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The inhibitor of activated protein C (APC), designated protein C inhibitor (PCI), was purified from human plasma. The inhibitor was a single chain polypeptide with molecular weight of 57, 000, and with microheterogeneity in pI: six pIs exist between 8.6 and 7.4. PCI was different from already established seven plasma protease inhibitors chemically and immunologically. It seemed to be also different from the recently discovered heparin cofactor II judging from amino acid composition, amino-terminal sequence and pI. PCI reduced an amidolytic activity of APC noncompetitively by forming a 1:1 molar complex with the enzyme, and also blocked the prolongation of plasma clotting time (APTT) by APC. PCI inhibited thrombin, Factor Xa, trypsin and chymotrypsin as well. The values of Ki of PCI against APC, thrombin and Factor Xa were 5.8×10-8 M, 6.7×10-8M and 3.1×10-7M, respectively. The rate of inactivation of APC by PCI was enhanced about 30 fold in the presence of a relatively higher concentration of heparin (5-10units/ml). The rabbit antiserum to PCI completely abrogated the inhibitory activity to APC in plasma, thus suggesting that the PCI purified in this study is the sole inhibitor to APC in plasma. The purification and characterization of PCI is important to elucidate the mechanism of regulation of APC in blood coagulation and fibrinolysis in future studies.
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