Saxitoxin binding sites in frog-myocardial cytosol

Cytosolic fractions of frog heart homogenates contain large amounts of a soluble, large molecular weight protein that binds the specific neurotoxin saxitoxin with the same high affinity as does the plasma membrane. Another neurotoxin, tetrodotoxin, which ordinarily is competitive with saxitoxin, does not displace saxitoxin from the cytosolic sites or from plasma membrane-enriched vesicular fractions even when its concentration exceeds that of saxitoxin by a factor of 1000. Thus, cytosolic sites are similar to membrane sites in this respect. The vesicular fraction accounts quantitatively for the amount of saxitoxin bound by whole ventricles, so that no appreciable losses seem to occur. Therefore, the cytosolic site probably is a membrane site precursor, although other possibilities cannot be ruled out. In any case, the occurrence of a soluble molecule closely related to the sodium channel provides opportunities for further study of the structure of the sodium channel.