Biochemical Characterization of the Tetrodotoxin Binding Protein from Electrophorus electricus

Biochemical properties of a detergent-solubilized tetrodotoxin binding component from Electrophorus electricus have been examined and compared with those found for the membrane-bound protein. The toxin binding component was solubilized with high efficiency by a variety of nonionic detergents and with lower efficiency by sodium cholate and deoxycholate. Detergent-solubilized preparations bound tetrodotoxin and saxitoxin tightly and specifically, and this binding was observed to be rapidly and irreversibly blocked by carboxylate-modifying reagents. Inactivation by carbodiimide and glycine ester or by a trimethyloxonium salt could be prevented by tetrodotoxin occupancy of the binding site. Tetrodotoxin binding activity in both solubilized preparations and in membranes was found to be highly resistant to proteases. In contrast, the activity was extremely sensitive to the action of phospholipase A₂. The biochemical properties of the tetrodotoxin binding component solubilized in mixed lipiddetergent micelles are similar to those found in native membranes, with respect to the characteristics of equilibrium toxin binding and to the sensitivity of toxin binding activity to chemical modification and degradative enzymes. There were some differences with respect to the kinetics of tetrodotoxin binding. In addition, the tetrodotoxin binding component from eel is shown to behave as a glycoprotein, being selectively absorbed to resins coupled to concanavalin A, wheat germ agglutinin, Lens culinaris lectin, and ricin with the appropriate glycoside.