Snake Venoms in Haemostasis
The composition of snake venom differs from species to species and even within a single species depending on age, season and temperature but generally it is a complex mixture of molecules including cytotoxins, neurotoxins and haemostatins (coagulation modifiers). The latter may be activators or inhibitors of coagulation and some have found their way into the laboratory as the basis for specialised haemostasis tests. The most common are listed below in the Table and their target in the coagulation cascade are shown in Figure.
Prothrombin activators are probably the best studied snake venom haemostatins. They are currently named according to the taxonomic name for the snake of origin with the suffix –arin or –activase and further classified on the basis of their cofactor requirement as shown below.
|Group A (no cofactor requirement)||Echis Carinatus||Ecarin A|
|Group B (requires calcium)||Echis Carinatus||Carinactivase B|
|Group C (requires calcium and phospholipid)||Pseudonaja textilis
|Group D (requires calcium and phospholipids and Factor Va)||Pseudonaja textilis||Pseutarin D|
So, for example, metalloproteinases that convert prothrombin to meizothrombin are classified as group A (requiring no cofactors for activity) or group B (requiring calcium only for activity) and these include ecarin A (the basis of the Ecarin Clotting Test) and carinactvase B isolated from Echis carinatus. Group C prothrombin activators are serine proteases that do not require mammalian Factor Va for the cleavage of prothrombin because they contain their own non-catalytic factor Va-like molecule complexed with a factor Xa-like protein but do require calcium and phospholipids. Examples include pseutarin C (the basis of the textarin test, from the venom of the brown snake, Pseudonaja textilis) and oscutarin C (the basis of the taipan viper venom test, from the coastal taipan, Oxyuranus scutellatus). Group D prothrombin activators are also serine proteases but require calcium, phospholipids, and mammalian factor Va for activity. This explains the utility of the Textarin:Ecarin test for evaluating the presence of antiphospholipid antibodies since both ecarin and textarin activate prothrombin but Ecarin A is a group A activator so has no phospholipid requirement and its action will be unaffected by such antibodies whereas textarin (Pseutarin C) is group C so requires phospholipids and its action will be affected.
Less Common Uses
Snake venom haemostatins also have more esoteric laboratory roles. Snake venom thrombin-like enzymes (SVTLE) are used for fibrinogen/fibrinogen breakdown product assay and for the detection of fibrinogen dysfunction. SVTLE are not inhibited by heparin and can thus can be used for assaying antithrombin and other haemostatic variables in heparin-containing samples. Prothrombin activators may be utilised in prothrombin assays or for studying dysprothrombinaemias. von Willebrand factor can be studied with botrocetin from venom which enhances the affinity of the A1 domain of VWF for the platelet glycoprotein GP1ba. Some snake venoms also contain disintegrins which interact with, and may be useful for studying, other platelet glycoprotein receptors.
Snake Venoms used in the Diagnostic Haemostasis Laboratory
|Image||Common Name||Taxonomic Name||Location||Clotting Compound||Action||Use|
|Saw Scaled Viper||Echis carinatus||Middle east and Central Asia||Ecarin A||Prothrombin activator (generates meizothrombin)||Textarin:Ecarin ratio for screening for lupus anticoagulants
Ecarin time to monitor hirudin activity and other direct thrombin inhibitors.
|Eastern Brown Snake||Pseudonaja textilis||Australia (kills more humans than any other Australian snake)||Pseutarin C||Prothrombin activator||Textarin:Ecarin time (test for lupus anticoagulant)|
|Costal Taipan||Oxyuranus scutellatus||Australia (4th most potent venom by LD50 of any snake in the world)||Oscutarin C||Prothrombin activator||Taipan snake venom test (test for lupus anticoagulant)|
|Common Lancehead||Bothrops atrox||South America (kills more humans than any other American snake)||Reptilase||Fibrinogen activator, (uninhibited by antithrombin so unaffected by Heparin)||Reptilase time (commonly used to screen for heparin contamination of a plasma sample)|
|Copperhead||Agkistrodon contortrix||North America||Protac||Protein C activator||The basis for functional Protein C and S assays and for activated protein C resistance|
|Russell's Viper||Daboia russelii||Asia (kills more humans than any other snake)||Dilute Russell's Viper Venom [dRVVT - test for lupus anticoagulant]||Activates factors V and X||dRVVT - test for lupus anticoagulant|
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