Characterization, solubilization, affinity labeling and purification of the cardiac Na+ channel using Tityus toxin gamma

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Abstract
Saturable, high-affinity binding of iodinated toxin .gamma. from T. serrulatus scorpion venom (TiTx.gamma.) to Na+ channel receptor was identified in sarcolemma membrane of chick heart. A binding capacity of 450-600 fmol/mg of protein was found similar to that of tetrodotoxin-binding component. The enrichment of these membrane-bound toxin binding sites follows that of other sarcolemma markers. Kinetic data and displacement of 125I-TiTx.gamma. from its binding sites by unlabeled TiTx.gamma. gave an equilibrium dissociation constant (Kd) of 1-3 pM. The gating component and the selectivity filter of the voltage-sensitive Na+ channel, identified as binding sites of TiTx.gamma. and of tetrodotoxin, respectively, have been efficiently solubilized with Nonidet P-40. Purification was achieved by ion-exchange chromatography on DEAE-Sephadex A-25, affinity chromatography on wheat-germ-agglutinin-Sepharose and sucrose density gradient centrifugation. An enrichment of 1400-fold from the original detergent extract was measured for both toxin binding sites (1120-1230 pmol/mg of protein). Sodium dodecyl sulfate gel electrophoresis reveals a single large polypeptide component of MW 230,000-270,000. The purified material exhibits an apparent sedimentation coefficient of 8.8 S. Covalent cross-linking of 125I-TiTx.gamma. to its membrane-embedded cardiac receptor shows that the cross-linked material, solubilized and purified by the same procedure comprises a single polypeptide chain of the same MW of 230,000-270,000. Furthermore, as seen for Electrophorous electricus electroplax and rat brain, the tetrodotoxin-binding component and the TiTx.gamma.-binding component are carried by the same polypeptide chain. The functional Na+ channel might be an oligomer of this subunit of MW 230,000-270,000.