Actions of 6-epitetrodotoxin and 11-deoxytetrodotoxin on the frog skeletal muscle fiber.
6-Epitetrodotoxin (6-epiTTX) and 11-deoxytetrodotoxin (11-deoxyTTX), isolated from an Okinawan newt, Cynops ensicauda, were tested for sodium-channel blocking effects on the voltage-clamped frog skeletal muscle fiber. In 6-epiTTX, the C-6 -OH is in an epimeric position; in 11-deoxyTTX, C-11 has a methyl in place of a hydroxymethyl group. At pH 7.25, the ED50s for reducing INa are: 4.1 nM (TTX), 96 nM (6-epiTTX), and 445 nM (11-deoxyTTX). In each analogue, the lowered potency can be attributed energetically to the loss of a hydrogen bond. By complementarity, in the sodium-channel receptor for TTX, there must be a hydrogen-acceptor group for the C-6 -OH, and another for the C-11 -OH. Therefore, the TTX molecule is bound to the receptor through an ion-pair (for the guanidinium), and five hydrogen bonds, one each for the -OH on C-9, C-10, C-4, and, as now identified, for C-6 and C-11. Considering the three-dimensional structure of the toxin molecules, these binding sites must be located in a fold or a crevice of the channel protein. If glutamate 387 of rat brain sodium channel II is the ion-pairing site for the guanidinium group, then the carbonyl oxygen of asparagine 388 is the hydrogen acceptor for the C-9 and C-10 -OHs.
Yang
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Department of Pharmacology, State University of New York, Brooklyn 11203.
Kao
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Yasumoto
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D000818:Animals; D066298:In Vitro Techniques; D015394:Molecular Structure; D009132:Muscles / Q000187:drug effects*; D015221:Potassium Channels / Q000187:drug effects; D011896:Rana temporaria; D015222:Sodium Channels / Q000187:drug effects*; D013779:Tetrodotoxin / Q000494:pharmacology*
D015221:Potassium Channels; D015222:Sodium Channels; C076431:6-epitetrodotoxin; C064101:11-deoxytetrodotoxin; D013779:Tetrodotoxin