Size characteristics of the solubilized sodium channel saxitoxin binding site from mammalian sarcolemma

Pharmacological and biochemical properties of saxiphilin, a soluble saxitoxin-binding protein from the bullfrog (Rana catesbeiana)

Supernatant fractions of various tissues and plasma from the North American bullfrog, Rana catesbeiana, specifically bind saxitoxin with high affinity. Binding of [3H]saxitoxin to bullfrog plasma follows single-site behavior with an equilibrium dissociation constant of Kd = 0.16 +/- 0.03 nM at 0 degrees C and a maximum binding capacity of 380 +/- 60 pmole/ml plasma. High-affinity binding of [3H]saxitoxin is chemically specific since it is unaffected by tetrodotoxin and a variety of cationic peptides, amino acids and drugs. The structure-activity dependence of binding to this site was investigated with eight different natural and synthetic derivatives of saxitoxin. Substitution of the carbamoyl side chain or the C-12 beta-hydroxyl group of saxitoxin with a hydrogen atom had little effect on binding affinity, but addition of a hydroxyl group at the N-1 position decreased the binding affinity from 430- to 710-fold in three different molecular pairs. High performance size exclusion chromatography of supernatant from bullfrog skeletal muscle showed that the [3H]saxitoxin-binding component migrates with an apparent molecular weight of Mr = 74,000 +/- 8000 or a Stokes radius of 35 +/- 2A. The [3H]saxitoxin-binding protein in skeletal muscle extract or plasma is retained on a cation-exchange column at pH 6.0, suggesting that the protein contains a region of exposed basic residues. Column isoelectric focusing of a sample from plasma indicated that the protein has a basic isoelectric point near pH = 10.7.(ABSTRACT TRUNCATED AT 250 WORDS)

Functional reconstitution of the purified sodium channel protein from rat sarcolemma

The purified saxitoxin (STX) binding component of the rat sarcolemmal sodium channel (SBC) has been reconstituted into phospholipid vesicles. The reconstituted SBC displays the pharmacological properties and the ability to control sodium fluxes expected of a functional sodium channel. Batrachotoxin (BTX) increases 22Na+ influx into reconstituted SBC vesicles by greater than 100% over control at early time points. The BTX-stimulated 22Na+ influx is specifically and quantitatively blocked by STX. Veratridine and aconitine also stimulate Na+-flux--although less effectively than BTX--in the order: BTX greater than veratridine greater than aconitine. The logarithmic dose--response curves for BTX and veratridine are sigmoidal with a K0.5 of 1.5 microM and 35 microM, respectively. Vesicles containing the reconstituted SBC demonstrate 3H-labeled STX binding to a single class of high affinity sites witha Kd of 5--7 nM at 0 degrees C; the thermal stability of the STX receptor is markedly enhanced by reconstitution. Our results confirm that the purified STX binding component from rat sarcolemma constitutes the sodium channel itself and contains at least those components sufficient for channel activation, transmembrane ion movement, and inhibition by STX.

Molecular Properties of neurotoxin receptors sites associated with sodium channels from mammalian brain

Neurotoxins that act at specific receptor sites on voltage-sensitive sodium channels have been used as molecular probes to identify and purify protein components of sodium channels from mammalian brain. Photoreactive derivatives of scorpion toxin have been prepared and used to covalently label sodium channels in intact synaptosomes. Two polypeptides, alpha with Mr approximately 270,000 and beta with Mr approximately 38,000, are specifically labeled indicating that they are components of the scorpion toxin receptor site on the sodium channel. The sodium channel can be solubilized with retention of specific binding of [3H] saxitoxin using nonionic detergents such as Triton X-100. The solubilized saxitoxin receptor has molecular weight of 316,000 +/- 63,000 and binds 0.9 g of Triton X-100 and phospholipid per g of protein. The solubilized receptor can be purified 750-fold by ion exchange chromatography, wheat germ lectin/Sepharose chromatography and sucrose gradient sedimentation to a final specific activity of 1488 pmol/mg. Analysis of the polypeptide chain composition of the most highly purified fractions indicates that alpha and beta comprise 65% of the protein of these fractions and are only the polypeptides whose presence correlates with saxitoxin binding activity. These studies lead to a working hypothesis of sodium channel structure in which the intact channel is comprised of a complex with Mr of approximately 316,000 containing one mole of alpha (Mr approximately 270,000) and one to three moles of beta (Mr approximately 38,000).

Modification of the tetrodotoxin receptor in Electrophorus electricus by phospholipase A2

The effects of phospholipase A2 treatment on the tetrodotoxin receptors in Electrophorus electricus was studied. (1) The binding of [3H]tetrodotoxin to electroplaque membranes was substantially reduced by treatment of the membranes with low concentrations of phospholipase A2 from a number of sources, including bee venom, Vipera russelli and Crotalus adamanteus and by beta-bungarotoxin. (2) Phospholipase A2 from bee venom and from C. adamanteus both caused extensive hydrolysis of electroplaque membrane phospholipids although the substrate specificity differed. Analysis of the phospholipid classes hydrolyzed revealed a striking correlation between loss of toxin binding and hydrolysis of phosphatidylethanolamine but not of phosphatidylserine. (3) The loss of toxin binding could be partially reversed by treatment of the membranes with bovine serum albumin, conditions which are known to remove hydrolysis products from the membrane. (4) Equilibrium binding studies on the effects of phospholipase A2 treatment of [3H]tetrodotoxin binding showed that the reduction reflected loss of binding sites and not a change in affinity. (5) These results are interpreted in terms of multiple equilibrium states of the tetrodotoxin-receptors with conformations determined by the phospholipid environment.

Glycoprotein characteristics of the sodium channel saxitoxin-binding component from mammalian sarcolemma

The saxitoxin-binding component of the excitable membrane sodium channel exhibits glycoprotein characteristics as evidenced by its specific interaction with various agarose-immobilized lectins. The detergent-solubilized saxitoxin-binding component interacts quantitatively with immobilized wheat germ agglutinin and concanavalin A and fractionally with immobilized Lens culinaris hemagglutinin and Ricinus communis agglutinin. These lectins preferentially bind N-acetylglucosamine and sialic acid (wheat germ agglutinin), mannose (concanavalin A and Lens cunilaris) and galactose (Ricinus communis). Removal of terminal sialic acid residues by neuraminidase markedly decreases binding to immobilized wheat germ agglutinin but uncovers sites capable of interacting with lectins specific for galactose and N-acetylgalactosamine. beta-N-acetylglucosaminidase, an exoglycosidase, has no effect on the binding of the channel protein to wheat germ agglutinin. Similarly, phospholipase C has no effect on binding of the solubilized toxin binding component to this lectin. Neither wheat germ agglutinin nor concanavalin A free in solution alters the number of toxin binding sites or their affinity for toxin. The sodium channel saxitoxin-binding component to wheat germ agglutinin. Similarly, phospholipase C has no effect on binding of the solubilized toxin binding component to this lectin. Neither wheat germ agglutinin nor concanavalin A free in solution alters the number of toxin binding sites or their affinity for toxin. The sodium channel saxitoxin-binding component to wheat germ agglutinin. Similarly, phospholipase C has no effect on binding of the solubilized toxin binding component to this lectin. Neither wheat germ agglutinin nor concanavalin A free in solution alters the number of toxin binding sites or their affinity for toxin. The sodium channel saxitoxin-binding component appears to be a glycoprotein containing terminal sialic acid residues and internal mannose, galactose, N-acetylglucosamine, and N-acetylgalactosamine residues. The toxin binding site is spatially separated from the binding sites for the lectins studied. The effect of these sugar moieties must be considered when evaluating the biophysical parameters of the sodium channel.

Estimate of the molecular weight of the sarcolemmal sodium channel using H2O-D2O centrifugation

The sodium channel saxitoxin binding component from rat sarcolemma was solubilized with NP-40 and centrifuged on sucrose gradients constructed in either D2O or H2O. When compared with a series of standard proteins the sedimentation behavior of the solubilized channel complex changed from an apparent S20,w of 9.1 in H2O to 6.1 in D2O. From these observations, a true partial specific volume of 0.83 ml/g was calculated for the complex. A Stokes radius of 8.6 nm was estimated from Sepharose 6-B chromatography in NP-40. The calculated protein molecular weight of the lipid-protein-detergent complex based on these data is 560,000. The complex contains about 56% protein, and the calculated molecular weight of this component is 314,000 if a v for the protein of 0.74 ml/g is assumed.