Summary
Purified snake venom prothrombin activators were used to probe the procoagulant properties
of platelet membranes. Human platelets were able to stimulate prothrombin activation
by the venom activators from Oxyuranus scutellatus and Notechis scutatus, while the
prothrombin activator from Echis carinatus was not affected by the presence of platelets.
The prothrombinconverting activity of platelets was further studied with the venom
activator from Oxyuranus scutellatus and with the factor Xa-Va complex as prothrombin
activating enzymes. Stimulation of platelets with collagen, collagen plus thrombin
or with the Ca-ionophore A23187 resulted in a considerable increase of platelet prothrombin
converting activity probed with the factor Xa-Va complex as well as with the prothrombin
activator from Oxyuranus scutellatus. The stimulatory effect of activated platelets
on the rates of prothrombin activation by Oxyuranus scutellatus was similar to that
determined for factor Xa-Va-catalyzed prothrombin activation. Compared to non-stimulated
platelets, platelets stimulated with thrombin plus collagen exposed 20-times more
procoagulant sites for as well the factor Xa-Va complex, as for the venom activator
from Oxyuranus scutellatus. The actual number of procoagulant sites per platelet determined
with the factor Xa-Va complex was in close agreement with the number of sites determined
with the venom activator. Also the time course of appearance of procoagulant activity
during platelet stimulation by collagen plus thrombin was comparable for both activator
complexes. Phospholipase A2 treatment of stimulated platelets resulted in an almost complete loss of their ability
to stimulate prothrombin activation by the enzyme from Oxyuranus scutellatus or by
factor Xa-Va complex. The findings presented in this paper suggest: a) that the factor
Xa-Va complex and the prothrombin activator from Oxyuranus scutellatus recognize the
same procoagulant sites on both stimulated and unstimulated platelets and b) that
negatively-charged phospholipids are essential components of these procoagulant sites.
Keywords
Platelet procoagulant activity - Prothrombin activators - Snake venoms - Notechis
scutatus - Oxyuranus scutellatus