Protease inhibitor. Plasma proteins HP-20, HP-25, HP-27 and HP-55 form a 140 kDa complex via disulfide bonds in the plasma. Plasma; synthesized in the liver. The protein complex disappears from the plasma at onset of hibernation and reappears as hibernation ceases. The reactive center loop (RCL) extends out from the body of the protein and directs binding to the target protease. The protease cleaves the serpin at the reactive site within the RCL, establishing a covalent linkage between the serpin reactive site and the active site of the protease. The resulting inactive serpin-protease complex is highly stable (By similarity). The N-terminus is blocked. Belongs to the serpin family.