Role of the sulphate charge center in irreversible interactions of holothurin A with chemoreceptors

Abstract

Further studies with the crystalline neurotoxin holothurin A and the rat phrenic nerve-diaphragm preparation (PN-D) have shown that potency and degree of irreversibility of agonistic actions are sharply dependent on the presence of the charged sulfate residue in the toxin structure. With a selectively desulfated derivative (DeH) of the natural toxin (H) to establish relative indices of potency, it has been found that H is approximately one order of magnitude more powerful than DeH in direct contractural action on the muscle, and in blockade of the directly or indirectly elicited twitch response. But strikingly, the blocking actions of DeH are largely reversible on washing, in contrast with the clear irreversibility with which the parent toxin H destroys excitability in the preparation. Additionally, the uncharged species DeH at the 1–5 × 10 −5 M level is able to afford significant elements of protection against irreversible destruction of twitch response normally evoked in the PN-D by the charged anion H at a 1 × 10 −4 M level in the bathing medium. In the intact mouse studied for acute toxic response on i.V. administration of DeH or H, the syndromes leading to lethality are quite similar for the two agents, except for their relative time courses. Neutral DeH is characterized by an extensive lag time in onset of its peripheral and central effects and a very prolonged interval of tissue interactions, in contrast with the very much accelerated rate at which anionic H sweeps through the same spectrum of toxic signs. These findings are discussed in terms of the effects that anionic charge, imparted to H by virtue of its half-esterified sulfate function, may have on the strength and progress of interactions with receptor loci in neuromuscular and other tissues.