ANTICOAGULANT REPERTOIRE OF THE HOOKWORM ANCYLOSTOMA-CANINUM

Hookworms are hematophagous nematodes that infect a wide range of mamm alian hosts, including humans, There has been speculation for nearly a century as to the identity of the anticoagulant substance(s) used by these organisms to subvert host hemostasis. Using molecular cloning, w e describe a family of potent small protein (75-84 amino acids) antico agulants from the hookworm Ancylostoma caninum termed AcAP (A. caninum anticoagulant protein). Two recombinant AcAP members (AcAP5 and AcAP6 ) directly inhibited the catalytic activity of blood coagulation facto r Xa (fXa), while a third form (AcAPc2) predominantly inhibited the ca talytic activity of a complex composed of blood coagulation factor VII a and tissue factor (fVIIa/TF), The inhibition of fVIIa/TF was by a un ique mechanism that required the initial formation of a binary complex of the inhibitor with fXa at a site on the enzyme that is distinct fr om the catalytic center (exo-site), The sequence of AcAPc2 as well as the utilization of an exo-site on fXa distinguishes this inhibitor fro m the mammalian anticoagulant TFPI (tissue factor pathway inhibitor), which is functionally equivalent with respect to fXa-dependent inhibit ion of fVIIa/TF, The relative sequence positions of the reactive site residues determined for AcAP5 with the homologous regions in AcAP6 and AcAPcZ as well as the pattern of 10 cysteine residues present in each of the inhibitors suggest that the AcAPs are distantly related to the family of small protein serine protease inhibitors found in the nonhe matophagous nematode Ascaris lumbricoides var. suum.