The purification, complete amino acid sequence, and
biological activity are described for several homologous
snake venom proteins that are
platelet glycoprotein (
GP) IIb-IIIa antagonists and potent inhibitors of platelet aggregation. The primary structures of
kistrin (from Agkistrodon rhodostoma), bitan (from Bitis arietans), three
isoforms of
trigramin (from Trimeresusus gramineus), and an
isoform of
echistatin (from Echis carinatus) were determined by automated sequence analysis and fast atom bombardment mass spectrometry analysis. Each of the
protein in this family, which range from 47 to 83 residues, contains an
Arg-Gly-Asp amino acid sequence found in
protein ligands that binds to GPIIb-IIIa, a high (17 +/- 1%)
cysteine content conserved in the primary sequence, and a homologous N-terminal region absent only in the
echistatin isoforms. Each
protein directly inhibits the interaction of purified platelet GPIIb-IIIa to immobilized
fibrinogen about 100 times more effectively than does the pentapeptide
Gly-Arg-Gly-Asp-Ser; IC50 values range from 1.1 to 3.0 nM. The IC50 value for the inhibition of platelet aggregation, using human platelet-rich plasma stimulated with
ADP, ranges from 110 to 550 nM for the various
proteins, about 1000-fold more potent than
Gly-Arg-Gly-Asp-Ser.
Kistrin binds reversibly to both resting and
ADP-activated human platelets with high affinity (Kd = 10.8 nM and 1.7 nM, respectively) and to purified GPIIb-IIIa with a lower affinity (Kd = approximately 100 nM). Finally,
kistrin injected at 1.0 mg/kg into rabbits reversibly inhibits platelet aggregation ex vivo over 30 min without induction of
thrombocytopenia. We propose that these
proteins are members of a general class of
proteins found in the
venom of pit vipers that inhibit platelet aggregation by antagonism of the GPIIb-IIIa-
fibrinogen interaction and as such serve as potential
antithrombotic agents.