Complete primary structure and biochemical properties of gilatoxin, a serine protease with kallikrein-like and angiotensin-degrading activities.

The activity and the complete primary structure of gilatoxin, a glycoprotein component from the venom of the Mexican beaded lizard (Heloderma horridum horridum) has been elucidated. Gilatoxin, a serine protease, showed kallikrein-like activity, releasing bradykinin from kininogen; toxin-treated kininogen also produced lowered blood pressure in rats and contraction of isolated rat uterus smooth muscle. Gilatoxin catalyzed the hydrolysis of various arginine ester substrates for trypsin and thrombin and degraded both angiotensin I and II by cleavage of the dipeptide Asp-Arg from the NH2-terminal end. Fibrinogen was degraded but a fibrin clot was not produced, indicating that gilatoxin has specificities different from thrombin and snake venom thrombin-like proteases. The complete amino acid sequence of gilatoxin (245 residues) was deduced from NH2-terminal sequencing of overlapping peptide fragments cleaved from the reduced and alkylated toxin by enzymatic and chemical methods. The toxin is extensively glycosylated, containing approximately 8 mol of monosaccharide/mol of toxin, but appears to lack O-glycosylation sites. Amino acid sequence alignment of gilatoxin with batroxobin, crotalase, kallikrein, thrombin, trypsin, and several partial sequences of other Heloderma toxins reveals that there is considerable homology between these enzymes, particularly in the regions of the presumed catalytic site. Gilatoxin contains an additional 7 residues in the highly conserved catalytic region of serine proteases (including Asp-96, in the basic specificity pocket of thrombin) which may contribute to the unusual substrate specificity of the toxin.