Alpha-1-antitrypsin-Pittsburgh A Potent Inhibitor of Human Plasma Factor XIa , Kallikrein , and Factor

Alpha-l-antitrypsin-Pittsburgh is a human variant that resulted from a point mutation in the plasma protease inhibitor, a,-antitrypsin (358 Met Arg). This defect in the a,-antitrypsin molecule causes it to have greatly diminished anti-elastase activity but markedly increased antithrombin activity. In this report, we demonstrate that this variant protein also has greatly increased inhibitory activity towards the arginine-specific enzymes of the contact system of plasma proteolysis (Factor XIa, kallikrein, and Factor XIIf), in contrast to normal cal-antitrypsin, which has modest to no inhibitory activity towards these enzymes. Wedetermined the second-order-inactivation rate constant (k") of purified, human Factor XIa by purified a,-antitrypsin-Pittsburgh and found it to be 5.1 X 10' Ms'1 (230C), which is a 7,700-fold increase over the kV for Factor XIa by its major inhibitor, normal purified al-antitrypsin (i.e., 6.6 X 101 M-l s-1). Human plasma kallikrein, which is poorly inhibited by a,-antitrypsin (k" = 4.2 M-l s1), exhibited a k" for a,-antitrypsinPittsburgh of 8.9 X 104 Ms'1 (a 21,000-fold increase), making it a more efficient inhibitor than either of the naturally occurring major inhibitors of kallikrein (Cl-inhibitor and a2-macroglobulin). Factor XIIf, which is not inhibited by normal a,-antitrypsin, displayed a k" for a,-antitrypsin-Pittsburgh of 2.5 X 104 Ms'1. This enhanced inhibitory activity is similar to the effect of a,-antitrypsin-Pittsburgh that has been reported for thrombin. In addition to its potential as an anticoagulant, this recently cloned protein may prove to be clinically valuable in the management of septic shock, hereditary angioedema, or other syndromes involving activation of the surface-mediated plasma pro-