Isolation and characterization of a protein C activator from tropical moccasin venom

A protease from the venom of the tropical moccasin (Agkistrodon bilineatus) that activates protein C was purified to homogeneity by ion-exchange and gel permeation chromatography. The purified protease is a glycoprotein, and exhibited a molecular weight of 35,000 and 38,000 in SDS-PAGE under non-reducing and reducing conditions, respectively. The purified protease readily activated human protein C and steady-state kinetic parameters indicated an apparent Km for human protein C of 1.7 microM and an apparent kcat of 0.02 sec-1. Calcium inhibited the activation of human protein C by the venom protease (Ki = 93 microM). Amino-terminal sequence analysis revealed that the tropical moccasin protein C activator was highly homologous to the protein C activator isolated from Southern copperhead venom.