVs displayed dominantly spherical vesicles that varied between 50 and 300 nm in diameter. Conclusions Increased plasma levels of PSGL-1 MVs are associated with the risk of unprovoked VTE. Large population-based prospective studies are required to validate our findings.

Factor X activation by the intrinsic Xase complex, composed of factor IXa bound to factor VIIIa on membranes, is essential for the amplified blood coagulation response. The biological significance of this step is evident from bleeding arising from deficiencies in factors VIIIa or IXa in hemophila. Here, we assess the mechanism(s) that enforce the distinctive specificity of intrinsic Xase for its biological substrate. Active site function of IXa was assessed with a tripeptidyl substrate (PF-3688). The reversible S1 site binder, 4-aminobenzamidine (pAB), acted as a classical competitive inhibitor of PF-3688 cleavage by Xase. In contrast, pAB acted as a noncompetitive inhibitor of factor X activation. This disconnect between peptidyl substrate and protein substrate cleavage indicates a major role for interactions between factor X and extended sites on Xase in determining substrate affinity. Accordingly, an uncleavable factor X variant, not predicted to engage the active site of IXa within Xase, acted as a classical competitive inhibitor of factor X activation. Fluorescence studies confirmed the binding of factor X to Xase assembled with IXa with a covalently-blocked active site. Our findings suggest that the recognition of factor X by the intrinsic Xase complex occurs through a multistep “dock-and-lock” pathway in which the initial interaction between factor X and intrinsic Xase occurs at exosites distant from the active site, followed by active site docking and bond cleavage.