Release of a small two-chain form of antithrombin III from a conformationally changed antithrombin III-thrombin complex

Reaction of antithrombin III (AT) with thrombin results in the formation of stable antithrombin III-thrombin (AT-T) complex with a M r of 92.5-kDa, accompanied by the appearance of a proteolytically modified form of the inhibitor (AT M). Under these conditions AT-T is also transformed to a smaller complex (AT-T S). This smaller complex (81-kDa), a product of a conformational change at the AT moiety of the AT-T complex, is further transformed to a very small complex (AT-T VS) with a M r of 71-kDa. Along with this process, AT-T S slowly dissociates to a free enzyme and a small, presumably to-chain product of AT (AT MS) with a M r of 49-kDa. The newly described component, AT MS, naturally occurs in plasma and serum and accumulates significantly in plasma of patients suffering from cardiovascular disease.