Bovine Plasma Protein C Inhibitor with Structural and Functional Homologous Properties to Human Plasma Protein C Inhibitor1

Bovine plasma protein C inhibitor was purified; it was then characterized in comparison with human protein C inhibitor. The specific inhibitory activity of the purified inhibitor for bovine activated protein C was 8,600 times that of the inhibitor in plasma. The purified inhibitor showed a single band with Mr 56,000 by SDS-PAGE at pH 7.0, and two bands at pH 8.8, a major one with Mr 56,000 and a minor one with Mr 105,000, under both unreduced and reduced conditions. The pi range of the inhibitor was between 4.4 and 6.1. The Mr of the inhibitor was reduced by treatment with neuraminidase, O-glycanase, and also with glycopeptidase-A, suggesting that the inhibitor has both Asn-linked and Ser/Thr-linked carbohydrate chains. Twenty-seven of the NH2-terminal 49 amino acid residues of the bovine inhibitor, which lacks the first 4 residues from the NH2 -terminal amino acid sequence of human inhibitor, were identical to those of the human inhibitor. The bovine inhibitor inhibited bovine and human activated protein C, human thrombin, Factor Xa, Factor XIa, and plasma kallikrein with K, = 1.0, 5.2, 2.6, 3.0, 1.3 ×10-8M, and 4.5 X10-9M, respectively. The inhibitory rates for activated protein C and thrombin were accelerated significantly in the presence of heparin or negatively charged dextran sulfate. However, the acceleration by heparin or dextran sulfate for the inhibition of Factor Xa, Factor XIa, and plasma kallikrein was not significant. The bovine inhibitor did not inhibit human Factor Xlla or plasmin. During the inhibition of bovine activated protein C, both the A£56,000 inhibitor and 105,000 inhibitor formed a complex with Mr 100,000. This complex was dissociated by treatment with hydroxyamine or ammonium, suggesting that the bond between the inhibitor and activated protein C is an acyl-bond. These results indicate that bovine protein C inhibitor is structurally and functionally homologous to human inhibitor and may be involved in the regulation of the bovine anticoagulant protein C pathway.