ere are two principal mechanisms to stop the loss of blood in higher organisms following vascular injury. Initially, platelets are activated and adhere to the site of injury. The platelets then aggregate and form a platelet plug that reduces or tem-

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Thrombotic thrombocytopenic purpura is associated with acquired or congenital deficiency of a plasma von Willebrand factor-cleaving protease (VWFCP). Based on partial amino acid sequence, VWFCP was identified recently as a new member of the ADAMTS family of metalloproteases and designated ADAMTS13. The 4.6-kilobase pair cDNA sequence for VWFCP has now been determined. By Northern blotting, full-length VWFCP mRNA was detected only in liver. VWFCP consists of 1427 amino acid residues and has a signal peptide, a short propeptide terminating in the sequence RQRR, a reprolysin-like metalloprotease domain, a disintegrinlike domain, a thrombospondin-1 repeat, a Cys-rich domain, an ADAMTS spacer, seven additional thrombospondin-1 repeats, and two CUB domains. VWFCP apparently is made as a zymogen that requires proteolytic activation, possibly by furin intracellularly. Sites for Zn 2؉ Thrombotic thrombocytopenic purpura (TTP)1 is a syndrome characterized by microangiopathic hemolytic anemia and thrombocytopenia, and it may be accompanied by neurological dysfunction, renal failure, and fever (1, 2). TTP often strikes young adults, mainly females, suggesting an autoimmune etiology. If untreated, the mortality may exceed 90% (2), but plasma exchange therapy has reduced the mortality to less than 20% (3).The basis for the efficacy of plasma exchange remains unknown, although a plausible model has been suggested in which the proteolysis of von Willebrand factor (VWF) plays a central role. In 1982, patients with chronic relapsing TTP were reported to have "unusually large" VWF multimers (UL-VWF) because of the absence of a VWF depolymerase activity. Binding of UL-VWF to platelets could promote microvascular thrombosis, platelet consumption, and hemolysis. Therapeutic infusion of plasma was proposed to replace the missing depolymerase activity or other factors and thereby limit VWF-dependent platelet thrombosis (4). Later discoveries have strengthened the relationship between VWF proteolysis and TTP. A plasma metalloprotease was identified that requires both calcium and zinc ions and cleaves the Tyr 1605 -Met 1606 bond 2 in the central A2 domain of the VWF subunit (5, 6). Cleavage was stimulated by shear forces like those occurring at sites of arterial thrombosis or by low concentrations of urea or guanidine. Furthermore, most adult patients with TTP were found to have congenital deficiency or an acquired autoantibody inhibitor of this VWF-cleaving protease (VWFCP) (7-9). These findings are consistent with the UL-VWF model of TTP pathogenesis. Alternatively, it is possible that failure to cleave another unknown substrate contributes to the pathogenesis of TTP.VWFCP was recently purified from human plasma sources, and an N-terminal amino acid sequence was obtained (10,11). This information showed it to be a member of the ADAMTS family of metalloproteases, named for the characteristic combination of a disintegrin-like and metalloprotease (reprolysintype), with thrombospondin type 1 motif (12-14). Furthermore, the VWFCP gene wa...