Bradykinin and its metabolite, Arg-Pro-Pro-Gly-Phe, are selective inhibitors of alpha-thrombin-induced platelet activation.
BACKGROUND
Plasma kininogens are selective inhibitors of alpha-thrombin activation of platelets and endothelial cells. In the present study, we localized the alpha-thrombin inhibitory sequence of kininogens and describe its mechanism of action.


RESULTS
Bradykinin and an analogue, MKRPPGFSPFRSSRIG, inhibited alpha-thrombin-induced platelet aggregation and secretion with an IC50 of 0.25 and 1 mmol/L and of 0.23 and 0.5 mmol/L, respectively. The minimal inhibitory peptide was RPPGF. Bradykinin and its analogues did not inhibit ADP-, collagen-, U46619-, or SFLLRN-induced platelet activation or the ability of alpha-thrombin to cleave chromogenic substrates, clot fibrinogen, or block alpha-thrombin binding to platelets. Bradykinin, MKRPPGFSPFRSSRIG, and RPPGF abolished alpha-thrombin-induced (1 nmol/L) calcium mobilization. On flow cytometry, bradykinin and MKRPPGFSPFRSSRIG blocked alpha-thrombin from removing the epitope of its cleavage site on the cloned thrombin receptor. Furthermore, peptide RPPGF or high-molecular-weight kininogen prevented alpha-thrombin from cleaving the thrombin receptor peptide, NATLDPRSFLLR, between arginine and serine.


CONCLUSIONS
These results indicate that bradykinin and its metabolites are selective antithrombins by preventing alpha-thrombin cleavage of the cloned thrombin receptor between arginine-41 and serine-42. These newly recognized antithrombin peptides, which are termed thrombostatins, contribute to the cardioprotective nature of kinins.
Hasan
A A
AA
Department of Internal Medicine, University of Michigan, Ann Arbor 48109-0724, USA.
Amenta
S
S
Schmaier
A H
AH
D000595:Amino Acid Sequence; D000991:Antithrombins / Q000494:pharmacology; D001920:Bradykinin / Q000235:genetics / Q000378:metabolism / Q000494:pharmacology*; D003001:Cloning, Molecular; D006801:Humans; D008969:Molecular Sequence Data; D010446:Peptide Fragments / Q000235:genetics / Q000494:pharmacology*; D015539:Platelet Activation / Q000187:drug effects*; D018179:Receptors, Thrombin / Q000187:drug effects / Q000235:genetics; D013917:Thrombin / Q000494:pharmacology*
D000991:Antithrombins; D010446:Peptide Fragments; D018179:Receptors, Thrombin; C079000:bradykinin (1-5); D013917:Thrombin; D001920:Bradykinin