Preparations of the tetrodotoxin (TTX) and saxitoxin binding protein isolated from the electroplax of Electrophorus electricus are of high specific activity (≥2000 pmol of TTX binding sites/mg of protein) and appear to be homogeneous in that they contain only the large polypeptide previously identified to make up part of the voltage-sensitive sodium channel [Agnew, W. S., Moore, A. C., Levinson, S. R., & Raftery, M. S. (1980) Biochem. Biophys. Res. Commun. 92, 860–866]. This permits the inference that the TTX binding site, thought to be associated with the mouth of the ion channel, is located on this peptide. This peptide presumably corresponds to the large peptide, designated the α-peptide subunit, of the synaptosomal sodium channel [Hartshorne, R. P., & Catterall, W. A. (1981) Proc. Natl. Acad. Sci. US.A. 78, 4620–4624]. No convincing evidence for lower molecular weight peptides has yet been found for the electroplax protein. A rapid and convenient method is described for preparation of milligram quantities of the pure, sodium dodecyl sulfate (NaDodSO4) denatured form of the peptide, and its amino acid and carbohydrate compositions are reported. The peptide behaved anomalously on NaDodSO4-polyacrylamide gels. It was demonstrated that the molecular weight cannot be accurately quantified by this method but that the true value likely exceeds the value of 260 000 reported previously. The denatured peptide displayed an electrophoretic microheterogeneity which may be ascribed to variations in bulky carbohydrate substituents and an extremely high free mobility which is inferred to result from binding of unusually large amounts of NaDodSO4.
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