The aim of this study was to systematically examine the inhibitory mechanisms of 
C-phycocyanin (C-PC), one of the major phycobiliproteins of Spirulina platensis 
(a blue-green alga), in platelet activation. In this study, C-PC 
concentration-dependently (0.5-10 nM) inhibited platelet aggregation stimulated 
by agonists. C-PC (4 and 8 nM) inhibited intracellular Ca2+ mobilization and 
thromboxane A2 formation but not phosphoinositide breakdown stimulated by 
collagen (1 microg/mL) in human platelets. In addition, C-PC (4 and 8 nM) 
markedly increased levels of cyclic GMP and cyclic GMP-induced 
vasodilator-stimulated phosphoprotein (VASP) Ser(157) phosphorylation. Rapid 
phosphorylation of a platelet protein of Mw 47,000 (P47), a marker of protein 
kinase C activation, was triggered by phorbol-12,13-dibutyrate (150 nM). This 
phosphorylation was markedly inhibited by C-PC (4 and 8 nM). In addition, C-PC 
(4 and 8 nM) markedly reduced the electron spin resonance (ESR) signal intensity 
of hydroxyl radicals in collagen (1 microg/mL)-activated platelets. The present 
study reports on a novel and very potent (in nanomolar concentrations) 
antiplatelet agent, C-PC, which is involved in the following inhibitory 
pathways: (1) C-phycocyanin increases cyclic GMP/VASP Ser157 phosphorylation and 
subsequently inhibits protein kinase C activity, resulting in inhibition of both 
P47 phosphorylation and intracellular Ca2+ mobilization, and (2) C-PC may 
inhibit free radicals (such as hydroxyl radicals) released from activated 
platelets, which ultimately inhibits platelet aggregation. These results 
strongly indicate that C-PC appears to represent a novel and potential 
antiplatelet agent for treatment of arterial thromboembolism.