C-terminus loop 13 of Na+ glucose cotransporter SGLT1 contains a binding site for alkyl glucosides

Recently, we identified the extramembranous C-terminus loop 13 of SGLT1 as a binding site for the aromatic glucoside phlorizin, which competitively inhibits sodium D-glucose cotransport. Alkyl glucosides are also competitive inhibitors of the transport. Therefore, in this study, we searched for potential binding sites for alkyl glucosides in loop 13. To this end, we synthesized a photoaffinity label (2'-Azi-n-octyl)-beta-D-glucoside and analyzed the region of attachment using MALDI mass spectrometry, producing wild-type recombinant truncated loop 13. Furthermore, we prepared four single-Trp mutants of the loop and determined their fluorescence, its change in the presence of alkyl glucosides, and their accessibility to acrylamide. Photolabeling of truncated loop 13 with (2'-Azi-n-octyl)-beta-D-glucoside revealed an attachment of the C2 group of the alkyl chain to Gly-Phe-Phe-Arg (amino acid residues 598-601). In the presence of n-hexyl-beta-D-glucoside, all mutants (R601W, D611W, E621W, and L630W) exhibited a significant decrease in Trp fluorescence with an apparent binding affinity of 8-14 microM. Only L630W exhibited a significant blue shift, and only in R601W was a change in acrylamide quenching (protection) observed. No quenching or protection was found for D-glucose; however, 1-hexanol produced the same results as n-hexyl-beta-D-glucoside. The interaction shows stereoselectivity for n-hexyl-beta-D-glucoside binding; the beta-configuration of the sugar moiety at C1, the cis conformation of the unsaturated alkenyl side chain in the C3-C4 bond, and the alkyl chain length of six to eight carbon atoms lead to an optimum interaction. A schematic two-dimensional model was derived in which C2 interacts with the region around residue 601, C3 and C4 interact with the region between residues 614 and 619, and C6-C8 interact with the region between residues 621 and 630. The data demonstrate that loop 13 provides binding sites for alkyl glucosides as well as for phlorizin; thus, loop 13 of SGLT1 seems to be a major binding domain for the aglucone residues of competitive D-glucose transport inhibitors.     

Interaction of fatty acids with lipid bilayers

We present a method by which it is possible to describe the binding of fatty acids to phospholipid bilayers. Binding constants for oleic acid and a number of fatty acids used as spectroscopic probes are deduced from electrophoresis measurements. There is a large shift in $\text{p}\text{K}$ value for the fatty acids on binding to the phospholipid bilayers, consistent with stronger binding of the uncharged form of the fatty acid. For dansylundecanoic acid, fluorescence titrations are consistent with the binding constants derived from the electrophoresis experiments. For 12-(9-anthroyloxy)stearic acid, fluorescence and electrophoresis data are inconsistent, and we attribute this to quenching of fluorescence at high molar ratios of 12-anthroylstearic acid to phospholipid in the bilayer.     

Interaction of protegrin-1 with lipid bilayers: membrane thinning effect

Protegrins (PG) are important in defending host tissues, preventing infection via an attack on the membrane surface of invading microorganisms. Protegrins have powerful antibiotic abilities, but the molecular-level mechanisms underlying the interactions of their beta-sheet motifs with the membrane are not known. Protegrin-1 (PG-1) is composed of 18 amino acids with a high content of basic residues and two disulfide bonds. Here we focused on the stability of PG-1 at the amphipathic interface in lipid bilayers and on the details of the peptide-membrane interactions. We simulated all-atom models of the PG-1 monomer with explicit water and lipid bilayers composed of both homogeneous POPC (palmitoyl-oleyl-phosphatidylcholine) lipids and a mixture of POPC/POPG (palmitoyl-oleyl-phosphatidylglycerol) (4:1) lipids. We observed that local thinning of the lipid bilayers mediated by the peptide is enhanced in the lipid bilayer containing POPG, consistent with experimental results of selective membrane targeting. The beta-hairpin motif of PG-1 is conserved in both lipid settings, whereas it is highly bent in aqueous solution. The conformational dynamics of PG-1, especially the highly charged beta-hairpin turn region, are found to be mostly responsible for disturbing the membrane. Even though the eventual membrane disruption requires PG-1 oligomers, our simulations clearly show the first step of the monomeric effects. The thinning effects in the bilayer should relate to pore/channel formation in the lipid bilayer and thus be responsible for further defects in the membrane caused by oligomer.     

A proteomic study of sodium/D-glucose cotransporter 1 (SGLT1): topology of loop 13 and coverage of other functionally important domains

In order to obtain further information about the structure and function of human sodium/D-glucose cotransporter 1 (hSGLT1), the recombinant protein was subjected, either after reconstitution into liposomes or in its free form, to proteolysis followed by nanoscale microcapillary liquid chromatography electrospray ionization tandem mass spectrometry (LC-MS/MS). The peptides released from SGLT1 proteoliposomes by trypsin bead digestion represented the early N-terminal, loop 7, and loop 9, supporting topology models that place these domains on the extracellular side of the protein. Trypsin bead digestion generated, however, also a number of peptides derived from loop 13 whose topology with regard to the membrane is hitherto a point of debate. Sequence coverage was provided from amino acids 559 to 644, suggesting that loop 13 is almost completely accessible at the extravesicular face of the proteoliposomes. These results support the notion that major parts of loop 13, essential for the interaction with transport inhibitors in vivo, are located extracellularly in intact cells. In-gel trypsin, chymotrypsin, and in particular trypsin/chymotrypsin digestion of recombinant SGLT1 in combination with LC-MS/MS provide extensive sequence coverage of the protein, including domains involved in sugar and inhibitor binding and potential phosphorylation sites. These studies demonstrate that proteomic analysis combined with mass spectrometry is a useful tool to characterize regions of SGLT1 that are important for its function and regulation.     

Interaction of small peptides with lipid bilayers.

Molecular dynamics simulations of the tripeptide Ala-Phe-Ala-O-tert-butyl interacting with dimyristoylphosphatidylcholine lipid bilayers have been carried out. The lipid and aqueous environments of the peptide, the alkyl chain order, and the lipid and peptide dynamics have been investigated with use of density profiles, radial distribution functions, alkyl chain order parameter profiles, and time correlation functions. It appears that the alkyl chain region accommodates the peptides in the bilayer with minimal perturbation to this region. The peptide dynamics in the bilayer bound form has been compared with that of the free peptide in water. The peptide structure does not vary on the simulation time scale (of the order of hundreds of picoseconds) compared with the solution structure in which a random structure is observed.     

Interaction of GAPR-1 with lipid bilayers is regulated by alternative homodimerization

Golgi-Associated Plant Pathogenesis-Related protein 1 (GAPR-1) is a mammalian protein that belongs to the superfamily of plant pathogenesis related proteins group 1 (PR-1). GAPR-1 is a peripheral membrane-binding protein that strongly associates with lipid-enriched microdomains at the cytosolic leaflet of Golgi membranes. Little is known about the mechanism of GAPR-1 interaction with membranes. We previously suggested that dimerization plays a role in the function of GAPR-1 and here we report that phytic acid (inositol hexakisphosphate) induces dimerization of GAPR-1 in solution. Elucidation of the crystal structure of GAPR-1 in the presence of phytic acid revealed that the GAPR-1 dimer differs from the previously published GAPR-1 dimer structure. In this structure, one of the monomeric subunits of the crystallographic dimer is rotated by 28.5°. To study the GAPR-1 dimerization properties, we investigated the interaction with liposomes in a light scattering assay and by flow cytometry. In the presence of negatively charged lipids, GAPR-1 caused a rapid and stable tethering of liposomes. [D81K]GAPR-1, a mutant predicted to stabilize the IP6-induced dimer conformation, also caused tethering of liposomes. [A68K]GAPR-1 however, a mutant predicted to stabilize the non-rotated dimer conformation, is capable of binding to liposomes but did not cause liposome tethering. Our combined data suggest that the charge properties of the lipid bilayer can regulate GAPR-1 dynamics as a potential mechanism to modulate GAPR-1 function.     

Membrane topology of loop 13-14 of the Na/glucose cotransporter (SGLT1): A SCAM and fluorescent labelling study

The accessibility of the hydrophilic loop between putative transmembrane segments XIII and XIV of the Na⁺/glucose cotransporter (SGLT1) was studied in Xenopus oocytes, using the substituted cysteine accessibility method (SCAM) and fluorescent labelling. Fifteen cysteine mutants between positions 565 and 664 yielded cotransport currents of similar amplitude than the wild-type SGLT1 (wtSGLT1). Extracellular, membrane-impermeant MTSES⁽⁻⁾ and MTSET⁽⁺⁾ had no effect on either cotransport or Na⁺ leak currents of wtSGLT1 but 9 mutants were affected by MTSES and/or MTSET. We also performed fluorescent labelling on SGLT1 mutants, using tetramethylrhodamine-5-maleimide and showed that positions 586, 588 and 624 were accessible. As amino acids 604 to 610 in SGLT1 have been proposed to form part of a phlorizin (Pz) binding site, we measured the K_i^Pz and K_m^αMG for wtSGLT1 and for cysteine mutants at positions 588, 605-608 and 625. Although mutants A605C, Y606C and D607C had slightly higher K_i^Pz values than wtSGLT1 with minimal changes in K_m^αMG, the effects were modest and do not support the original hypothesis. We conclude that the large, hydrophilic loop near the carboxyl terminus of SGLT1 is thus accessible to the external solution but does not appear to play a major part in the binding of phlorizin.     

Membrane topology of loop 13-14 of the Na+/glucose cotransporter (SGLT1): a SCAM and fluorescent labelling study

The accessibility of the hydrophilic loop between putative transmembrane segments XIII and XIV of the Na+/glucose cotransporter (SGLT1) was studied in Xenopus oocytes, using the substituted cysteine accessibility method (SCAM) and fluorescent labelling. Fifteen cysteine mutants between positions 565 and 664 yielded cotransport currents of similar amplitude than the wild-type SGLT1 (wtSGLT1). Extracellular, membrane-impermeant MTSES(-) and MTSET(+) had no effect on either cotransport or Na+ leak currents of wtSGLT1 but 9 mutants were affected by MTSES and/or MTSET. We also performed fluorescent labelling on SGLT1 mutants, using tetramethylrhodamine-5-maleimide and showed that positions 586, 588 and 624 were accessible. As amino acids 604 to 610 in SGLT1 have been proposed to form part of a phlorizin (Pz) binding site, we measured the K(i)(Pz) and K(m)(alphaMG) for wtSGLT1 and for cysteine mutants at positions 588, 605-608 and 625. Although mutants A605C, Y606C and D607C had slightly higher K(i)(Pz) values than wtSGLT1 with minimal changes in K(m)((alpha)MG), the effects were modest and do not support the original hypothesis. We conclude that the large, hydrophilic loop near the carboxyl terminus of SGLT1 is thus accessible to the external solution but does not appear to play a major part in the binding of phlorizin.     

Interaction of nonionic PEO-PPO diblock copolymers with lipid bilayers

The relationship between the molecular architecture of a series of poly(ethylene oxide)-b-poly(propylene oxide) (PEO-PPO) diblock copolymers and the nature of their interactions with lipid bilayers has been studied using small- and wide-angle X-ray scattering (SAXS and WAXS) and differential scanning calorimetry (DSC). The number of molecular repeat units in the hydrophobic PPO block has been found to be a critical determinant of the nature of diblock copolymer-lipid bilayer association. For dimyristoyl-sn-glycero-3-phosphocholine (DMPC)-based biomembrane structures, polymers whose PPO chain length approximates that of the acyl chains of the lipid bilayer yield highly ordered, expanded lamellar structures consistent with well-integrated (into the lipid bilayer) PPO blocks. Shorter diblock copolymers produce mixed lamellar and nonlamellar mesophases. The thermotropic phase behavior of the polymer-doped membrane systems is highly influenced by the presence and molecular architecture of the diblock copolymer, as evidenced by shifting of the main phase transition to higher temperatures, broadening of the main transition, and the appearance of other features. Studies of temperature-induced changes in the mesophase structure for compositions prepared with well-integrated PEO-PPO polymers indicate that they undergo reversible changes to a nonlamellar structure as the temperature is lowered. Increasing either the number of repeat units in the PEO block or the polymer concentration promotes a greater degree of structural ordering.     

Assessing the risk of ketoacidosis due to sodium-glucose cotransporter (SGLT)-2 inhibitors in patients with type 1 diabetes: A meta-analysis and meta-regression

BackgroundSodium-glucose cotransporter-2 (SGLT2) inhibitors (SGLT2i) showed benefits in type 1 diabetes mellitus (T1DM), but the risk of diabetic ketoacidosis (DKA) limits their use. Ability to predict DKA risk and therapeutic responses would enable appropriate patient selection for SGLT2i. We conducted a meta-analysis and meta-regression of randomized controlled trials (RCTs) evaluating SGLT2i in T1DM to assess moderators of the relative risk (RR) of DKA, of glycemic (HbA1c, fasting plasma glucose, continuous glucose monitoring parameters, insulin dose, and insulin sensitivity indices) and non-glycemic (body mass index (BMI), systolic BP, renal function, albuminuria, and diabetic eye disorders) efficacy, and of other safety outcomes (including hypoglycemia, infections, major adverse cardiovascular events, and death).Methods and findingsWe searched MEDLINE, Cochrane Library, EMBASE, ClinicalTrials.gov, Cochrane CENTRAL Register of Controlled Trials, and other electronic sources through August 30, 2020, for RCTs comparing SGLT2i with active comparators or placebo in adult patients with T1DM. Reviewers extracted data for relevant outcomes, performed random effects meta-analyses, subgroup analyses, and multivariable meta-regression. The strength of evidence was summarized with the GRADE approach. Among 9,914 records identified, 18 placebo-controlled RCTs (7,396 participants, 50% males, mean age 42 y (range 23 to 55 y), 5 different SGLT2i evaluated), were included. Main outcome measures were effect sizes and moderators of glycemic and non-glycemic efficacy and of safety outcomes. In a multivariable meta-regression model, baseline BMI (β = 0.439 [95% CI: 0.211, 0.666], p < 0.001) and estimated glucose disposal rate (eGDR) (β = −0.766 [−1.276, −0.256], p = 0.001) were associated with the RR of DKA (RR: 2.81; 95% CI:1.97, 4.01; p < 0.001, R² = 61%). A model including also treatment-related parameters (insulin dose change-to-baseline insulin sensitivity ratio and volume depletion) explained 86% of variance across studies in the risk of DKA (R² = 86%). The association of DKA with a BMI >27 kg/m² and with an eGDR <8.3 mg/kg/min was confirmed also in subgroup analyses. Among efficacy outcomes, the novel findings were a reduction in albuminuria (WMD: −9.91, 95% CI: −16.26, −3.55 mg/g, p = 0.002), and in RR of diabetic eye disorders (RR: 0.27[0.11, 0.67], p = 0.005) associated with SGLT2i. A SGLT2i dose-response gradient was consistently observed for main efficacy outcomes, but not for adverse events (AEs). Overall, predictors of DKA and of other AEs differed substantially from those of glycemic and non-glycemic efficacy. A limitation of our analysis was the relatively short (≤52 weeks) duration of included RCTs. The potential relevance for clinical practice needs also to be confirmed by real-world prospective studies.ConclusionsIn T1DM, the risk of DKA and main therapeutic responses to SGLT2i are modified by baseline BMI and insulin resistance, by total insulin dose reduction-to-baseline insulin sensitivity ratio, and by volume depletion, which may enable the targeted use of these drugs in patients with the greatest benefit and the lowest risk of DKA.

Giovanni Musso and colleagues conduct a meta-analysis to identify risk factors of diabetic ketoacidosis in patients with Type 1 diabetes taking SGLT2 inhibitors.     

Interaction of local anesthetics with lipid bilayers investigated by 1H MAS NMR spectroscopy

The membrane location of the local anesthetics (LA) lidocaine, dibucaine, tetracaine, and procaine hydrochloride as well as their influence on phospholipid bilayers were studied by ³¹P and ¹H magic-angle spinning (MAS) NMR spectroscopy. The ³¹P NMR spectra of the LA/lipid preparations confirmed that the overall bilayer structure of the membrane remained preserved. The relation between the molecular structure of the LAs and their membrane localization and orientation was investigated quantitatively using induced chemical shifts, nuclear Overhauser enhancement spectroscopy, and paramagnetic relaxation rates. All three methods revealed an average location of the aromatic rings of all LAs in the lipid-water interface of the membrane, with small differences between the individual LAs depending on their molecular properties. While lidocaine is placed in the upper chain/glycerol region of the membrane, for dibucaine and procaine the maximum of the distribution are slightly shifted into the glycerol region. Finally for tetracaine the aromatic ring is placed closest to the aqueous phase in the glycerol/headgroup region of the membrane. The hydrophobic side chains of the LA molecules dibucaine and tetracaine were located deeper in the membrane and showed an orientation towards the hydrocarbon core. In contrast the side chains of lidocaine and procaine are oriented towards the aqueous phase.     

Interaction of glycosylated human myelin basic protein with lipid bilayers

Myelin basic protein (MBP), isolated from normal human myelin, was glycosylated with UDP-N-acetyl-D-galactosamine and a glycosyltransferase isolated from porcine submaxillary glands. MBP containing 0.85 mol of N-acetyl-D-galactosamine per mole of protein was oxidized at carbon 6 by galactose oxidase and complexed with a spin-label, Tempoamine, in order to study its interactions with lipids. When the spin-labeled MBP was reacted with lipid vesicles consisting of DSPG, DPPG, and DMPG, most of the spin-label was motionally restricted in the gel phase, with a correlation time greater than 10(-8)s. The motion increased with increasing temperature and was sensitive to the lipid phase transition. Interaction with the gel phase of DPPA caused much less motional restriction of the probe. However, melting of the lipid allowed increased interaction and motional restriction of the probe, which was only partially reversed on cooling back to the gel phase. The motional restriction of the probe in these lipids is attributed to its penetration partway into the lipid bilayer in both the gel and liquid-crystalline phases. The fact that the probe bound to the protein can penetrate partway into the bilayer suggests that other hydrophobic side chains and residues of the protein can similarly penetrate into the bilayer. Additional evidence for penetration was provided by digestion of the lipid-bound protein with endoproteinase Lys-C. When nonglycosylated and glycosylated MBP in solution was treated with Lys-C, extensive digestion occurred. A single radioactive peptide which eluted at 25 min was identified as residues 92-105.(ABSTRACT TRUNCATED AT 250 WORDS)     

Interaction of tea catechins with lipid bilayers investigated with liposome systems

Interaction of tea catechins with lipid bilayers was investigated with liposome systems, which enabled us to separate liposomes from the external medium by centrifugation. We found that epicatechin gallate had the highest affinity for lipid bilayers, followed by epigallocatechin gallate, epicatechin, and epigallocatechin. Epicatechin gallate and epigallocatechin gallate in the surface of lipid bilayer perturbed the membrane structure.     

Interaction of local anesthetics with lipid bilayers investigated by (1)H MAS NMR spectroscopy

The membrane location of the local anesthetics (LA) lidocaine, dibucaine, tetracaine, and procaine hydrochloride as well as their influence on phospholipid bilayers were studied by (31)P and (1)H magic-angle spinning (MAS) NMR spectroscopy. The (31)P NMR spectra of the LA/lipid preparations confirmed that the overall bilayer structure of the membrane remained preserved. The relation between the molecular structure of the LAs and their membrane localization and orientation was investigated quantitatively using induced chemical shifts, nuclear Overhauser enhancement spectroscopy, and paramagnetic relaxation rates. All three methods revealed an average location of the aromatic rings of all LAs in the lipid-water interface of the membrane, with small differences between the individual LAs depending on their molecular properties. While lidocaine is placed in the upper chain/glycerol region of the membrane, for dibucaine and procaine the maximum of the distribution are slightly shifted into the glycerol region. Finally for tetracaine the aromatic ring is placed closest to the aqueous phase in the glycerol/headgroup region of the membrane. The hydrophobic side chains of the LA molecules dibucaine and tetracaine were located deeper in the membrane and showed an orientation towards the hydrocarbon core. In contrast the side chains of lidocaine and procaine are oriented towards the aqueous phase.     

Aldosterone suppresses expression of an avian colonic sodium-glucose cotransporter

Transport in the colon of the domestic fowl switches from sodium-linked hexose and amino acid cotransport on high-salt intake to amiloride-sensitive sodium channel expression on low-salt (LS) diets. The present experiments were designed to investigate the role of aldosterone in suppression of the colonic sodium-glucose luminal cotransporter (SGLT). LS-adapted hens were resalinated with or without simultaneous aldosterone treatment. Changes in the electrophysiological responses and SGLT protein expression levels were examined at 1, 3, and 7 days of treatment. Serum aldosterone levels fell from approximately 400 pmol/l in LS-adapted hens to values below the detection limit (<44 pmol/l) after 1 day of resalination. At the same time, glucose-stimulated short circuit current (I(SC)) increased from 20.9 +/- 8.7 to 56.3 +/- 15.5 microA/cm(2), whereas amiloride-sensitive I(SC) decreased from -68.9 +/- 12.7 microA/cm(2) on LS to +0.6 +/- 12.0 microA/cm(2). Glucose-stimulated I(SC) increased further at 3 and 7 days of resalination, whereas amiloride-sensitive I(SC) remained suppressed. When resalinated birds were simultaneously treated with aldosterone, the LS pattern of high amiloride-sensitive I(SC) and low glucose-stimulated I(SC) was maintained. Immunoblotting results from the same tissues demonstrated that SGLT-like protein expression increased following resalination. Aldosterone treatment completely blocked this effect. These results demonstrate that aldosterone suppresses both activity and protein expression of hen colonic SGLT. Resalination either through decreased aldosterone or other factors may be able to activate SGLT activity independently of increases in protein expression.     

Interaction of a two-transmembrane-helix peptide with lipid bilayers and dodecyl sulfate micelles

To probe structural changes that occur when a membrane protein is transferred from lipid bilayers to SDS micelles, a fragment of bacteriorhodopsin containing transmembrane helical segments A and B was studied by fluorescence spectroscopy, molecular dynamics (MD) simulation, and stopped flow kinetics. In lipid bilayers, F?rster resonance energy transfer (FRET) was observed between tyrosine 57 on helix B and tryptophans 10 and 12 on helix A. FRET efficiency decreased substantially when the peptide was transferred to SDS. MD simulation showed no evidence for significant disruption of helix-helix interactions in SDS micelles. However, a cluster of water molecules was observed to form a hydrogen-bonded network with the phenolic hydroxyl group of tyrosine 57, which probably causes the disappearance of tyrosine-to-tryptophan FRET in SDS. The tryptophan quantum yield decreased in SDS, and the change occurred at nearly the same rate as membrane solubilization. The results provide a clear example of the importance of corroborating distance changes inferred from FRET by using complementary methods.     

Interaction of synaptotagmin with lipid bilayers, analyzed by single-molecule force spectroscopy

Synaptotagmin I is the major Ca²⁺ sensor for membrane fusion during neurotransmitter release. The cytoplasmic domain of synaptotagmin consists of two C2 domains, C2A and C2B. On binding Ca²⁺, the tips of the two C2 domains rapidly and synchronously penetrate lipid bilayers. We investigated the forces of interaction between synaptotagmin and lipid bilayers using single-molecule force spectroscopy. Glutathione-S-transferase-tagged proteins were attached to an atomic force microscope cantilever via a glutathione-derivatized polyethylene glycol linker. With wild-type C2AB, the force profile for a bilayer containing phosphatidylserine had both Ca²⁺-dependent and Ca²⁺-independent components. No force was detected when the bilayer lacked phosphatidylserine, even in the presence of Ca²⁺. The binding characteristics of C2A and C2B indicated that the two C2 domains cooperate in binding synaptotagmin to the bilayer, and that the relatively weak Ca²⁺-independent force depends only on C2A. When the lysine residues K189-192 and K326, 327 were mutated to alanine, the strong Ca²⁺-dependent binding interaction was either absent or greatly reduced. We conclude that synaptotagmin binds to the bilayer via C2A even in absence of Ca²⁺, and also that positively charged regions of both C2A and C2B are essential for the strong Ca²⁺-dependent binding of synaptotagmin to the bilayer.     

Interaction of sporidesmin,a mycotoxin fromPithomyces chartarum,with lipid bilayers

Sporidesmin, a mycotoxin fromPithomyces chartarum is a hydrophobic molecule. It can therefore be easily incorporated in the cell membrane, where it is likely to cause changes in the bilayer organization and the properties of membrane proteins. In order to understand the redox behaviour of sporidesmin in a hydrophobic environment, we have investigated the effects of oxidized and reduced sporidesmin on the phase transition properties of bilayers and on the susceptibility of bilayers to pancreatic phospholipase A₂ (PLA₂). The changes induced by sporidesmin in the thermotropic phase transition profiles of dimyristoyl-sn-3-phosphatidyl choline (DMPC) bilayers were similar to those caused by solutes known to localize in the glycerol-backbone region of the lipid bilayer, suggesting a similar localization for oxidized and reduced sporidesmin. Neither form of toxin disrupt the bilayer or membrane organization even at relatively high mole fractions. At concentrations <10 mole% both forms partitioned equally well in the gel and liquid-crystalline phases, whereas at higher concentrations (30 mole%) reduced sporidesmin is preferentially localized in the liquid-crystalline phase. These effects of sporidesmin on the phase properties of DMPC vesicles were also reported by the fluorescence behavior of 10-pyrenedecanoic acid (PDA). The effects of oxidized and reduced sporidesmins on PLA₂ kinetics are consistent with their ability to perturb bilayer organisation.