Enhancement of rabbit protein S anticoagulant cofactor activity in vivo by modulation of the protein S C4B binding protein interaction.

The carboxy-terminal region of protein S has been recently been observed to be involved in the interaction between protein S and C4b-binding protein (Walker, F. J. 1989. J. Biol. Chem. 264:17645-17658). A synthetic peptide, GVQLDLDEAI, corresponding to that region of protein S has been used to investigate the protein S/C4b-binding protein interaction in vitro and in vivo. Rabbit activated protein C possesses species-specific anticoagulant activity for which rabbit protein S functions as a cofactor. In plasma, rabbit protein S is found in complex with C4b-binding protein. GVQLDLDEAI can inhibit this interaction, resulting in enhancement of the anticoagulant activity of rabbit activated protein C. The effect of the peptide can be blocked by the concurrent addition of human or rabbit C4b-binding protein. When infused into rabbits, GVQLDLDEAI was cleared from the circulation with a half-life of 80 min. This is significantly less rapid than the clearance of similarly sized control peptides (half-life of 15 min), but much more than that of bovine protein S, a much larger protein (half-life of 15 h). Plasma samples removed from the rabbits after infusion with GVQLDLDEAI were found to have increased concentrations of free protein S and to show enhanced anticoagulation by rabbit activated protein C ex vivo in a dose-dependent manner. The concentration for half-maximal effect (5 microM) was very similar to that observed in vitro. These results suggest that the formation of a complex between protein S and C4b-binding protein is important in the regulation of protein S activity in vivo, and that modulation of this interaction allows one to influence the anticoagulant activity of the protein C pathway.