Protein C inhibitor (PCI) regulates the anticoagulant protein C pathway by
neutralizing activated protein C and thrombin-thrombomodulin complex in the
human hemostatic system. In this study, we cloned a full-length bovine PCI
cDNA encoding a putative 19-residue signal peptide and a 385-residue matur
e protein. this showed 70.6%. 70.6%, 57.5% and 59.6% amino acid sequence ho
mology with the human, rhesus monkey, rat and mouse PCIs, respectively. Bov
ine PCI mRNA (2.1 kb in size) was expressed strongly in the liver. and mode
rately in the kidney and testis. but not in other tissues tested. Bovine PC
I has a putative reactive site peptide bond, Lys-Ser, that is different fro
m the reactive site sequence (Arg-Ser) of other species' PCI. We found that
bovine PCI transiently inhibits bovine plasmin, but not human plasmin. Wes
tern blot analysis showed that the reactive site of bovine PCI is cleaved d
uring the course of complete formation with bovine plasmin: degraded PCI is
released from the complex gradually concomitant with the recovery of plasm
in activity. These findings suggest that bovine PCI plays a role not only i
n the protein C pathway but also in the fibrinolytic activity of bovine hem
ostatic system.