Action and binding of palytoxin, as studied with brain membranes

A revisited hemolytic assay for palytoxin detection: Limitations for its quantitation in mussels

Palytoxin (PLTX) and its analogues have been detected as seafood contaminants associated with a series of human foodborne poisonings. Due to a number of fatalities ascribed to the ingestion of PLTX-contaminated marine organisms, the development of methods for its detection in seafood has been recommended by the European Food Safety Authority (EFSA). Due to its feasibility, the spectrophotometric hemolytic assay is widely used to detect PLTX in different matrices, even though a standardized protocol is still lacking. Thus, on the basis of available assay procedures, a new standardized protocol was set up using purified human erythrocytes exposed to PLTX (working range: 3.9 × 10(-10)-2.5 × 10(-8) M) in a K(+)-free phosphate buffered saline solution, employing a 5 h incubation at 41 °C. An intra-laboratory characterization demonstrated its sensitivity (limit of detection, LOD = 1.4 × 10(-10) M and quantitation, LOQ = 3.4 × 10(-10) M), accuracy (bias = -0.8%), repeatability (RSDr = 15% and 6% for intra- and inter-day repeatability, respectively) and specificity. However, the standardized method seems not to be suitable for PLTX quantitation in complex matrices, such as mussel (Mytilus galloprovincialis) extracts, at least below the limit suggested by EFSA (30 μg PLTXs/Kg shellfish meat). Thus, the hemolytic assay for PLTX quantitation in seafood should be used only after a careful evaluation of the specific matrix effects.

Palytoxin action on the Na(+),K(+)-ATPase and the disruption of ion equilibria in biological systems

Palytoxin-group toxins (PlTX) exert their potent biological activity by altering mechanisms of ion homeostasis in excitable and non-excitable tissues. This review will describe major aspects that led to the relatively early identification of the Na(+),K(+)-ATPase as the molecular target and receptor of the toxin in sensitive systems. The importance of this pump in the normal functioning of animal cells has driven extensive investigative efforts. The recognized molecular mechanism of action of PlTX involves its binding to the extracellular portion of alpha subunit of this plasma membrane protein, which converts an enzyme carrying ions against their concentration gradients at the expense of chemical energy (ATP) into a non-selective cation channel, allowing passive flow of ions following their concentration gradients. More recent findings have indicated that PlTX would interfere with the normal strict coupling between inner and outer gates of the pump controlling the ion access to the Na(+),K(+)-ATPase, allowing the gates to be simultaneously open. The ability of PlTX to make internal portions of the Na(+),K(+)-ATPase accessible to relatively large molecules has been exploited to characterize the structure-function relationship of the pump, leading to a better understanding of its ion translocation pathway. Thus, forty years from the isolation of this potent marine biotoxin, a considerable understanding of its mode of action and of its potential as a research tool have been achieved and are the basis for promising future advancement in the characterization of biological systems and their alteration by PlTX.

Palytoxin: membrane mechanisms of action

Palytoxin is a marine toxin originally isolated from the zoantharians of the genus Palythoa, but now is found in marine organisms ranging from dinoflagellates to fishes. With a MW of 2680, it is one of the largest nonpolymeric natural products ever found. Its complex structure has been elucidated and total synthesis has been achieved. With an LD(50) of 25 ng/kg for rabbits (the most sensitive species), it is one of the most lethal marine toxins. It binds to the Na,K-ATPase specifically with a K(D) of 20 pM. It has a unique action on the Na,K-ATPase, converting the pump into an ion channel and resulting in K(+) efflux, Na(+) influx and membrane depolarization. As a result palytoxin causes a wide spectrum of secondary pharmacological actions. By acting like a key to unlock the internal structure of the Na,K-ATPase, palytoxin holds promise as a useful tool for investigation of the pump molecule.

Channel induction by palytoxin in yeast cells expressing Na+,K+-ATPase or its chimera with sarco/endoplasmic reticulum Ca2+-ATPase

Palytoxin (PTX) induces a cation channel through interaction with Na(+),K(+)-ATPase. It is unclear how this action relates to the enzyme catalytic activity. We examined whether the action of PTX depends on the catalytic domain specific for Na(+),K(+)-ATPase. Wild-type Na(+),K(+)-ATPase alpha-subunit (NNN) or its chimera (NCN), in which the catalytic domain was replaced with that of sarcoplasmic/endoplasmic reticulum Ca(2+)-ATPase, was co-expressed with beta-subunit in the yeast Saccharomyces cerevisiae. PTX (0.1-100 nM) increased K(+) efflux in NNN- or NCN-transfected cells to a similar degree but not in non-transfected cells. When ouabain-resistant NNN and NCN were expressed, PTX also increased K(+) efflux. Ouabain inhibited the effect of PTX in NNN or NCN cells but not in ouabain-resistant cells. These data suggest that the channel-forming action of PTX does not depend on the catalytic domain species.

Palytoxin modulates cytosolic pH in human osteoblast-like Saos-2 cells via an interaction with Na(+)-K(+)-ATPase

Palytoxin (PTx) at nanomolar concentrations enhances the permeability of mammalian cell membranes to both Na+ and Ca2+. In basal human osteoblast-like Saos-2 cells, PTx (8 nM) caused a persistent decrease in cytosolic pH (pHi) of about 0.2 units, which required the presence of extracellular Ca2+ (Cae2+) and Na+ (Nae+). We acidified Saos-2 cells by incubation with nigericin to examine the action of PTx in cells with an activated Na+/H+ antiporter. Under these conditions, PTx increased the pHi without requiring Cae2+ or Nae+, and the alkalinization was unaffected by hexamethylene amiloride. We conclude that the PTx-induced rise in pHi did not involve the Na+/H+ antiporter. PTx increased the rate of 86Rb+ efflux. We propose that PTx induced alkalinization in nigericin-acidified cells by collapsing the K+ gradient. Exposure to ouabain had no effect on pHi, but it prevented the actions of PTx on PHi in both basal and nigericin-acidified cells. Ouabain-resistant mutant cells were less sensitive to PTx in extruding 86Rb+ than their ouabain-sensitive parents. We conclude that PTx interacts with the Na(+)-K(+)-adenosinetriphosphatase to regulate pHi in both basal and nigericin-acidified Saos-2 cells.

Inhibition of sodium-dependent uptake processes in purified rat brain synaptosomes by Lophozozymus pictor toxin and palytoxin

To get an insight into the mechanism of neurotoxicity exhibited by Lophozozymus pictor toxin (LPTX) and the toxin isolated from P.caribaeorum (C-PTX) studies were carried out on the effect of these toxins on the uptake of selected substrates (neurotransmitters, amino acids and glucose) in isolated nerve endings. The toxins were found to inhibit the uptake of gamma-aminobutyric acid (GABA), noradrenaline, choline, L-leucine and 2-deoxy-D-glucose in rat brain synaptosomes. LPTX- or C-PTX-induced inhibition of synaptosomal uptake was reduced in the absence of Na+ in the assay medium. Synaptosomes exposed to LPTX and C-PTX release K+ in a dose-dependent manner. Ouabain, a selective inhibitor of the plasma membrane Na+, K(+)-ATPase could inhibit LPTX- and C-PTX-induced K+ efflux from synaptosomes and alleviate the toxin-induced inhibition of synaptosomal GABA uptake. It appears that the induction of ionic flux is the primary cause of toxicity by these toxins leading to the inhibition of Na(+)-dependent uptake processes in synaptosomes. The antagonistic action of ouabain suggests the involvement of the membrane sodium pump in the development of cytotoxicity.

Actions of palytoxin on Na+ and Ca2+ homeostasis in human osteoblast-like Saos-2 cells

Palytoxin (PTx) is a potent membrane-active agent produced by marine coelenterates that acts to stimulate bone resorption in organ culture at nanomolar concentrations. We report here the actions of PTx on Na+ and Ca2+ homeostasis in human osteoblast-like Saos-2 cells. PTx induced a rise in the cytosolic free Na+ concentration ([Na]i) by causing entry of extracellular Na+ (Na(e)+). PTx also caused a concentration-dependent biphasic rise in the cytosolic free Ca2+ concentration ([Ca2+]i) by enhancing entry of extracellular Ca2+ (Ca(e)2+). Entry of Na+ was dependent on the presence of Ca(e)2+ and was prevented by the Na+/Ca2+ exchange antagonist 3,4-dichlorobenzamil (DCB). Entry of Ca2+ was dependent on the presence of Na(e)+ but was not prevented by DCB. The actions of PTx on [Na+]i and [Ca2+]i were completely inhibited by pretreatment of the cells with ouabain. Ouabain alone had no acute effect on [Na+]i or [Ca2+]i in Saos-2 cells. We propose that interaction of PTx with the Na+ pump created a channel that allowed influx of Na(e)+ and Ca(e)2+. The rise in [Ca2+]i then stimulated the activity of the plasma membrane Na+/Ca2+ exchanger, which further enhanced Na(e)+ entry.

Role of cell membrane Na,K-ATPase for survival of human lymphocytes in vitro

Lymphocytes are primordial immune cells with variable life times. Besides genetic programming, extracellular factors interacting with cell surface receptors might alter cell survival. We investigated whether the activity of the membrane-embedded Na,K-ATPase (EC3.6.1.37) or sodium pump (NKA) plays a role for cell survival since this ubiquitous system establishes the vital transmembrane Na and K gradients as well as the resulting high intracellular K/Na ratio required for macromolecule synthesis; furthermore, the system exposes an extracellular inhibitory receptors for cardioactive steroids and palytoxin. Isolated human lymphocytes were incubated in vitro and their viability assessed by exclusion of trypan blue. Various incubation conditions were compared; in RPMI-1640 medium cell viability was preserved for 30 h at 37 degrees C. Externally added ouabain, a hydrophilic cardioactive steroid, blocked the [86Rb]potassium uptake at nanomolar concentrations. Despite pump inhibition ouabain did not alter lymphocyte survival, even at 10 mM for 30 h. By contrast, the hydrophilic toxin palytoxin, the most potent animal poison described so far, killed all cells within 2 h at 10 nM; this toxin is known to act via the sodium pump and to provoke deadly cation-leaks by unmasking a channel component. Intracellular Na increased and K decreased as measured by atomic absorption spectrometry in presence of palytoxin; cell swelling was seen by electron microscopy. Ouabain protected the cells from the toxic effect of palytoxin. The results reveal a pivotal role of NKA integrity for lymphocyte survival.

Biological properties of a crude venom extract from the greater weever fish Trachinus draco

Crude venom of the greater weever fish, Trachinus draco was analyzed to assess its toxicity, stability and biological properties. The best yield of venom was obtained by extraction in physiological saline of the whole venom apparatus of the fish which were shock-frozen and stored at -70 degrees C. This extract had a mouse i.v. minimum lethal dose of 1.8 micrograms protein per gram mouse and a total of 61,000 minimum lethal doses were obtained from venom apparatus of one fish. The lethal activity was unstable at room temperature especially at lower protein concentrations. Stability was achieved either by storing the extract at -70 degrees C or by precipitation with ammonium sulfate at 50% saturation. Toxicity of the crude venom was abolished by trypsin treatment. The crude venom did not possess any proteolytic or histamine-releasing activities. The venom caused an outflow of tetraphenylphosphonium from preloaded rat brain particles in a concentration-dependent manner. Like toxicity, this effect was also abolished by trypsin treatment or by keeping the venom at higher temperatures. The crude venom also possessed hemolytic activity with an EC50 for rabbit erythrocytes of 75 ng/ml venom protein. The hemolytic activity was also sensitive to heat and proteolytic treatment. Rabbit erythrocytes were most sensitive to venom followed by rat erythrocytes. Mouse and cattle erythrocytes were only slightly sensitive, whereas human, chicken and guinea pig erythrocytes were totally resistant.

Increase of permeability of synaptosomes and liposomes by the heavy chain of tetanus toxin

In search of a role for the heavy chain of tetanus toxin in poisoning, its actions on natural and artificial membranes have been assessed. The heavy chain increases the permeability of synaptosomes to lactate dehydrogenase and potassium ions, and promotes the outward shift of the lipophilic cation tetraphenylphosphonium which is a particularly sensitive indicator for depolarization. Independent of the assay system the potency of the heavy chain is high, i.e. in the range of about 1 nM, whereas its efficacy is low. Its potency is decreased by the addition of the light chain and by treatment of the synaptosomes with the C-terminal fragment C of the heavy chain, but not with its N-terminal fragment beta 2. Single- or two-chain toxin itself is inactive, and so are the light chain or the two heavy chain fragments beta 2 and C. Liposomes were made from phosphatidylcholine and phosphatidylserine or gangliosides and loaded with calcein. At pH 6 the outflow of calcein is promoted in the order heavy chain greater than toxin much greater than fragment beta 2, and the action of toxin is promoted by ganglioside. At pH 5, fragment beta 2 is nearly as active as the heavy chain and more potent than the toxin. The heavy chain, but neither of the fragments, is strongly adsorbed in hydrophobic interaction chromatography and caused aggregation of polystyrene-divinylbenzene beads. Evidence for polymerization of heavy chains is lacking in zonal centrifugation. It is concluded that both domains of the heavy chain co-operate to exert the membranal events described, and that the heavy chain is partially hidden by the light chain in the complete toxin molecule.

Palytoxin acidifies chick cardiac cells and activates the Na+/H+ antiporter

The cardiotoxic action of palytoxin was investigated using embryonic chick ventricular cells. Under normal ionic conditions, palytoxin produced an intracellular acidification which is partially compensated for by the Na+/H+ antiporter thereby leading to an increased rate of ethylisopropylamiloride-sensitive 22Na+ uptake. Under depolarizing membrane conditions, palytoxin produced a cellular acidification, a cellular alkalinization or no change in intracellular pH depending on the value of the extracellular pH. We propose that palytoxin acidifies cardiac cells by opening preexisting H+ conducting pathways in the plasma membrane.

Palytoxin promotes potassium outflow from erythrocytes, HeLa and bovine adrenomedullary cells through its interaction with Na+, K+ -ATPase

Erythrocytes from four mammalian species were compared with regard to K+ loss triggered by palytoxin, to Na+, K+ -ATPase activity, and to ouabain sensitivity of both events. Palytoxin sensitivity (EC50) decreased in the order rat, man (approximately equal to 1 pM) greater than cattle (approximately equal to 500 pM) greater than dog (greater than 10 nM). Na+, K+ -ATPase activity, as measured by Rb uptake, was in the series rat greater than man greater than cattle greater than dog. The glycoside potently inhibited both palytoxin action and ATPase activity in man, cattle and dog erythrocytes, but weakly in those from rats. Ca2+ promoted the palytoxin effects on all erythrocytes. As shown for human erythrocytes, Sr2+ and Ba2+ but not Mg2+ can substitute for Ca2+, and sucrose can substitute for sodium chloride. Human HeLa and bovine adrenomedullary cells also lost their K+ within a few min when exposed to palytoxin (1-10 pM). Ouabain acted as a palytoxin antagonist on both cell types. We conclude that: (a) the ouabain binding site of Na+, K+ -ATPase is part of the palytoxin receptor in every cell type tested, (b) high palytoxin sensitivity is not necessarily accompanied by high ouabain sensitivity, and (c) active ion transport is not a precondition for the action of palytoxin or for its inhibition by ouabain.

Palytoxin acts through Na+,K+-ATPase

Palytoxin is the most potent animal toxin, with a unique structure. The author's group has searched for its mode of action with the following results: 1. Palytoxin (1 pM and less) causes a fast K+ outflow from erythrocytes; 2. Extracellular Ca2+ and borate, and intracellular ATP enhance, but ouabain potently inhibits the palytoxin effects; 3. Palytoxin increases the permeability for Na+ and K+ but not for Ca2+; 4. Palytoxin in comparatively high concentrations (100 nM and above) inhibits Na+,K+-ATPase; 5. Palytoxin can be radiolabeled with 125I. Its receptor is very similar to, but not identical to that of ouabain. A reaction scheme has been delineated which allows an explanation to be obtained for all the known actions of palytoxin. It centers on the hypothesis that palytoxin binds to Na+,K+-ATPase and converts the enzyme or its close vicinity into an open channel with the permselectivity measured on erythrocytes. Patch clamp data from myocytes were obtained in other laboratories. They prove the presence of the predicted palytoxin channel.

Magnesium and methionine deprivation affect the response of rats to boron deprivation

A series of nine experiments were done to obtain further evidence that boron might be involved in major mineral metabolism (Ca, P, and Mg), thus indicating that boron is an essential nutrient for animals. Eight factorially arranged experiments of 6-10 wk durations were done with weanling Sprague-Dawley male rats. One factorially arranged experiment was done with weanling spontaneously hypertensive rats. The variables in each experiment were dietary boron supplements of 0 and 3 micrograms g, and dietary magnesium supplements of either 200 (Experiments 1-3) or 100 (Experiments 4-9) and 400 micrograms/g. In Experiments 7 and 9, a third variable was dietary manganese supplements of 25 and 50 micrograms/g. Methionine status was varied throughout the series of experiments by supplementing the casein-based diet with methionine and arginine. Findings were obtained indicating that the severity of magnesium deprivation and the methionine status of the rat strongly influence the extent and nature of the interaction between magnesium and boron, and the response to boron deprivation. When magnesium deprivation was severe enough to cause typical signs of deficiency, a significant interaction between boron and magnesium was found. Generally, the interaction was characterized by the deprivation of one of the elements making the deficiency signs of the other more marked. The interaction was most evident when the diet was not supplemented with methionine and especially when the diet contained luxuriant arginine. Signs of boron deprivation were also more marked and consistent when the diet contained marginal methionine and luxuriant arginine. Among the signs of boron deprivation exhibited by rats fed marginal methionine were depressed growth and bone magnesium concentration, and elevated spleen wt/body wt and kidney wt/body wt ratios. Because the boron supplement of 3 micrograms/g did not make the dietary intake of this element unusual, it seems likely that the response of the rats to dietary boron in the present study were manifestations of physiological, not pharmacological, actions, and support the hypothesis that boron is an essential nutrient for the rat.

Depolarization increases inositolphosphate production in a particulate preparation from rat brain

We have studied the accumulation of inositol phosphates (InsP) due to depolarization. A particulate preparation of rat brain was introduced to rule out transmitter activated mechanisms and to allow free access for drugs of high molecular weights. Potassium depolarization doubled InsP within a few minutes. InsP accumulation depended on time and K+ concentration, and was affected neither by tetrodotoxin nor by atropine. Radioactive metabolites co-eluted with inositol mono-phosphate and inositol bis-phosphate, whereas only minor amounts appeared with inositol tris-phosphate. The content in phosphatidylinositols was decreased. No evidence was found for the involvement of a neurotransmitter. Sea anemone toxin II (around 1 mumol/l), which keeps the Na+-channels open, promoted the InsP accumulation in an atropine-resistant manner. Tetrodotoxin prevented it when given before, and inhibited it when given after initiation by sea anemone toxin II. Moreover the K+ channel blockers 4-aminopyridine, dendrotoxin and tetraethylammonium all caused InsP accumulation. Palytoxin was by far the most potent promoter of InsP accumulation with a detection limit below 10 pmol/l, and displayed a unique bell-shaped concentration-effect correlation. Ouabain (3 mumol/l and above) also elicited the InsP accumulation. The response to carbachol was not only inhibited completely by atropine, but also partially (more than 50%) by tetrodotoxin, which indicates the involvement of voltage-dependent sodium channels in the receptor-triggered InsP accumulation. Thus independent of the causative agent, depolarization promotes an InsP accumulation. We conclude that degradation of phosphatidylinositols is mediated not only by receptor occupation but also by a positive shift in membrane voltage.

Palytoxin-induced permeability changes in excitable membranes

Palytoxin, a toxin isolated from the Caribean corrall Palythoa caribaeorum, increases the cation permeability of excitable membranes in vitro. Three membrane systems have been investigated: axonal membranes from crayfish walking leg nerves, membranes rich in nicotinic acetylcholine receptor isolated from Torpedo californica electric tissue and, for control, artificial liposomes. Ion permeability of the latter was not affected by palytoxin, but with both biological membranes an increase in cation permeability was observed at a palytoxin concentration of 0.14 microM. Palytoxin-induced cation flow through the axonal membrane was not inhibited by tetrodotoxin, indicating that the voltage-dependent sodium channels were not involved. The effect of palytoxin on the receptor-rich membranes was not blocked by alpha-bungarotoxin, a competitive antagonist of the nicotinic acetylcholine receptor, nor by triphenylmethylphosphonium, a blocker of the receptor-ion channel. But with both the axonal and the receptor-rich membranes ouabain was an inhibitor of the palytoxin-induced cation flow. Evidence is presented that it is not the (Na+ + K+)-ATPase which is affected by palytoxin as has been postulated for similar observations with non-neuronal membranes (Chhatwal, G.S., Hessler, H.-J. and Habermann, E. (1983) Naunyn-Schmiedeberg's Arch. Pharmacol. 323, 261-268).

Electrophysiological and neurobiochemical evidence for the blockade of a potassium channel by dendrotoxin

The effects of dendrotoxin (DTX), a toxic peptide from Dendroaspis angusticeps venom, were studied electrophysiologically on peripheral frog nerve fibres, and biochemically on large synaptosomes from rat brain. On nerve fibres, DTX reduced the amplitude and prolonged the duration of the action potential; even at 0.1 nmol/l DTX produced significant effects. Maximum block of potassium currents occurred at about 30 nmol/l. Turning on of the remaining current was slowed. Reversibility was incomplete. The reduction of potassium currents was between 31% and 85% at 85 nmol/l DTX (n = 8). The remainder appeared to be resistant to DTX. Sodium channels were not affected. On large synaptosomes DTX (above 1 nmol/l) produced a slight depolarization, indicated by an outward shift of the lipophilic cation tetraphenylphosphonium, and promoted the release of radioactivity after preloading with [3H] GABA. DTX had similar potency but lower efficacy in this respect than sea anemone toxin II (ATX II). In contrast to the effects of ATX II, those due to DTX were only partially inhibited by tetrodotoxin. The actions of 4-aminopyridine resembled those of DTX, but the latter was about 500 times more potent. The electrophysiological data provide direct evidence for blockade of a potassium channel by DTX. This action is sufficient to explain the biochemical observations, although additional effects on synaptosomes cannot be excluded.

Characterization of depolarization induced by palytoxin and grayanotoxin-I in isolated cardiac tissues from dogs and guinea pigs

The mode of depolarizing action of palytoxin (PTX) was compared with that of grayanotoxin-I (GTX-I) to examine the site of action of PTX in cardiac tissues isolated from dogs and guinea pigs: PTX at above 1 X 10(-10) mol/l depolarized the membrane of canine Purkinje fibres, canine and guinea-pig ventricular muscles regardless of stimulation. The PTX-induced depolarization was resistant to 1 X 10(-5) mol/l tetrodotoxin (TTX) but was attenuated by low Na medium. GTX-I at 1 X 10(-5) mol/l depolarized the membrane of ventricular muscles from dogs and guinea pigs only when they were stimulated. Although GTX-I caused depolarization in Purkinje fibres in both stimulated and rested conditions, the effect was greater in stimulated fibres. In the presence of GTX-I, ventricular muscles generated long-lasting action potentials as a response to stimuli. TTX antagonized the GTX-I-action. After treatment of PTX, GTX-I still induced the long-lasting action potential and TTX could antagonize it. Ouabain at 1 X 10(-6) or 3 X 10(-6) mol/l partially inhibited the PTX-induced depolarization in guinea-pig papillary muscles. The data suggest that the TTX-sensitive Na channel is intact after treatment with PTX and that PTX acts on a site different from the Na channel, possibly on the Na+, K+-ATPase in cardiac cells.

Inhibitory effect of calmodulin inhibitors on palytoxin-induced K+ release from rabbit erythrocytes

Palytoxin (PTX) caused K+ release from rabbit erythrocytes at a concentration as low as 10(-11) M. The K+ release due to PTX at a concentration below 10(-9) M was dependent on Ca2+ in medium. The effect of Ca2+ was substituted fully by Sr2+ and partially by Ba2+. W-7 (2 X 10(-4) M), a known inhibitor of calmodulin, markedly inhibited the rate of K+ release due to PTX. W-5 (2 X 10(-4) M), an analog of W-7 with lower affinity to calmodulin than W-7, showed weaker inhibition. Other calmodulin antagonists, such as prenylamine, chlorpromazine and compound 48/80, also inhibited the PTX-induced K+ release. These results suggest that the K+ release induced by PTX involves the process(es) mediated by intracellular Ca2+ and calmodulin.

Palytoxin binds to and inhibits kidney and erythrocyte Na+, K+-ATPase

Hog kidney Na+, K+-ATPase, purified to the microsomal stage and activated with detergent, binds palytoxin, as shown by the nearly complete competition of the toxin with 3H-ouabain. The Ki-values of palytoxin, but not of ouabain, depend on the protein concentration; this indicates additional binding sites for the toxin on kidney membranes. - Palytoxin inhibits the enzymatic activity of the detergent-activated preparation nearly completely (IC50 8 X 10(-7) mol/l). Inhibition of ATPase activity and of ouabain binding are promoted by borate, a known activator of palytoxin. - Palytoxin also inhibits the Na+,K+-ATPase of erythrocyte ghosts in the same dose range. The data are discussed in context with the hypothesis (Chhatwal et al. 1983) that palytoxin raises the cellular permeability by altering the state of Na+,K+-ATPase or its environment.