Amino acid sequence of trocarin, a prothrombin activator from Tropidechis carinatus venom: Its structural similarity to coagulation factor Xa

Among snake venom procoagulant proteins, group II prothrombin activators ar e functionally similar to blood coagulation factor Xa. We have purified and partially characterized the enzymatic properties of trocarin, the group II prothrombin activator from the venom of the Australian elapid, Tropidechis carinatus (rough-scaled snake). Prothrombin activation by trocarin is enha nced by Ca2+, phospholipids, and factor Va, similar to that by factor Xa, H owever, its amidolytic activity on peptide substrate S-2222 is significantl y lower. We have determined the complete amino acid sequence of trocarin. I t is a 46,515-Dalton glycoprotein highly homologous to factor Xa and shares the same domain architecture. The light chain possesses an N-terminal Gla domain containing 11 gamma-carboxyglutamic acid residues, followed by two e pidermal growth factor (EGF)-like domains; the heavy chain is a serine prot einase. Both chains are likely glycosylated: the light chain at Ser 52 and the heavy chain at Asn 45. Unlike other types of venom procoagulants, troca rin is the first true structural homologue of a coagulation factor, It clot s snake plasma and thus may be similar, if not identical, to snake blood co agulation factor Xa. Unlike blood factor Xa, it is expressed in high quanti ties and in a nonhepatic tissue, making snake venom the richest source of f actor Xa-like proteins, It induces cyanosis and death in mice at 1 mg/kg bo dy weight. Thus, trocarin acts as a toxin in venom and a similar, if not id entical, protein plays a critical role in hemostasis. (C) 1999 by The Ameri can Society of Hematology.