Isolation of a venom factor devoid of proteolytic activity from Taiwan habu (Trimeresurus mucrosquamatus): N-terminal sequence homology and no functional similarity to factors IX/X-binding proteins and botrocetin

One novel venom factor was isolated and purified from the venom of Taiwan habu (Trimeresurus mucrosquamatus) using two consecutive anion-exchange and gel-filtration chromatographies followed by cation-exchange HPLC. Further characterization of the purified protein indicated that it lacks the proteolytic activity toward fibrinogen molecules, suggesting that this protein factor does not belong to the familes of metalloproteinases and thrombin-like serine proteases commonly found in the crude venoms of various crotalid snakes. The purified protein exists as a native dimeric protein of 26 kDa, consisting of two closely similar subunits of 16 and 13 kDa, held together by disulfide linkage. N-Terminal sequence analysis revealed that both chains are homologous to each other at the N-terminal fragment and also similar to the factors IX/X-binding protein isolated from Trimeresurus flavoviridis and botrocetin from Bothrops jararaca. This study points to the existence of one new two-chain venom factor without fibrinogenase activity from Taiwan habu, which, in contrast to botrocetin, promotes platelet agglutination even in the absence of von Willebrand factor, Unlike factors IX/X-binding proteins, it did not show affinity to coagulation factors IX and X in the presence of Ca2+ ion. It also shows no inhibition on thrombin, in contrast with bothrojaracin, a thrombin inhibitor isolated from Bothrops jararaca venom. We have therefore named this novel venom factor trimecetin to distinguish it from some structurally related venom factors present in various crotalid and viperid snakes.