Study of the transverse diffusion of spin-labeled phospholipids in biological membranes. II. Inner mitochondrial membrane of rat liver: use of phosphatidylcholine exchange protein.

Spin-labeled phosphatidylcholine was incorporated into the membrane of isolated "inner membrane+matrix" particles of rat liver mitochondria by incubation with sonicated spin-labeled phosphatidylcholine vesicles at 22 degrees C. When the spin label was on the acyl chain the incorporation of phosphatidylcholine into the membrane was stimulated by the presence of the phosphatidylcholine exchange protein extracted from rat or beef liver. On the other hand no stimulation was observed when the nitroxide was on the polar head-group. When spin-labeled phosphatidycholine was incorporated into the mitochondrial membrane in the absence of phosphatidylcholine exchange protein, ascorbate treatment at 0 degrees C reduced the EPR signal of the spin-labeled membranes by approximately 50%, indicating that fusion incorporates molecules equally on both sides of the membrane. On the other hand when spin-labeled phosphatidylcholine was incorporated in the presence of the exchange protein most of the EPR signal could be destroyed by the ascorbate treatment at 0 degrees C, indicating that the spin-labeled phosphatidylcholine had been selectively incorporated in the outer layer of the membrane. Finally when the label is on the polar head-group the inner content of mitochondria reduces the label facing the matrix, thus creating again an anisotropy of the labeling. The anisotropic distribution of spin-labeled phosphatidylcholine in the mitochondrial membrane was found to be stable at 25 degrees C for more than 2 h. It is therefore concluded that the rate of outside-inside and inside-outside transitions are extremely slow (half-life greater than 24 h).     

Study of the transverse diffusion of spin-labeled phospholipids in biological membranes. II. Inner mitochondrial membrane of rat liver: Use of phosphatidylcholine exchange protein

Spin-labeled phosphatidylcholine was incorporated into the membrane of isolated “inner membrane+matrix” particles of rat liver mitochondria by incubation with sonicated spin-labeled phosphatidylcholine vesicles at 22°C. When the spin label was on the acyl chain the incorporation of phosphatidylcholine into the membrane was stimulated by the presence of the phosphatidylcholine exchange protein extracted from rat or beef liver. On the other hand no stimulation was observed when the nitroxide was on the polar head-group.When spin-labeled phosphatidylcholine was incorporated into the mitochondrial membrane in the absence of phosphatidylcholine exchange protein, ascorbate treatment at O°C reduced the EPR signal of the spin-labeled membranes by approximately 50%, indicating that fusion incorporates molecules equally on both sides of the membrane. On the other hand when spin-labeled phosphatidylcholine was incoporated in the presence of the exchange protein most of the EPR signal could be destroyed by the ascorbate treatment at 0°C, indicating that the spin-labeled phosphatidylcholine had been selectively incorporated in the outer layer of the membrane. Finally when the label is on the polar head-group the inner content of mitochondria reduces the label facing the matrix, thus creating again an anisotropy of the labeling.The anisotropic distribution of spin-labeled phosphatidylcholine in the mitochondrial membrane was found to be stable at 25°C for more than 2 h. It is therefore concluded that the rate of outside-inside and inside-outside transitions are extremely slow (half-life greater than 24 h).     

Study of the transverse diffusion of spin labeled phospholipids in biological membranes. I. Human red bloods cells.

Spin labeled analogs of phosphatidylcholine were used to study the transverse diffusion (flip-flop) of phospholipids in the erythrocyte membrane. The nitroxide spin label was placed either on the beta acyl chain or on the choline group. These labeled phosphatidylcholine molecules were incorporated into the membrane by incubation of the red cells at 22 degrees C with sonicated spin-labed phosphatidylcholine vesicles from which all traces of free fatty acids and lyso derivatives were carefully removed by bovine serum albumin treatment. This incorporation did not provide any change in the morphology of the cell as indicated by scanning electron microscopy. When spin-labeled phosphatidylcholine, having a nitroxide on the beta chain but near the polar head-group, was incorporated into the erythrocyte membrane, ascorbate treatment at 0 degrees C allows selective reduction of the signal coming from the outer layer of the membrane. When the label was on the polar head-group, the inner content of the erythrocyte rapidly reduced the label facing the cytoplasm, thus creaging a spontaneous anisotropy of the labeling. The anisotropic distribution of spin-labeled phosphatidylcholine in the erythrocyte membrane was found to be stable at 22 and 37 degrees C for more than 4 h. It is therefore concluded that the rate of outside-inside and inside-outside transition is so slow that the anisotropic distribution of the phospholipids in the erythrocyte membrane can be maintained during cell life.     

Study of the transverse diffusion of spin labelled phospholipids in biological membranes. I. Human red blood cells

Spin labeled analogs of phosphatidylcholine were used to study the transverse diffusion (flip-flop) of phospholipids in the erythrocyte membrane. The nitroxide spin label was placed either on the β acyl chain or on the choline group. These labeled phosphatidylcholine molecules were incorporated into the membrane by incubation of the red cells at 22°C with sonicated spin-labeled phosphatidylcholine vesicles from which all traces of free fatty acids and lyso derivatives were carefully removed by bovine serum albumin treatment. This incorporation did not provide any change in the morphology of the cell as indicated by scanning electron microscopy. When spin-labeled phosphatidylcholine, having a nitroxide on the β chain but near the polar head-group, was incorporated into the erythrocyte membrane, ascorbate treatment at 0dgC allows selective reduction of the signal coming from the outer layer of the membrane. When the label was on the polar head-group, the inner content of the erythrocyte rapidly reduced the label facing the cytoplasm, thus creating a spontaneous anisotropy of the labeling. The anisotropic distribution of spin-labeled phosphatidylcholine in the erythrocyte membrane was found to be stable at 22 and 37°C for more than 4 h. It is therefore concluded that the rate of outside-inside and inside-outside transition is so slow that the anisotropic distribution of the phospholipids in the erythrocyte membrane can be maintained during cell life.     

Asymmetry of the site of choline incorporation into phosphatidylcholine of rat liver microsomes.

[14C]Choline was incorporated into microsomal membranes in vivo, and from CDP-[14C]choline in vitro, and the site of incorporation determined by hydrolysis of the outer leaflet of the membrane bilayer using phospholipase C from Clostridium welchii. Labelled phosphatidylcholine was found to be concentrated in the outer leaflet of the membrane bilayer with a specific activity approximately three times that of the inner leaflet. During incorporation of CDP-choline and treatment with phospholipase C the vesicles retained labelled-protein contents indicating that they remained intact. When the microsomes were opened with taurocholate after incorporation of [14C]choline in vivo, the labelled phosphatidylcholine behaved as a single pool. Selective hydrolysis of labelled phosphatidylcholine in intact vesicles is not, therefore, a consequence of specificity of phospholipase C. These results indicate that the phosphatidylcholine of the outer leaflet of the microsomal membrane bilayer is preferentially labelled by the choline-phosphotransferase pathway and that this pool of phospholipid does not equilibrate with that of the inner leaflet.     

The effect of phosphatidylcholine to sphingomyelin mole ratio on the dynamic properties of sheep erythrocyte membrane.

Sheep red blood cells are shown to incorporate phosphatidylchline when incubated in human plasma in the presence of EGTA. This treatment results in up to a 5-fold increase in mol ratio of phosphatidylcholine to sphingomyelin. By replacing EGTA with Ca+ the increase of phsphatidylcholine content is completely inhibited, due to the activation of the membrane bound lecithinase which rapidly degrades the incorporated phosphatidylcholine. Analogous treatments of the isolate membranes resulted in similar phosphatidylcholine incorporation but in the presence of Ca+ a residual phosphatidylcholine uptake was still oberved. These results suggest that in the isolated membranes small amounts of phosphatidylcholine can be incorporated into an additional region which is unavailable for the membrane lecithinase. The increase in the phosphatidylcholine to sphingomyelin mol ratio in sheep red blood cells is concomitant with an increase in lipid fluidity, as well as increase in osmotic fragility9     

Asymmetric exchange of vesicle phospholipids catalyzed by the phosphatidylcholine exhange protein. Measurement of inside--outside transitions.

Purified phosphatidylcholine exchange protein was used to exchange phosphatidylcholine between homogeneous single-walled phosphatidylcholine vesicles and human erythrocyte ghosts. When excess ghosts were present, it was found that only 70% of the vesicle phosphatidylcholine was available for exchange. This fraction corresponds closely to the amount of phosphatidycholine in the outer monolayer of these vesicles, indicating that only the outer surface of the vesicle is accessible to the exchange protein. Also, it was found that all phosphatidylcholine introduced into vesicles by the exchange protein was available for subsequent exchange. Using the exchange protein, asymmetrical vesicles were prepared in which the outer monolayer was either enriched or depleted in radioactive phosphatidylcholine as compared to the inner monolayer. Re-equilibration of the radioactivity between the two surfaces of the vesicle (flip-flop) could not be detected, even after 5 days at 37degrees. It is estimated that the half-time for flip-flop is in excess of 11 days at 37degrees. These results indicate that the properties of the exchange protein can be expolited to measure phosphatidylcholine flip-flop rates and possible phosphatidylcholine asymmetry in biological and model membranes, without altering the structure of the membrane.     

Stimulation of cholinephosphotransferase activity by phosphatidylcholine transfer protein. Regulation of membrane phospholipid synthesis by a cytosolic protein

The effect of rat liver phosphatidylcholine transfer protein on the incorporation of CDP-choline and dioleoylglycerol into phosphatidylcholine catalyzed by rat liver microsomal CDP-choline: 1,2-diacyl-sn-glycerol cholinephosphotransferase was studied. In the presence of phosphatidylcholine transfer protein, the incorporation of CDP-choline into phosphatidylcholine was markedly stimulated. Phosphatidylcholine transfer protein isolated from either rat or bovine liver was capable of this stimulatory effect; in contrast, phosphatidylinositol transfer protein from rat liver had no effect on phosphatidylcholine synthesis. Kinetic analysis showed that microsomal phosphatidylcholine synthesis increased 2.4-fold after 1 min and reached a maximum of approximately 10-fold within 10 min in the presence of phosphatidylcholine transfer protein; in the absence of this protein phosphatidylcholine synthesis stopped after 2-4 min. These results suggest that phosphatidylcholine transfer protein permits phosphatidylcholine synthesis to proceed further. With the addition of phospholipid vesicles, as an acceptor membrane in the reaction mixture, there was a significant amount of protein-mediated transfer of synthesized phosphatidylcholine to the vesicles. Measurable transfer of synthesized phosphatidylcholine to vesicles could only be detected after a lag of 2-4 min. The stimulation of cholinephosphotransferase could be nearly abolished by increasing the amount of added phospholipid vesicles; concurrently, a greater transfer to the vesicles was observed. These results describe a new property of phosphatidylcholine transfer protein which may be of physiological significance in the regulation of phosphatidylcholine synthesis in mammalian tissues.     

Membrane proteins exposed on the external side of the intestinal brush-border membrane have fusogenic properties

The intestinal brush-border membrane contains one or several membrane proteins that mediate fusion and/or aggregation of small unilamellar egg phosphatidylcholine vesicles. The fusion is accompanied by a partial loss of vesicle contents. Proteolytic treatment of the brush-border membrane with proteinase K abolishes the fusogenic property. This finding suggests that the fusogenic activity is associated with a membrane protein exposed on the external or luminal side of the brush-border membrane. Activation of intrinsic proteinases of the brush-border membrane liberates water-soluble proteins (supernate proteins). These proteins behave in an analogous way to intact brush-border membrane vesicles; they induce fusion of egg phosphatidylcholine vesicles and render the egg phosphatidylcholine bilayer permeable to ions and small molecules (Mr less than or equal to 5000). Furthermore, supernate proteins mediate phosphatidylcholine and cholesterol exchange between two populations of small, unilamellar phospholipid vesicles. Supernate proteins are fractionated on Sephadex G-75 SF yielding three protein peaks of apparent Mr greater than or equal to 70,000, Mr = 22,000 and Mr = 11,500. All three protein fractions show similar phosphatidylcholine-exchange activity, but they differ in their effects on the stability of egg phosphatidylcholine vesicles. The protein fraction with an apparent Mr greater than or equal to 70,000 has the highest fusogenic activity while the protein fraction of apparent Mr = 11,500 appears to be most effective in rendering the egg phosphatidylcholine bilayer permeable.     

The effect of the inhibitor diphenylene iodonium on the superoxide-generating system of neutrophils. Specific labelling of a component polypeptide of the oxidase.

Horse kidney brush border membrane proteins were incorporated into phosphatidylcholine vesicles. Structural analysis of proteoliposomes prepared with various lipid:protein ratios showed that: (a) only a few of the proteins present in the crude brush border extract are integrated, (b) all known membrane hydrolases are integrated, and (c) these proteoliposomes are homogeneous vesicles. Papain solubilization of brush border membrane hydrolases, i.e. aminopeptidase M, neutral alpha-glucosidase, gamma-glutamyltransferase and alkaline phosphatase, performed in parallel on native membrane vesicles and proteoliposomes, revealed similar kinetics. Analysis of membrane vesicles and proteoliposomes on sucrose density gradients either without any treatment, or after papain treatment showed that: (a) in proteoliposomes, neutral alpha-glucosidase is associated with radiolabelled phosphatidylcholine, and (b) papain-treated vesicles and proteoliposomes released enzyme activity in the same way. These results suggest that the integration mechanism of brush border membrane proteins may be similar in proteoliposomes and native membrane vesicles. Transport experiments under equilibrium exchange conditions showed that the uptake properties of proteoliposomes are similar to those of brush border membrane vesicles.     

Asymmetry and transposition rates of phosphatidylcholine in rat erythrocyte ghosts.

Purified phospholipid exchange protein from beef heart cytosol is used to accelerate the exchange of phospholipids between labeled sealed ghosts and phosphatidylcholine/cholesterol liposomes. The purified protein accelerates the transfer of phosphatidylcholine and, to a lesser degree, that of sphingomyelin, phosphatidylinositol, and lysophosphatidylcholine. The presence of exchange protein does not accelerate the exchange of phospholipids between intact red blood cells and liposomes, but 75% of the phosphatidylcholine of sealed ghosts is readily available for exchange. The remaining 25% is also exchangeable but at a slower rate. When the exchange is assayed between inside-out vesicles and liposomes, 37% of the phosphatidylcholine is readily available, and 63% is exchanged at a slower rate. These results are consistent with an asymmetric distribution of phosphatidylcholine in isolated erythrocyte membrane fractions. The sum of the forward and backward transposition of phosphatidylcholine between the inside and outside layers of sealed ghost membranes amounts to 11% per hour, and the half-time for equilibration is 2.3 h. Significatnly lower values are obtained for the inside-out vesicles (half-time for equilibration: 5.3 h). These results suggest that, during the formation of the vesicles, the asymmetry of phosphatidylcholine is partially preserved, but structural changes occur in the membrane that affect the rate of membrane transposition of phosphatidylcholine.     

Effect of 1,25-dihydroxyvitamin D3 on phospholipid metabolism in chick duodenal mucosal cell. Relationship to its mechanism of action

Pretreatment of the D-deficient chick with 1,25-dihydroxyvitamin D3 increases de novo synthesis of phosphatidylcholine by a stimulation of CDP-choline: sn-1,2-diacylglycerol choline-phosphotransferase reaction. The time course of change in the incorporation of [3H]choline and [14C]ethanolamine into the brush border lipid fraction after 1,25-dihydroxyvitamin D3 treatment correlates closely with the time course of change in calcium uptake into the brush border membrane vesicles. Prior treatment with cycloheximide does not block this increase in phosphatidylcholine synthesis. In addition, 1,25-dihydroxyvitamin D3 administration increases the incorporation of [3H]arachidonic acid into the phosphatidylcholine fraction of the brush border to a great extent but does not increase the incorporation of [3H]palmitic acid into the phosphatidylcholine fraction. The incorporation of these 3H labeled fatty acids into diacylglycerol is not changed by 1,25-dihydroxyvitamin D3. These data indicate that 1,25-dihydroxyvitamin D3 enhances the synthesis of phosphatidylcholine independent of new protein synthesis, and also increases the incorporation of unsaturated fatty acids into phosphatidylcholine. From these results we suggest that changes in phospholipid metabolism in the enterocyte are the mechanisms by which 1,25-dihydroxyvitamin D3 acts to enhance calcium entry across the brush border membrane.     

The mechanism of the solute-induced chain interdigitation in phosphatidylcholine vesicles and characterization of the isothermal phase transitions by means of dynamic light scattering.

A new method is introduced for the detection of chain interdigitation in phospholipid bilayers. The same method is used to measure the hydrocarbon tilt in the dipalmitoylphosphatidylcholine membranes as a function of the bulk concentration of the interdigitation-inducing solutes, such as ethanol. The hydrocarbon tilt in the phosphatidylcholine bilayers is demonstrated to be limited to angles below approx. 51 degrees. The need for higher tilt values leads to bilayer interdigitation. Solute-induced chain interdigitation is shown to be a cooperative process provoked by the excessively large lateral repulsion in the interfacial region and the concomitant excessive chain tilt. Ethanol-induced phosphatidylcholine interdigitation, for example, proceeds via interdigitated domains formation and finally gives rise to the bilayers with fully intercalated chains tilted by at least 30 degrees (and sometimes as much as 50 degrees) with respect to the membrane normal.     

The lipid environment of the glucagon receptor regulates adenylate cyclase activity.

1. The lipids composition of rat liver plasma membranes was substantially altered by introducing synthetic phosphatidylcholines into the membrane by the techniques of lipid substitution or lipid fusion. 40-60% of the total lipid pool in the modified membranes consisted of a synthetic phosphatidylcholine. 2. Lipid substitution, using cholate to equilibrate the lipid pools, resulted in the irreversible loss of a major part of the adenylate cyclase activity stimulated by F-, GMP-P(NH)P or glucagon. However, fusion with presonicated vesicles of the synethic phosphatidylcholines causes only small losses in adenylate cyclase activity stimulated by the same ligands. 3. The linear form of the Arrhenius plots of adenylate cyclase activity stimulated by F- or GMP-(NH)P was unaltered in all of the membrane preparations modified by substitution or fusion, with very similar activation energies to those observed with the native membrane. The activity of the enzyme therefore appears to be very insensitive to its lipid environment when stimulated by F- or gmp-p(nh)p. 4. in contrast, the break at 28.5 degrees C in the Arrhenius plot of adenylate cyclase activity stimulated by glucagon in the native membrane, was shifted upwards by dipalmitoyl phosphatidylcholine, downwards by dimyristoyl phosphatidylcholine, and was abolished by dioleoyl phosphatidylcholine. Very similar shifts in the break point were observed for stimulation by glucagon or des-His-glucagon in combination with F- or GMP-P(NH)P. The break temperatures and activation energies for adenylate cyclase activity were the same in complexes prepared with a phosphatidylcholine by fusion or substitution. 5. The breaks in the Arrhenius plots of adenylate cyclase activity are attributed to lipid phase separations which are shifted in the modified membranes according to the transition temperature of the synthetic phosphatidylcholine. Coupling the receptor to the enzyme by glucagon or des-His-glucagon renders the enzyme sensitive to the lipid environment of the receptor. Spin-label experiments support this interpretation and suggest that the lipid phase separation at 28.5 degrees C in the native membrane may only occur in one half of the bilayer.     

Substrate specificities of the enzymes of the oleate desaturase system from photosynthetic tissue.

In the microsomal fraction from young pea (Pisum sativum L.) leaves, the oleoyl moieties from oleoyl-CoA are principally transferred to the sn-2 position of phosphatidylcholine by oleoyl-CoA:1-acyl-lysophosphatidylcholine acyltransferase. The major product of this acyl transfer is 1-palmitoyl(stearoyl)-2-oleoyl phosphatidylcholine. The 1-palmitoyl(stearoyl)-2-oleoyl phosphatidylcholine is subsequently converted into 1-palmitoyl(stearoyl)-2-linoleoyl phosphatidylcholine by the oleate desaturase complex without equilibrating with the bulk membrane phosphatidylcholine pool. Hence, both the acyl transfer to phosphatidylcholine and the subsequent desaturation of oleoyl moieties occur on the sn-2 position of phosphatidylcholine, and there is also a functional coupling of the acyltransferase and oleate desaturase.     

The effect of phosphatidylcholine to sphingomyelin mole ratio on the dynamic properties of sheep erythrocyte membrane

Sheep red blood cells are shown to incorporate phosphatidylcholine when incubated in human plasma in the presence of EGTA. This treatment results in up to a 5-fold increase in mol ratio of phosphatidylcholine to sphingomyelin. By replacing EGTA with Ca²⁺ the increase of phosphatidylcholine content is completely inhibited, due to the activation of the membrane bound lecithinase which rapidly degrades the incorporated phosphatidylcholine. Analogous treatments of the isolated erythrocyte membranes resulted in similar phosphatidylcholine incorporation but in the presence of Ca²⁺ a residual phosphatidylcholine uptake was still observed. These results suggest that in the isolated membranes small amounts of phosphatidylcholine can be incorporated into an additional region which is unavailable for the membrane lecithinase. The increase in the phosphatidylcholine to sphingomyelin mol ratio in sheep red blood cells is concomitant with an increase in lipid fluidity, as well as increase in osmotic fragility.     

Transbilayer distribution of lipids in a population of sarcoplasmic-reticulum vesicles sealed with their cytoplasmic side outwards.

A population of sarcoplasmic-reticulum vesicles, all of which were sealed with their cytoplasmic side outwards, was obtained by density-gradient centrifugation after loading the vesicles with calcium oxalate. The calcium oxalate could subsequently be removed from the vesicles by the reverse action of the calcium-transport system. Measurements of the catalysed exchange of the phosphatidylcholine in the sarcoplasmic-reticulum cytoplasmic monolayer with an exogenous phosphatidylcholine pool suggested that phosphatidylcholine is symmetrically distributed across the sarcoplasmic-reticulum membrane. A similar result was obtained for phosphatidylethanolamine when sarcoplasmic-reticulum lipids were labelled with trinitrobenzenesulphonic acid. Further catalysed lipid-exchange reactions showed that the transverse movement of phosphatidylcholine across the membrane was exceedingly slow (t 1/2 greater than 15 days).     

Effect of streptolysin S on liposomes. Influence of membrane lipid composition on toxin action

The effect of the bacterial cytolytic toxin, streptolysin S, on liposomes composed of various phospholipids was investigated. Large unilamellar vesicles containing [14C]sucrose were prepared by reverse-phase evaporation, and membrane damage produced by the toxin was measured by following the release of labeled marker. The net charge of the liposomes had little or no effect on their susceptibility to steptolysin S and the toxin was about equally effective on liposomes composed of phosphatidylcholine, phosphatidylethanolamine, phosphatidylserine and phosphatidylglycerol. Experiments with liposomes composed of synthetic phospholipids showed that the ability of the toxin to produce membrane damage depended on the degree of unsaturation of the fatty acyl chains. The order of sensitivity was C18 : 2 phosphatidylcholine greater than C18: I phosphatidylcholine greater than C18 : 0 phosphatidylcholine = C16 : 0 phosphatidylcholine. Liposomes containing the latter two phospholipids were virtually unaffected by streptolysin S, and experiments with C18 : 0 phosphatidylcholine suggested that toxin activity does not bind to liposomes composed of phospholipids with saturated fatty acyl chains. The inclusion of 40 mol% cholesterol in C16 : 0 phosphatidylcholine and C18 : 0 phosphatidylcholine liposomes made these vesicles sensitive to streptolysin S. Egg phosphatidylcholine liposomes, which were unaffected at 0 degrees C and 4 degrees C became susceptible to the toxin at these temperatures when cholesterol was included. Liposomes composed of C14 : 0 phosphatidylcholine were unaffected by streptolysin S at temperatures below the chain-melting transition temperature (23 degrees C) of this phospholipid, but became increasingly susceptible above this temperature. The results suggest that the fluidity of the phospholipid hydrocarbon chains in the membrane is important in streptolysin S action.     

Probing the lipid membrane dipole potential by atomic force microscopy

The electrostatic properties of biological membranes can be described by three parameters: the transmembrane potential, the membrane surface potential, and the membrane dipole potential. The first two are well characterized in terms of their magnitudes and biological effects. The dipole potential, however, is not well characterized. Various methods to measure the membrane dipole potential indirectly yield different values, and there is not even agreement on the source of the membrane dipole moment. This ambiguity impedes investigations into the biological effects of the membrane dipole moment, which should be substantial considering the large interfacial fields with which it is associated. Electrostatic analysis of phosphatidylcholine lipid membranes with the atomic force microscope reveals a repulsive force between the negatively charged probe tips and the zwitterionic lipids. This unexpected interaction has been analyzed quantitatively to reveal that the repulsion is due to a weak external field created by the internal membrane dipole potential. The analysis yields a dipole moment of 1.5 Debye per lipid with a dipole potential of +275 mV for supported phosphatidylcholine membranes. This new ability to quantitatively measure the membrane dipole moment in a noninvasive manner with nanometer scale spatial resolution will be useful in identifying the biological effects of the dipole potential.     

Influence of enzymatic phospholipid cleavage on the permeability of the erythrocyte membrane. I. Transport of non-electrolytes via the lipid domain.

In order to further elucidate the influence of membrane lipids on transport via the lipid domain of the erythrocyte membrane, simple non-electrolyte diffusion was investigated by tracer flux measurements in whole cells after cleavage of up to 65% of phosphatidylcholine or sphingomyelin by phospholipase A2 from Naja naja, or by sphingomyelinase. A new type of labelled model non-electrolyte was used in this study, readily available by reacting a non-labelled thiol with a labelled alkylating SH-reagent. In spite of the marked enzymatic alterations of the membrane, which lead to the occurrence of large quantities of lysophosphatidylcholine and long chain fatty acids, or of ceramide, the permeability of the lipid domain remained unaffected. This finding is very surprising, since the physical properties of the lipid phase (microviscosity, structure of the membrane interface) are likely to be perturbed in the enzyme-treated membranes. Sphingomyelinase-treated cells undergo stomatocytic shape changes followed by deep invaginations of the membrane and finally endocytosis, while phospholipase A2-treated cells essentially maintain their normal shape.