Is able to inhibit all four classes of proteinases by a unique 'trapping' mechanism. This protein has a peptide stretch, called the 'bait region' which contains specific cleavage sites for different proteinases. When a proteinase cleaves the bait region, a conformational change is induced in the protein which traps the proteinase. The entrapped enzyme remains active against low molecular weight substrates (activity against high molecular weight substrates is greatly reduced). Following cleavage in the bait region a thioester bond is hydrolyzed and mediates the covalent binding of the protein to the proteinase. Homotetramer; disulfide-linked. Highest constitutive expression in ovary. Low level in testis, uterus and non-acute phase liver. Protein found in plasma. By inflammatory stimulus in liver. The level of this protein increases during acute phase, then decreases again. Belongs to the protease inhibitor I39 (alpha-2-macroglobulin) family.