Actions of chiriquitoxin on frog skeletal muscle fibers and implications for the tetrodotoxin/saxitoxin receptor.

Abstract

Chiriquitoxin (CqTX) from the Costa Rican frog Atelopus chiriquensis differs from tetrodotoxin (TYX) only in that a glycine residue replaces a methylene hydrogen of the C-11 hydroxymethyl function. On the voltage-clamped frog skeletal muscle fiber, in addition to blocking the sodium channel and unrelated to such an action, CqTX also slows the activation of the fast potassium current in ~ 40% of the muscle fiber population. At pH 7.25, CqTX is as potent as TI'X in blocking the sodium channel, with an ED~0 of 3.8 nM. Its ED50's at pH 6.50 and 8.25 are 6.8 and 2.3 nM, contrasted with 3.8 and 4.3 nM for "ITX. These differences are attributable to changes in the chemical states in the glycine residue. The equipotency of CqTX with TIX at pH 7.25 is explainable by an intramolecular salt bridge between the amino and carboxyl groups of the glycine function, all other surface groups in TI'X and CqTX being the same. From available information on these groups and those in saxitoxin (STX), the TI'X/STX binding site is deduced to be in a pocket 9.5/~ wide, 6/~ high, and 5 ~ deep. The glycine residue of CqTX probably projects out of the entrance to this pocket. Such a view of the binding site could also account for the actions of STX analogues, including the C-11 sulfated gonyautoxins and the 21-sulfocarbamoyl analogues. In the gonyautoxins the sulfate groups are equivalently placed as the glycine in CqTX, whereas in the sulfocarbamoyl toxins the sulfate groups extend the carbamoyl side-chain, leading to steric hinderance to productive binding.

Differential binding of tetrodotoxin and its derivatives to voltage‐sensitive sodium channel subtypes (Nav1.1 to Nav1.7)

“…CHTX (2) is a TTX analogue with a glycine functionality connected to C11 (Kao et al, 1981;Yotsu et al, 1990) that is known to inhibit both voltage-sensitive potassium channels and sodium channels (Kao et al, 1981;Yang and Kao, 1992). Our study presents the first investigation of CHTX (2) subtype selectivity.…”

Differential effects of sulfhydryl reagents on saxitoxin and tetrodotoxin block of voltage-dependent Na channels

“…Instead, we suggest an alternative explanation. Because of the multiple determinants involved in binding (Yang and Kao, 1992) and the fact that the STX/ITX association rate is -1000 times slower than free diffusion, the rate-limiting step for toxin-receptor association may be a slow transition to a stable toxin-receptor complex that occurs after the initial toxin-receptor collision (Guo et al, 1987). A specific, two-step model in which the initial collision is followed by a conformational change in the TTX receptor has been proposed previously (Green et al, 1987) and may be useful in explaining our results.…”

Tetrodotoxin: Chemistry, Toxicity, Source, Distribution and Detection

“…[35], crabs: Demania cultripes , Demania toxica , D. reynaudi , Lophozozymus incisus , L. Pictor and A. germaini [62,139,170] and in frogs and toads: Atelopus [70,143] and Brachycephalus [33,34,72]. …”

“…Besides puffer fish, other species known to harbour TTX include: gastropods [4,5,7,8,10,12,13,25,28,40,41,42,43,44,45,46,47,48], newts [31,32,35,36,37,49,50,51,52,53,54,55,56,57,58,59,60], crabs [61,62,63,64,65,66,67,68,69], frogs [30,33,34,70,71,72], sea slugs [73,74], star fishes [75], blue-ringed octopuses [76,77,78], ribbon worms [22,79] and bacteria [74,80,81,82,83,84,85,86,87,88]. The distribution of TTX and its analogues are known to be organism and/or tissue specific […”