125I labelled inulin: a convenient marker for deposition of liposomal contents in vivo

The utility of an iodinated derivative of inulin (125I-tyraminyl-inulin, 125ITI) for reporting in vivo tissue distributions of liposomal contents is described. It is shown, employing a rat model, that this probe satisfies the criteria that the free form is rapidly cleared from the circulation and excreted, whereas 125ITI encapsulated in large unilamellar vesicle (LUV) systems and subsequently taken up in various tissues exhibits a long (greater than 3 days) retention time. Further, high specific activities (greater than 1 muCi per microliter) are easily achievable, allowing low LUV dose levels (less than or equal to 2.5 mumole phospholipid/kg body weight) to be employed. Minimal tissue workups for quantitation of 125ITI distributions are required. It is concluded that from criteria of sensitivity, expense and simplicity, 125ITI is a most convenient probe for characterizing liposome deposition in vivo.

Detergent-induced solubilization of cytochrome c oxidase as detected in a novel reconstituted system

A preparation of reconstituted cytochrome oxidase vesicles in which the enzyme is oriented facing inwards (such that it cannot interact with external cytochrome c) is described. No oxidase activity is expressed by these vesicles unless they are disrupted, allowing influx of cytochrome c or exposure of the oxidase-binding site to the external medium. We have exploited this property to follow detergent-induced solubilization of the membrane, a technique which allows membrane disruption and enzyme activity to be monitored simultaneously. This protocol can be employed to investigate the properties and mechanism of action of detergents as is illustrated for several ionic and nonionic detergents.

Cation-dependent segregation phenomena and phase behavior in model membrane systems containing phosphatidylserine: influence of cholesterol and acyl chain composition

The addition of Ca2+ to model membrane systems containing phosphatidylserine (PS) can have remarkable effects on the distribution of PS and the overall polymorphic phase [bilayer or hexagonal (HII)] assumed by the lipid mixture. In this study, we examine the influence of Ca2+ on lipid mixtures composed of well-defined (synthetic) species of PS, phosphatidylethanolamine (PE), and phosphatidylcholine (PC) in the presence and absence of cholesterol by employing 31P and 2H NMR, freeze-fracture, and X-ray techniques. It is shown that whereas Ca2+ can segregate PS into crystalline cochleate domains in equimolar mixtures of dioleoyl-PE and dioleoyl-PS (DOPS), such effects are not observed for mixtures containing more unsaturated (dilinoleoyl) species of PS. The addition of cholesterol to these PE-PS systems inhibits Ca2+-induced segregation of DOPS and facilitates Ca2+-triggered hexagonal (HII) phase formation for both the PE and the PS components. In contrast, in equimolar mixtures of DOPS with dioleoyl-PC, Ca2+-induced segregation of phospholipid is not affected by the presence of up to 33 mol % cholesterol. These and related effects suggest that, in multicomponent biomembrane systems containing both PE and cholesterol, phase segregation of PS by Ca2+ may not be readily achievable. These results are discussed with regard to the reliability of 31P NMR phase identifications of phospholipid structure in model and biological membranes and demonstrate that in mixed lipid systems the influence of divalent cations on lipid distribution and structure can be exquisitely sensitive to details of the local lipid composition.(ABSTRACT TRUNCATED AT 250 WORDS)

Non-bilayer structures in membrane fusion

Membrane fusion is an ubiquitous event in cell biology. One can distinguish two types of fusion: (i) outside/outside fusion, for example endocytosis, and (ii) inside/inside fusion, for example exocytosis. In spite of this difference in types of fusion in relation to membrane asymmetry and in spite of the large variety of lipid compositions encountered in biological membranes, a universal mechanism can be postulated for the role of lipids in membrane fusion. In this concept the lipids leave the bilayer configuration temporarily and locally. This notion, and the fact that any biological membrane contains a substantial amount of lipids which prefer the non-bilayer hexagonal II phase in physiological conditions, has led to the hypothesis that such hexagonal II phase lipids play a crucial role in membrane fusion. This proposition is strongly supported by model membrane experiments in which it has been demonstrated that factors such as Ca2+ and temperature, which trigger the transition from bilayer to hexagonal II phase, in fact induce membrane fusion.

Influence of vesicle size and oxidase content on respiratory control in reconstituted cytochrome oxidase vesicles

Previous work has shown that the respiratory control or coupling exhibited by reconstituted cytochrome oxidase systems can be markedly sensitive to the lipid composition, the lipid to protein ratio, and the vesicle size. In this work we have attempted to ascertain which, if any, of these factors plays a definitive role in determining the observed coupling. Vesicles prepared from dioleoylphosphatidylcholine-dioleoylphosphatidylethanolamine (1:4) were fractionated by DEAE chromatography to obtain a population homogeneous with respect to their lipid to protein ratio. This subpopulation was then applied to a Sepharose 4B-CL column to separate the vesicles by size. Fractions eluted from the column were assayed for enzyme activity in the presence and absence of carbonyl cyanide ( trifluoromethoxy )phenylhydrazone plus valinomycin. The coupling ratio was found to be dependent upon vesicle size; the smaller the vesicles, the higher the ratio. This suggests that lipid composition per se does not determine coupling characteristics. Reducing vesicle size for a given lipid to protein ratio has two effects. First, the radius of curvature is increased and, second, the average number of oxidase molecules per vesicle is reduced. In order to identify which of these factors was responsible for the observed tighter coupling, the oxidase was reconstituted with dioleoylphosphatidylcholine into vesicles of a defined size and the number of oxidase molecules per vesicle varied. The highest coupling ratios were observed for vesicles containing on average only one oxidase dimer. As the fraction of vesicles containing more than one protein was increased, the coupling ratio rapidly declined.(ABSTRACT TRUNCATED AT 250 WORDS)

Polymorphism of phosphatidylethanolamine-phosphatidylserine model systems: influence of cholesterol and Mg2+ on Ca2+-triggered bilayer to hexagonal (HII) transitions

Previous work has shown that Ca2+ can trigger bilayer to hexagonal (HII) polymorphic phase transitions in (unsaturated phosphatidylserine (PS)-phosphatidylethanolamine (PE) model systems. In this work we examine the influence of cholesterol and Mg2+ on the phase preferences of PS-PE systems. Subsequently, the influence of cholesterol and Mg2+ on the levels of Ca2+ required to trigger bilayer-HII transitions in these mixed systems is studied. It is shown that at 30 degrees C the presence of equimolar (with respect to phospholipid) levels of cholesterol engenders formation of the HII phase for PE-PS systems containing 15 and 30 mol% PS, whereas bilayer structure is maintained for PE-PS-cholesterol (1:1:2) dispersions. However, the polymorphic phase preferences of the latter system are much more sensitive to the presence of monovalent and divalent cations. In the absence of cholesterol, Mg2+ and high salt concentrations do not affect the polymorphic phase preferences of PE-PS (1:1) systems. In contrast, 8 mM or higher Mg2+ levels or salt concentrations greater than 1.0 M induce HII-phase formation in PE-PS-cholesterol (1:1:2) systems. Further, lower Mg2+ concentrations (2 mM) act as a powerful adjunct to Ca2+ triggering of HII-phase structure in such systems, reducing the Ca2+ concentration required from 4 to 0.25 mM. These results are discussed in terms of Ca2+ concentrations required for fusion events and the influence of cholesterol on the structural preferences of the inner monolayer lipids of the erythrocyte membrane.

Lipid requirements for coupled cytochrome oxidase vesicles

Cytochrome c oxidase has been reconstituted with two synthetic phospholipids, dioleoylphosphatidylcholine and dioleoylphosphatidylethanolamine. Vesicles prepared from either of these two lipids alone showed no stimulation of enzyme activity upon addition of carbonyl cyanide (trifluoromethoxy)phenylhydrazone and valinomycin, indicating that they were leaky to small ions. However, when mixtures of the two lipids were used for the reconstitution, tightly coupled vesicles could be obtained. The coupling ratio was dependent upon the ratio of dioleoylphosphatidylcholine to dioleoylphosphatidylethanolamine and also on the lipid-to-protein ratio. Maximal rates of enzyme activity were not significantly different with different lipid mixtures. The results are discussed in terms of both the size distribution of the reconstituted vesicles and the possible requirement for a variety of lipid species to ensure tight sealing at the lipid-protein interface.

Polymorphic phase preferences of phosphatidic acid: A 31P and 2H NMR study

31P NMR, 2H NMR and freeze fracture techniques have been employed to investigate the structural preferences of dioleoyl phosphatidic acid (DOPA) under various conditions of pH and divalent cation content. It is shown that DOPA increasingly prefers the HII organization below pH 5, and that low levels of Ca2+ and Mg2+ (Me2+/DOPA = 0.5) induce HII phase structure for pH less than 6.0. Higher Mg2+ and Ca2+ levels (Me2+/DOPA greater than 1.0) induce more complex structures at pH greater than 5.0 which may correspond to intermediates between lamellar and HII organization. This work illustrates the utility of 2H NMR techniques in conjunction with suitable 2H labelled lipids to provide structural information on lipid-water systems, and suggests that 31P NMR techniques for determinations of lipid organization can be applied to advantage in PA containing membranes.

Ca2+ and pH induced fusion of small unilamellar vesicles consisting of phosphatidylethanolamine and negatively charged phospholipids: a freeze fracture study

Fusion processes in small (sonicated) unilamellar vesicle (SUV) systems composed of 80 mol % unsaturated phosphatidylethanolamine and 20 mol % of phosphatidylserine, phosphatidylinositol, phosphatidic acid or cardiolipin have been examined by freeze-fracture techniques. All these mixtures have the property that Ca2+ and, in some cases low pH, can trigger bilayer to hexagonal (HII) phase transitions in large multilamellar dispersions of these lipid mixtures. It is shown that when the SUV systems are subjected to similar protocols, the vesicles first fuse to form larger systems prior to HII phase formation and this fusion is accompanied by the appearance of lipidic particle structures, often localized to the fusion interface. We conclude that factors promoting HII phase structure initially result in fusion of unilamellar systems and that this fusion proceeds via intermediary formation of non-lamellar, possibly inverted micellar, structure.

Phospholipids as adjuncts for calcium ion stimulated release of chromaffin granule contents: implications for mechanisms of exocytosis

Structure-function relationships for the lipid component of chromaffin granules isolated from the bovine adrenal medulla have been investigated by employing 31P nuclear magnetic resonance (NMR), freeze-fracture, and spectrophotometric techniques. Two aspects have been studied in detail, namely, the structural preferences of lipids in the isolated granule membrane and derived liposomal model membrane systems as well as the influence of exogenous lipid (in the form of sonicated vesicle systems) on the Ca2+-stimulated release of granule contents. It is shown that at least 90% of endogenous granule membrane phospholipids assume a liquid-crystalline bilayer configuration at physiological temperatures. Liposomal dispersions of total granule lipid also exhibit bilayer structure, consistent with a structural role of phospholipids in vivo. Incubation of intact isolated granules in the presence of up to 10 mM Ca2+ does not induce significant release of contents above background levels. However, it is shown that incubation of granules in the presence of sonicated phospholipid systems which undergo structural transitions in the presence of Ca2+ can cause immediate and total release of granule contents at Ca2+ levels of 2 mM or more. This behavior is attributed to disruption of granule membrane integrity due to fusion of the vesicle systems with the chromaffin granules. Direct evidence for such fusion is obtained by freeze-fracture electron microscopy. On the basis of this information and with the assumption that the inner leaflet of the adrenal cell plasma membrane is composed predominantly of phosphatidylethanolamine and phosphatidylserine, a mechanism of Ca2+-stimulated exocytotic release of catecholamines in vivo is proposed.

Influence of cholesterol on the structural preferences of dioleoylphosphatidylethanolamine-dioleoylphosphatidylcholine systems: a phosphorus-31 and deuterium nuclear magnetic resonance study

The polymorphic phase behavior of mixtures of synthetic dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) and the influence of cholesterol on these phase preferences have been investigated by employing nuclear magnetic resonance (NMR) techniques. In particular, 31P NMR procedures are utilized to study the overall phase preferences of these mixed systems, whereas 2H NMR is employed to monitor the structural preferences of individual components of these systems by using versions of DOPE and DOPC which are deuterium (2H) labeled at the C11 position of the acyl chains. The results obtained show that DOPE-DOPC systems containing as little as 20 mol % DOPC initially assume lamellar structure at 40 degrees C, even though DOPE in isolation prefers the hexagonal (HII) organization at this temperature. However, this lamellar organization appears to represent a metastable state, as incubation for extended periods at 40 degrees C results in formation of a structure, possibly the cubic phase, in which the phospholipids experience isotropic motional averaging. The addition of cholesterol induces hexagonal (HII) phase organization. 2H NMR studies of appropriately labeled versions of these systems indicate that cholesterol does not produce such effects by associating preferentially with either DOPE or DOPC. Further, in situations where bilayer, hexagonal, or "isotropic" phases coexist in the same sample, the phospholipids exhibit apparently ideal mixing behavior.

Structural preferences of phosphatidylinositol and phosphatidylinositol-phosphatidylethanolamine model membranes. Influence of Ca2+ and Mg2+

The structural preferences of soya phosphatidylinositol in isolation and in mixtures with soya phosphatidylethanolamine, and the influence of Ca2+ and Mg2+ on these preferences, have been examined employing 31P-NMR and freeze-fracture techniques. It is shown that phosphatidylinositol assumes the bilayer organization on hydration both in the presence and absence of Ca2+ and Mg2+. In mixed systems with (HII phase) phosphatidylethanolamine, phosphatidylinositol induces lipidic particle structure at low (less than 10 mol%) concentrations and bilayer structure at higher levels. In systems containing 15 or 20 mol% phosphatidylinositol, Ca2+ (but not Mg2+) can induce HII phase structure. The results indicate that phosphatidylinositol is a more effective agent than other acidic phospholipids for stabilizing bilayer structure, particularly when high levels of divalent cations are present. These findings are discussed in terms of functional roles of phosphatidylinositol and mechanisms whereby Ca2+ induces structural reorganizations in mixed systems containing acidic phospholipids and phosphatidylethanolamine.

Stabilization of bilayer structure for unsaturated phosphatidylethanolamines by detergents

The structural preferences of mixed lipid systems containing egg yolk or 18:1c/18:1c phosphatidylethanolamine and representative detergents (Triton X-100, deoxycholate, octylglucoside and lyso-phosphatidylcholine) have been examined. It is shown that all these detergents exhibit an ability to stabilize a bilayer organization for the phosphatidylethanolamine at detergent to phosphatidylethanolamine molar rations of 0.05 to 0.5, depending on the detergent and/or phosphatidylethanolamine species. These results are interpreted in terms of molecular shape, where the 'inverted cone' shape detergents combine in a complementary fashion with 'cone shaped' phosphatidylethanolamine to result in net bilayer structure.

Influence of local and neutral anaesthetics on the polymorphic phase preferences of egg yolk phosphatidylethanolamine

(1) The polymorphic phase preferences of egg phosphatidylethanolamine have been examined in the presence of normal alcohols and alkanes of varying chain length, as well as charged amine anaesthetics. (2) It is shown that the charged anaesthetics, ethanol and butanol can stabilize a bilayer arrangement for egg phosphatidylethanolamine. In contrast, longer chain (C greater than or equal to 6) normal alcohols and alkanes induce the hexagonal (HII) phase. (3) The relative potency of local anaesthetics in vitro (chlorpromazine, dibucaine, tetracaine and procaine) is mirrored by their relative ability to stabilize bilayer structure for hydrated egg phosphatidylethanolamine. Further, the aqueous concentrations of anaesthetic required to affect phospholipid polymorphism is sensitive to the lipid composition. For example, the inclusion of 20 mol% egg phosphatidylserine in egg phosphatidylethanolamine dispersions can reduce the aqueous concentrations of dibucaine required to induce appreciable bilayer stabilization effects from 5.0 mM to 0.5 mM. (4) It is suggested that the ability of amphipathic molecules such as anaesthetics to influence phosphatidylethanolamine polymorphism arises from their molecular shape. The possibility that anaesthetic molecules may exert their effects by virtue of this shape property is raised.

The polymorphic phase behaviour of mixed phosphatidylserine-phosphatidylethanolamine model systems as detected by 31P-NMR

1. The influence of divalent cations and pH on the polymorphic phase behaviour of aqueous dispersions of phosphatidylethanolamine-phosphatidylserine systems have been investigated employing 31P-NMR techniques. 2. Phosphatidylserines, derived from both egg and soya phosphatidylcholines, stabilize a bilayer organization at 30 degrees C in mixtures with soya phosphatidylethanolamine (which assumes the hexagonal (HII) phase on hydration) when the phosphatidylserine constitutes 15 mol% or more of the phospholipid. 3. The addition of Ca2+ to equimolar soya phosphatidylserine/soya phosphatidylethanolamine mixtures triggers complete HII phase formation as detected by 31P-NMR at Ca2+:phosphatidylserine ratios, R, of 1.0 or larger. In contrast, Mg2+ is ineffective even at Mg2+:phosphatidylserine ratios of 10.0. In mixtures containing 15 mol% phosphatidylserine, Ca2+ triggers HII phase formation at R = 0.25. The Ca2+-induced polymorphic phase transitions appear to occur as a result of a structural segregation of phosphatidylserine by Ca2+ into crystalline domains, leaving the phosphatidylethanolamine free to adopt the HII phase it prefers in isolation. 4. The polymorphism of soya phosphatidylserine/soya phosphatidylethanolamine systems is markedly sensitive to the pH of the aqueous medium. At 30 degrees C equimolar mixtures exhibit a bilayer-HII transition as the pH is decreased below 4.0, whereas mixtures containing 15 mol% phosphatidylserine exhibit detectable HII phase structure at pH values below 5.5. 5. 31P-NMR studies suggest that the binding of Ca2+ to phosphatidylserine to produce crystalline structures is sensitive to the unsaturation of the acyl chains, with more unsaturated species requiring higher Ca2+:phosphatidylserine ratios for formation of crystalline Ca2+-phospholipid complexes. Studies of the binding of Ca2+ with soya phosphatidylserine indicate half maximal binding at 0.3 mM in the absence of salt, which is increased to approx. 0.8 mM in the presence of 100 mM NaCl. 6. These results suggest that the effectiveness of phosphatidylserine as a bilayer-stabilizing agent can be modulated by local changes in such biologically relevant parameters as pH, ionic strength and/or cation concentrations, and are discussed in relation to membrane fusion processes.

The role of nonbilayer lipid structures in the fusion of human erythrocytes induced by lipid fusogens

(1) A comparative study of the polymorphic phase behaviour of reconstituted erythrocyte phospholipase with 'fusogens' and chemically related 'non-fusogens' using 31P-NMR techniques has been made. We demonstrate that only fusogens can induce a conformational change in the membrane lipids from the bilayer of the hexagonal (HII) phase. (2) The tendency of erythrocyte (ghost) membranes to adopt nonbilayer structures is correlated with the extent of fusion in erythrocytes at various membrane concentrations of glycerol monooleate. Above a certain critical membrane concentration of glycerol monooleate, there is an increase in the tendency of ghost membranes to adopt the HII phase, which is paralleled by a marked increase in fusion processes in the intact erythrocytes. (3) These observations are consistent with a common mechanism of action for fusogenic lipids whereby inverted lipid structures such as long cylinders (HII configuration) or inverted micelles are induced, which serve as intermediates in the fusion event.

Effects of tumbling and lateral diffusion on phosphatidylcholine model membrane 31P-NMR lineshapes

Two factors determining the isotropic motional averaging of NMR spectra obtained from lipids in model and biological membranes systems are particle tumbling and lateral diffusion. The influence of these motions (of which the magnitudes are detemined by the medium viscosity and temperature) on the 31P-NMR spectra arising from unilamellar dioleoyl phosphatidylcholine vesicles of a defined size are examined. It is shown that the lineshapes obtained are in good agreement with those predicted by the theory of motional narrowing. These results are discussed with regard to order parameter determinations and polymorphic phase identifications as obtained by NMR techniques.

Cytochrome c specifically induces non-bilayer structures in cardiolipin-containing model membranes

(1) The effect of cytochrome c addition on the phospholipid structure of liposomes composed of cardiolipin, phosphatidylserine, phosphatidylglycerol, phosphatidylcholine or phosphatidylethanolamine in a pure form or in mixtures was investigated by 31P-NMR and freeze-fracture techniques. (2) Cytochrome c specifically induces the hexagonal Hii phase and possibly an inverted micellar structure of part of the phospholipids in cardiolipin-containing model membranes. (3) These results are compared with the effect of Ca2+ on cardiolipin and are discussed in relation to the structure and function of the inner mitochondrial membrane.

Phospholipids and membrane transport

The ability of membrane lipids to adopt nonbilayer configurations suggests dynamic roles for lipids in many functional abilities of biological membranes. In this work evidence supporting the involvement of lipids in three types of membrane transport process is presented and discussed. These transport processes include facilitated transbilayer transport of polar molecules, transport mechanisms involving fusion events, and transport possibilities arising from alternative membrane morphology. In particular it is shown that lipids such as cardiolipin, which adopt the hexagonal HII phase in the presence of Ca2+, may be logically proposed to facilitate Ca2+ transport across membranes via an inverted micellar intermediate. Alternatively, in transport processes such as exocytosis the ability of Ca2+ to generate membrane instabilities favouring nonbilayer alternatives suggests a crucial role of phospholipid in the fusion event vital to exocytotic release. Finally, nonbilayer lipid structures may be suggested to favour formation of isolated compartments connected by a continuous membrane where lateral diffusion processes can lead to transport. These various possibilities are summarized in a "metamorphic mosaic" model of biological membranes.

The influence of poly(L-lysine) on phospholipid polymorphism. Evidence that electrostatic polypeptide-phospholipid interactions can modulate bilayer/non-bilayer transitions

31P-NMR shows that poly(L-lysine) binding to cardiolipin, phosphatidylserine or phosphatidylglycerol does not affect the macroscopic structure or local order (in the phosphate region) of the phospholipids. In the case of cardiolipin poly(L-lysine) inhibits the ability of Ca2+ to induce the hexagonal HII phase. Alternatively, poly(L-lysine) induces the hexagonal HII phase for a fraction of the phospholipids in phosphatidylethanolamine-cardiolipin (2:1) dispersions.

The lipidic particle as an intermediate structure in membrane fusion processes and bilayer to hexagonal HII transitions

Small unilamellar vesicles comprised of a mixture of phosphatidylethanolamine/phosphatidylcholine/cholesterol (3 : 1 : 2) fuse to form large multilamellar vesicles on increasing the temperature from 0 to 50 degrees C. This event is associated with the appearance of lipidic particles at the fusion sites, consistent with a role as intermediary structures during the fusion process. Further, for phosphatidylcholine/cardiolipin (1 : 1) liposomes in the presence of Mn2+ a direct relationship between lipidic particles and the hexagonal (HII) phase is demonstrated which suggests that lipidic particles can also occur as intermediaries between bilayer and hexagonal (HII) structures.

Structural properties of phospholipids in the rat liver inner mitochondrial membrane

1. The 31P-NMR characteristics of intact rat liver mitochondria, mitoplasts and isolated inner mitochondrial membranes, as well as mitochondrial phosphatidylethanolamine and phosphatidylcholine, have been examined. 2. Rat liver mitochondrial phosphatidylethanolamine hydrated in excess aqueous buffer undergoes a bilayer-to-hyexagonal (HII) polymorphic phase transition as the temperature is increased through 10 degrees C, and thus prefers the HII arrangement at 37 degrees C. Rat liver mitochondiral phosphatidylcholine, on the other hand, adopts the bilayer phase at 37 degrees C. 3. Total inner mitochondrial membrane lipids. dispersed in an excess of aqueous buffer, exhibit 31P-NMR spectra consistent with a bilayer arrangment for the majority of the endogeneous phospholipids; the remainer exhibit spectra consistent with structure allowing isotropic motional averaging. Addition of Ca2+ results in hexagonal (HII) phase formation for a portion of the phospholipids, as well as formation of 'lipidic particles' as detected by freeze-fracture techniques. 4. Preparations of inner mitochondrial membrane at 4 and 37 degrees C exhibit 31P-NMR spectra consistent with a bilayer arrangement of the large majority of the endogenous phospholipids which are detected. Approx. 10% of the signal intensity has characteristics indicating isotropic motional averaging processes. Addition of Ca2+ results in an increase in the size of this component, which can become the domiant spectral feature. 5. Intact mitochondria, at 4 degrees C, exhibit 31P-NMR spectra arising from both phospholid and small water-soluble molecules (ADP, Pi, etc.). The phospholipid spectrum is characteristic of a bilayer arrangement. At 37 degrees C the phospholipids again give spectra consistent with a bilayer; however, the labile nature of these systems is reflected by increased isotropic motion at longer (at least 30 min) incubation times. 6. It is suggested that the uncoupling action of high Ca2+ concentrations on intact mitochondria may be related to a Ca2+-induced disruption of the integrity of the inner mitochondrial phospholipid bilayer. Further, the possibility that non-bilayer lipid structures such as inverted micelles occur in the inner mitochondrial membrane cannot be excluded.