THE INHIBITOR OF ACTIVATED PROTEIN C: STRUCTURE AND FUNCTION
Authors: Suzuki Koji , Deyashiki Yoshihiro , Nishioka Junji , Toma Kazunori , Yamamoto Shuji 
Published: August 2018

In the final step of protein C pathway, activated protein C (APC) is neutralized with a plasma inhibitor, termed protein C inhibitor (PCI). PCI was first described by Marlar and Griffin (1980) and then isolated from human plasma as a homogeneous form and characterized by the authors (1983). PCI is a single chain glycoprotein with M 57,000 and a plasma concentration of 5 ug/ml. Analysis of a cDNA nucleotide sequence has clarified that a precursor of human PCI consists of a mature protein of 387 amino acid residues (M 43,759) and a signal peptide of 19 amino acid residues. Only one cysteine residue is present in the entire protein as in α1antitrypsin (α1AT) and α1antichymotrypsin (α1ACT). Three Asn-X-Ser/Thr sequences and two Ser/Thr-X-X-Pro sequences are present as potential attachment sites of carbohydrate chains. Based on the amino acid sequence of the carboxyl-terminal peptide released from the inhibitor by APC digestion, the reactive site peptide bond of PCI was found to be Arg(354)-Ser(355). It is similar to the reactive sites of the other serine protease inhibitors which are located to their carboxyl-terminal Arg(393)-Ser (394), Met(358)-Ser(359) and Leu(358)-Ser(359) in antithrombin III, α1AT and α1ACT, respectively. The alignment of the amino acid sequence of PCI with heparin cofactor II, α1plasmin inhibitor, ovalbumin, angiotensinogen and the above noted plasma inhibitors showed that PCI is a member of serine protease inhibitor superfamily. PCI inhibits APC noncompetitively in a 1:1 stoichiometry and forms a covalent acyl-bond with a Ser residue in the active center of APC. The half life of APC in plasma approximately 30 min, which is rather slow compared with the other protease inhibitors. However, optimal concentrations of heparin, dextran sulfate and its derivatives potentiate the rate of inhibition 30-60 fold. PCI has Ki of 10-8m for APC, and can inhibit thrombin, Factor Xa, urokinase and tissue plasminogen activator as well in the presence of heparin or dextran sulfate, though the Ki for these enzymes is slightly higher. During the complex formation with APC, PCI is cleaved by the complexed APC to form a modified form with M 54,000. PCI is synthesized in several hepatoma cell lines and decreased in plasma of patients with liver cirrhosis. It is also decreased in patients with DIC or those during cardiopulmonary bypass in parallel with the decrease in protein C, suggesting that PCI participates in regulation of the protein C pathway in intravascular coagulation. Recently, we have obtained the recombinant PCI from COS-1 cells which were transfected with expression vector pSV2 containing the cDNA of PCI. The recombinant PCI had the same Mr and specific activity as the protein purified from plasma. It also had an affinity for heparin and dextran sulfate. Moreover, we have predicted a three dimentional structure of the proteolytically modified PCI with computer graphics based on its amino acid sequence homology with the modified α1AT whose structure had been elucidated with X-ray crystallography. All potential carbohydrate attachment sites were estimated to exist on the surface of the protein. Succesively we have constructed the interaction model between the intact PCI predicted from the modified form and the active center of APC which was simulated from that of trypsin. From the model, it was observed that the amino-group of Arg (354, PI site) of PCI could strongly interact with the carboxy1-group of Asp (88, SI site) of the heavy chain of APC at the base of the active center pocket of the enzyme.