Habutobin recognizes Thr(7) in the sequence of fibrinopeptide A of rabbit fibrinogen.

Habutobin, a thrombin-like enzyme from Trimeresurus flavoviridis venom, cleaves only the Arg(16)-Gly(17) bond in the rabbit Aalpha chain and releases fibrinopeptide A (FPA). To investigate the role of amino acid residues in the rabbit FPA sequence upon habutobin action, we examined the inhibitory effects of FPA and peptides containing partial sequences of FPA on the habutobin action. Fibrinopeptides from rabbit, human, bovine and dog were isolated and rabbit FPA was fragmented using dilute HCl. Rabbit FPA inhibited the action of habutobin although FPA from human, bovine and dog did not. Among the fragments of rabbit FPA, a heptapeptide Aalpha 3-9, the N-terminal region of rabbit FPA, competitively inhibited the release of FPA by habutobin, whereas the C-terminal hexapeptide of FPA (Aalpha 11-16) exerted no effect on the habutobin action. Synthetic tripeptides Ser-Thr-Phe corresponding to Aalpha 6-8 and Ala-Thr-Phe also inhibited the habutobin action, but Ser-Asp-Phe and Ala-Thr-Gly did not. It is concluded that habutobin would recognize the region around Thr(7)-Phe(8) in the sequence of rabbit FPA (Aalpha 1-16) prior to the cleavage of the Arg(16)-Gly(17) bond.