Kallikrein-like enzymes from Crotalus atrox venom

The symptoms which immediately follow envenomation by many crotalid snakes include hypotension, hypovolemia, hemoconcentration, and shock. We have isolated and characterized two proteases (EI and EII) from the venom of Crotalus atrox which may be involved in the onset of these symptoms. EI and EII have molecular weights of 27,500 and 29,200 and isoelectric points of 4.7 and 4.3, respectively. Specific esterolytic activities of EI and EII on N alpha-p-tosyl-L-arginine methyl ester are 51.5 mumol min-1mg-1 and 48.1 mumol min-1 mg-1, respectively. Both enzymes are rather specific in their substrate requirements in that neither was demonstrated to have any proteolytic activity against either of the oxidized chains of insulin, or glucagon. Neither enzyme was shown to have plasmin or fibrinolytic activity. Both enzymes are able to cleave a kininogen analog to release bradykinin. This proteolytic activity is inhibited by aprotinin and phenylmethanesulfonyl fluoride but not by ethylenediaminetetraacetate. The enzymes are active upon the kallikrein substrates S2666 and S2302. The Km values of the enzymes with these substrates are similar to those reported for kallikrein. Structural similarity between the two enzymes was demonstrated by ultraviolet and circular dichroic spectroscopy, and amino acid analysis. Tryptic peptide mapping of the two native enzymes also suggested a large degree of structural similarity. Furthermore, sequence studies on the NH2-terminal regions of the enzymes indicate that they share a significant degree of sequence homology with porcine kallikrein and crotalase, a kallikrein-like enzyme from Crotalus adamanteus. The main physical difference between the two kallikreins reported here appears to be due to the carbohydrate moieties on the enzymes. At present the in vivo role of venom kallikreins in envenomation pathology is uncertain; however, it is possible that they play an important part in giving rise to the initial symptoms of hypotension and shock.