Objective: The Australian snake venom ptFV (Pseudonaja textilis venom-derived factor V) variant retains cofactor function despite APC (activated protein C)-dependent proteolysis. Here, we aimed to... Show moreObjective: The Australian snake venom ptFV (Pseudonaja textilis venom-derived factor V) variant retains cofactor function despite APC (activated protein C)-dependent proteolysis. Here, we aimed to unravel the mechanistic principles by determining the role of the absent Arg306 cleavage site that is required for the inactivation of FVa (mammalian factor Va). Approach and Results: Our findings show that in contrast to human FVa, APC-catalyzed proteolysis of ptFVa at Arg306 and Lys507 does not abrogate ptFVa cofactor function. Remarkably, the structural integrity of APC-proteolyzed ptFVa is maintained indicating that stable noncovalent interactions prevent A2-domain dissociation. Using Molecular Dynamics simulations, we uncovered key regions located in the A1 and A2 domain that may be at the basis of this remarkable characteristic. Conclusions: Taken together, we report a completely novel role for uniquely adapted regions in ptFVa that prevent A2 domain dissociation. As such, these results challenge our current understanding by which strict regulatory mechanisms control FVa activity. Show less
Summary. Background: It takes many years to obtain reliable values for the risk of venous thrombosis of hormonal contraceptive users from clinical data. Measurement of activated proteinC (APC)... Show moreSummary. Background: It takes many years to obtain reliable values for the risk of venous thrombosis of hormonal contraceptive users from clinical data. Measurement of activated proteinC (APC) resistance via thrombin generation is a validated test for determining the thrombogenicity of hormonal contraceptives. Sex hormone-binding globulin (SHBG) might serve as a marker for the risk of venous thrombosis, and can be easily and rapidly measured in routine laboratories. Objective: To determine whether SHBG is a useful marker for the thrombotic risk of hormonal contraceptive users by comparing plasma SHBG levels with normalized APC sensitivity ratio (nAPCsr) values and thrombosis risks reported in the recent literature. Methods: We conducted an observational study in 262 users of different contraceptives, and measured nAPCsr and SHBG levels. Results: Users of contraceptives with a higher risk of causing venous thrombosis, i.e. combined hormonal contraceptives containing desogestrel, cyproterone acetate or drospirenone, and the transdermal patch, had higher SHBG levels than users of combined hormonal contraceptives containing levonorgestrel, which carry a lower thrombosis risk. Users of the patch had the highest SHBG levels, with a mean difference of 246nmolL−1 (95% confidence interval179–349) from that in users of levonorgestrel-containing combined hormonal contraceptives. SHBG levels were positively associated with both the nAPCsr and the risks of venous thrombosis reported in the recent literature. Conclusion: SHBG is a useful marker with which to estimate the thrombotic safety of a preparation. Show less
Raps, M.; Helmerhorst, F.; Fleischer, K.; Thomassen, S.; Rosendaal, F.; Rosing, J.; ... ; Vliet, H. van 2012
