Protease nexin-2/amyloid beta protein precursor. A tight-binding inhibitor of coagulation factor IXa.

Protease nexin-2/amyloid beta protein precursor (PN-2/A beta PP) is an abundant, secreted platelet protein which is a potent inhibitor of coagulation Factor XIa. We examined other potential anticoagulant activities of PN-2/A beta PP. Purified Kunitz protease inhibitor domain of PN-2/A beta PP and PN-2/A beta PP itself were found to prolong the coagulation time of plasma and pure Factor IXa. The Kunitz protease inhibitor domain also inhibited the ability of Factor IXa to activate Factor X. PN-2/A beta PP inhibited Factor IXa with a Ki of 7.9 to 3.9 x 10(-11) M in the absence and presence of heparin, respectively. When the second-order rate constant of PN-2/A beta PP's inhibition of Factor IXa (2.7 x 10(8) M-1min-1) was compared to that of antithrombin III (3.8 x 10(6) M-1min-1), PN-2/A beta PP was at least a 71-fold more potent inhibitor of Factor IXa than antithrombin III. PN-2/A beta PP formed a complex with Factor IXa as detected by gel filtration and ELISA. The finding that PN-2/A beta PP is a potent inhibitor of Factor IXa could help to explain the spontaneous intracerebral hemorrhages seen in patients with hereditary cerebral hemorrhage with amyloidosis Dutch-type where there is an extensive accumulation of PN-2/A beta PP in their cerebral blood vessels.