Carbon monoxide releasing molecule-2 inhibition of snake venom thrombin-like activity: novel biochemical "brake"?

A complication of defibrinogenation therapy with snake venom enzymes such as ancrod is hypofibrinogenemia associated bleeding secondary to no human-derived inhibitor being available to inactivate or diminish the activity of such enzymes. Of interest, ancrod contains a critical histidine residue without which enzymatic activity is inhibited, and carbon monoxide has been demonstrated to inhibit biomolecular function by interacting with histidine moieties in ion channels. We tested the hypothesis that exposure of three different snake venoms containing serine proteases with thrombin-like activity (which included ancrod) to carbon monoxide derived from carbon monoxide releasing molecule-2 would diminish their effects on plasmatic coagulation as assessed by thrombelastography. In the case of the Malayan pit viper and Eastern diamondback rattlesnake venoms, carbon monoxide diminished the effects of thrombin-like activity. In contrast, timber rattlesnake venom demonstrated enhancement of "thrombin-generating" activity with simultaneous loss of thrombin-like activity in response to carbon monoxide exposure. These findings may serve as the rational basis for not just continuing to investigate the potential of snake venom enzymes as clinical defibrinogenating agents, but to also to assess the potential to stop such agents from becoming a catalytic "runaway train" by judicious application of a biochemical "brake" such as carbon monoxide.

Thrombomodulin-independent activation of protein C and specificity of hemostatically active snake venom serine proteinases: crystal structures of native and inhibited Agkistrodon contortrix contortrix protein C activator.

Protein C activation initiated by the thrombin-thrombomodulin complex forms the major physiological anticoagulant pathway. Agkistrodon contortrix contortrix protein C activator, a glycosylated single-chain serine proteinase, activates protein C without relying on thrombomodulin. The crystal structures of native and inhibited Agkistrodon contortrix contortrix protein C activator determined at 1.65 and 1.54 A resolutions, respectively, indicate the pivotal roles played by the positively charged belt and the strategic positioning of the three carbohydrate moieties surrounding the catalytic site in protein C recognition, binding, and activation. Structural changes in the benzamidine-inhibited enzyme suggest a probable function in allosteric regulation for the anion-binding site located in the C-terminal extension, which is fully conserved in snake venom serine proteinases, that preferentially binds Cl(1-) instead of SO(4)(2-).

Antithrombin activity of intact human platelets.

Antithrombin activity has been identified in intact washed human platelets. An apparent activity was demonstrated at platelet concentrations above 0.31 X 10(9)/ml, when platelet suspensions were incubated with 2.0 NIH units/ml of thrombin. Neither red cells nor white cells revealed antithrombin activity. No significant loss of the platelet antithrombin activity was observed after ten successive washings or after treatment of platelets with antibodies to antithrombin III or alpha2-macroglobulin. Almost the same amount of antithrombin activity as normal platelets was demonstrated in the platelets from an afibrinogenemic patient. Pre-treatment of platelets with trypsin, papain, and neuroaminidase reduced the activity significantly, whereas lipase was without effect. The platelet antithrombin reacted with thrombin in less than 3 seconds, and this rapid reaction of platelet antithrombin was different from that of plasma antithrombin III or fibrinogen. The thrombin-like clotting activity of ancrod was inhibited by fibrinogen but not platelets. Also, unlike plasma antithrombin III or fibrinogen, brief exposure to heat (56 degrees C or 60 degrees C) reduced considerable amounts of platelet antithrombin activity. These results suggest that platelets possess a specific antithrombin with different characteristics from other known antithrombins.