Interaction of cholesterol, phospholipid and apoprotein in high density lipoprotein recombinants.

To examine the effect of incorporation of cholesterol into high density lipoprotein (HDL) recombinants, multilamellar liposomes of 3H cholesterol/14C dimyristoyl phosphatidylcholine were incubated with the total apoprotein (apoHDL) and principal apoproteins (apoA-1 and apoA-2) of human plasma high density lipoprotein. Soluble recombinants were separated from unreacted liposomes by centrifugation and examined by differential scanning calorimetry and negative stain electron microscopy. At 27 degrees C, liposomes containing up to approx. 0.1 mol cholesterol/mol dimyristoyl phosphatidylcholine (DMPC) were readily solubilized by apoHDL, apoA-1 or apoA-2. However, the incorporation of DMPC and apoprotein into lipoprotein complexes was markedly reduced when liposomes containing a higher proportion of cholesterol were used. For recombinants prepared from apoHDL, apoA-1, or apoA-2, the equilibrium cholesterol content of complexes was approx. 45% that of the unreacted liposomes. Electron microscopy showed that for all cholesterol concentrations, HDL recombinants were predominantly lipid bilayer discs, approx. 160 X 55 A. Differential scanning calorimetry of cholesterol containing recombinants of DMPC/cholesterol/apoHDL or DMPC/cholesterol/apoA-1 showed, with increasing cholesterol content, a linear decrease in the enthalpy of the DMPC gel to liquid crystalline transition, extrapolating to zero enthalpy at 0.15 cholesterol/DMPC. The enthalpy values were markedly reduced compared to control liposomes, where the phospholipid transition extrapolated to zero enthalpy at approx. 0.45 cholesterol/DMPC. The calorimetric and solubility studies suggest that in high density lipoprotein recombinants cholesterol is excluded from 55% of DMPC molecules bound in a non-melting state by apoprotein.     

Interaction of cholesterol,phospholipid and apoprotein in high density lipoprotein recombinants

To examine the effect of incorporation of cholesterol into high density lipoprotein (HDL) recombinants, multilamellar liposomes of ³H cholesterol/¹⁴C dimyristoyl phosphatidylcholine were incubated with the total apoprotein (apoHDL) and principal apoproteins (apoA-1 and apoA-2) of human plasma high density lipoprotein. Soluble recombinants were separated from unreacted liposomes by centrifugation and examined by differential scanning calorimetry and negative stain electron microscopy. At 27°C, liposomes containing up to approx. 0.1 mol cholesterol/mol dimyristoyl phosphatidylcholine (DMPC) were readily solubilized by apoHDL, apoA-1 or apoA-2. However, the incorporation of DMPC and apoprotein into lipoprotein complexes was markedly reduced when liposomes containing a higher proportion of cholesterol were used. For recombinants prepared from apoHDL, apoA-1 or apoA-2, the equilibrium cholesterol content of complexes was approx. 45% that of the unreacted liposomes. Electron microscopy showed that for all cholesterol concentrations, HDL recombinants were predominantly lipid bilayer discs, approx. $\text{160 × 55}\text{A}\text{?}$. Differential scanning calorimetry of cholesterol containing recombinants of DMPC/cholesterol/apoHDL or DMPC/cholesterol/apoA-1 showed, with increasing cholesterol content, a linear decrease in the enthalpy of the DMPC gel to liquid crystalline transition, extrapolating to zero enthalpy at 0.15 cholesterol/DMPC. The enthalpy values were markedly reduced compared to control liposomes, where the phospholipid transition extrapolated to zero enthalpy at approx. 0.45 cholesterol/DMPC. The calorimetric and solubility studies suggest that in high density lipoprotein recombinants cholesterol is excluded from 55% of DMPC molecules bound in a non-melting state by apoprotein.     

Dye permeability at phase transitions in single and binary component phospholipid bilayers

By encapsulating a pH-sensitive dye, phenol red, in multilamellar liposomes of DMPC, DPPC and DMPC/DPPC mixtures, the permeability of these phospholipid bilayers to dye as a function of temperature has been studied. For both DMPC and DPPC liposomes, dye release begins well below the main gel-to-liquid-crystalline phase transition (24°C and 42°C, respectively) at temperatures corresponding to the onset of the pretransition (about 14°C and 36°C, respectively) with DPPC liposomes exhibiting a permeability anomaly at the main phase transition (42°C). The perturbation occurring in the bilayer structure that allows the release of encapsulated phenol red (approx. 5 Å diameter) is not sufficient to permit the release of encapsulated haemoglobin (approx. 20 Å diameter, negatively charged). In liposomes composed of a range of DMPC/DPPC mixtures, dye release commences at the onset of the pretransition range (determined by optical absorbance measurements) and increases with increasing temperature until the first appearance of liquid crystalline phase after which no further dye release occurs. Interestingly, the dye retaining properties of DMPC and DPPC liposomes well below their respective pretransition temperature regions are very different: DMPC liposomes release much encapsulated dye at incubation temperatures of 5°C whilst DPPC liposomes do not.     

A fluorescent sterol probe study of cholesterol/phospholipid membranes

The behavior of dehydroergosterol in -α-dimyristoylphosphatidylcholine (DMPC) unsonicated multilamellar liposomes was characterized by absorption spectroscopy and fluorescence measurements. Dehydroergosterol exhibited a lowered absorption coefficient in multilamellar liposomes whiel the steady-state fluorescence anisotropy of dehydroergosterol in these membranes decreased significantly with increasing dehydroergosterol concentration, suggesting membrane sterol-sterol interactions. The comparative steady-state anisotropy of 0.9 mole percent dehydroergosterol in multilamellar liposomes was lower than in small unilamellar vesicles suggesting different sterol environments for dehydroergosterol. Dehydroergosterol fluorescence lifetime was relatively independent of membrane sterol content and yielded similar values in sonicated and unsonicated model membranes. In multilamellar liposomes containing 5 mole percent cholesterol, the gel-to-liqui crystalline phase transition of DMPC detected by 0.9 mole percent dehydroergosterol was significantly broadened when compared to the phase transition detected by dehydroergosterol in the absence of membrane cholesterol (Smutzer, G. et al. (1986) Biochim. Biophys. Acta 862, 361–371). In multilamellar liposomes containing 10 mole percent cholesterol, the major fluorescence lifetime of dehydroergosterol did not detect the gel-to-liquid crystalline phase transition of DMPC. Time-correlated fluorescence anisotropy decays of dehydroergosterol in DMPC multilamellar liposomes in the absence and presence of 5 mole percent cholesterol exhibited a single rotational correlation time near one nanosecond that was relatively independent of temperature and low concentrations of membrane cholesterol. The limiting anisotropy of 0.9 mole percent dehydroergosterol decreased above the gel-to-liquid crystalline phase transition in membranes without cholesterol and was not significantly affected by the phase transition in membranes containing 5 mole percent cholesterol. These results suggested hindered rotational diffusion of dehydroergosterol in multilamellar liposomes. Lifetime and time-correlated fluorescence measurements of 0.9 mole percent dehydroergosterol in multilamellar liposomes further suggested this fluorophore was detecting physical properties of the bulk membrane phospholipids in membranes devoid of cholesterol and was detecting sterol-rich regions in membranes of low sterol concentration.     

Spectroscopic and calorimetric studies on trazodone hydrochloride–phosphatidylcholine liposome interactions in the presence and absence of cholesterol

The interaction of antidepressant drug trazodone hydrochloride (TRZ) with dipalmitoyl phosphatidylcholine (DPPC) multilamellar liposomes (MLVs) in the presence and absence of cholesterol (CHO) was investigated as a function of temperature by using Electron Paramagnetic Resonance (EPR) spin labeling, Fourier Transform Infrared (FTIR) Spectroscopy and Differential Scanning Calorimetry (DSC) techniques. These interactions were also examined for dimyristoyl phosphatidylcholine (DMPC) multilamellar liposomes by using Electron Paramagnetic Resonance (EPR) spin labeling technique. In the EPR spin labeling studies, 5- and 16-doxyl stearic acid (5-DS and 16-DS) spin labels were used to monitor the head group and alkyl chain region of phospholipids respectively. The results indicated that TRZ incorporation causes changes in the physical properties of PC liposomes by decreasing the main phase transition temperature, abolishing the pre-transition, broadening the phase transition profile, and disordering the system around the head group region. The interaction of TRZ with unilamellar (LUV) DPPC liposomes was also examined. The most pronounced effect of TRZ on DPPC LUVs was observed as the further decrease of main phase transition temperature in comparison with DPPC MLVs. The mentioned changes in lipid structure and dynamics caused by TRZ may modulate the biophysical activity of membrane associated receptors and in turn the pharmacological action of TRZ.     

Comparison of steady-state fluorescence polarization and urea permeability of phosphatidylcholine and phosphatidylsulfocholine liposomes as a function of sterol structure

The well-known reduction in the permeability properties of liposomes of dimyristoylphosphatidylcholine (DMPC) by sterols has also been demonstrated for its sulfonium analog (DMPSC) in which the N+(CH3)3 group of choline is replaced by S+(CH3)2. We have now compared the effects of 25 mol% 24-methylenecholesterol and cholesterol on the initial rates of urea permeation into dipalmitoyl-PC (DPPC) and dipalmitoyl-PSC (DPPSC) liposomes above the gel-to-liquid-crystalline phase transition temperature and found a greater reduction with 24-methylenecholesterol/DPPSC than with cholesterol/DPPSC liposomes but little difference between the two sterols in DPPC liposomes. Fluorescence polarization studies, using diphenylhexatriene as a probe, show that polarization (P) values are considerably higher in DMPSC liposomes containing 20 and 30 mol% 24-methylenecholesterol than in DMPC liposomes containing 20 and 30 mol% cholesterol. Higher P values were also obtained in DMPSC liposomes containing other 24-alkyl-substituted sterols (beta-sitosterol, ergosterol and campesterol) than in DMPC liposomes containing the same sterols. Reduced permeability rates in PSC liposomes containing 24-alkyl-substituted sterols are correlated with higher polarization values, reflecting an increased degree of order and/or motion in these liposomes compared with liposomes from the corresponding PC. These results suggest that alkyl substitution at C-24 of the sterol molecule results in tighter interactions with the sulfonium analog of PC than with PC.     

Physical biochemistry of a liposomal amphotericin B mixture used for patient treatment

There seems little doubt now that intravenous liposomal amphotericin B can be a useful treatment modality for the management of immunocompromised patients with suspected or proven disseminated fungal infections. Interestingly, the very significant reduction in toxicity reported when amphotericin B is part of a bilayer membrane is closely tied to the physical characteristics of the liposomes involved, although these are poorly understood at the molecular level. We record here an examination by spectroscopy and freeze-etch electron microscopy of unsonicated amphotericin B multilamellar vesicles prepared along the lines that we and others have followed for samples used in clinical trials and preclinical in vivo or in vitro studies. Our study has focussed on liposomes of 7:3 dimyristoylphosphatidylcholine/dimyristoylphosphatidylglycerol (DMPC/DMPG) bearing 0-25 mol% amphotericin B, since this lipid mixture has been the choice for the first clinical trials. Phase transition behaviour of these liposomes was examined by electron paramagnetic resonance (EPR) spectroscopy of a nitroxide spin label partitioning into the bilayers. The same experiments were then performed on similarly prepared liposomes of the disaturated species, dipalmitoylphosphatidylcholine (DPPC), and the diunsaturated species, dielaidoylphosphatidylcholine (DEPC). Partial phase diagrams were constructed for each of the lipid/drug mixtures. Melting curves and derived phase diagrams showed evidence that amphotericin B is relatively immiscible with the solid phase of bilayer membranes. The phase diagram for DEPC/amphotericin B was very similar to that of DPPC/amphotericin B, and both exhibited less extensive temperature ranges of phase separation than did the 7:3 DMPC/DMPG mixture with amphotericin B. Between 25 and 37 degrees C the measured fluidity of the 7:3 DMPC/DMPG liposomes was similar to that of the (unsaturated fatty acid) DEPC liposomes, and considerably higher than that seen for (saturated fatty acid) DPPC liposomes. Preparations of 7:3 DMPC/DMPG, DPPC, and DEPC containing 0-25 mol% amphotericin B were examined by freeze-etch electron microscopy at 35 and 22 degrees C (to cover the temperature range of the mammalian body core and periphery). The same liposome features were present in all three liposome types studied. The appearance of individual liposomes at x 100,000 magnification reflected their molecular characteristics, which were found to be significantly heterogeneous within each batch. The lipid/drug structures were bilayer in nature, although liposomes showing considerable disruption were common, particularly at the highest drug concentrations.(ABSTRACT TRUNCATED AT 400 WORDS)     

The effects of cholesterol and β-sitosterol on the structure of saturated diacylphosphatidylcholine bilayers

The structures of DMPC and DPPC bilayers in unilamellar liposomes, in the presence of 33.3 mol% cholesterol or the plant sterol β-sitosterol, have been studied by small-angle neutron scattering. The bilayer thickness d _L increases in a similar way for both sterols. The repeat distance in multilamellar liposomes, as determined by small-angle X-ray diffraction, is larger in the presence of β-sitosterol than in the presence of cholesterol. We observe that each sterol modifies the interlamellar water layer differently, cholesterol reducing its thickness more efficiently than β-sitosterol, and conclude that cholesterol suppresses bilayer undulations more effectively than β-sitosterol.     

The antioxidant activity of alpha-tocopherol-bovine serum albumin complex in micellar and liposome autoxidations

A comparison is made of the antioxidant activity of a water-soluble form of alpha-tocopherol complexed with bovine serum albumin (alpha-T X BSA) with that of micellar alpha-tocopherol and aqueous 6-hydroxy-2,5,7,8-tetramethylchroman-2-carboxylate (Trolox) to inhibit autoxidation of linoleic acid in sodium dodecyl sulfate micelles. The peroxyl radical trapping ability of alpha-T X BSA compares favorably with that of alpha-tocopherol and Trolox, and all three can be used in quantitative measurements of the susceptibility of the micellar substrate to undergo autoxidation: the oxidizability, for reactions initiated in the micellar phase by di-tertbutylhyponitrite (DBHN) or in the aqueous phase by azobisamidinopropane hydrochloride (ABAP). alpha-Tocopherol and Trolox are also effective antioxidants to inhibit DBHN- or ABAP-initiated autoxidations of dilinoleoylphosphatidylcholine (DLPC) liposomes prepared as multilamellar or unilamellar bilayers characterized by 31P NMR spectra. The oxidizability of DLPC liposomes is determined by various combinations of water-soluble and lipid-soluble initiators and the antioxidants, alpha-tocopherol and Trolox. In contrast, alpha-T X BSA does not effectively trap peroxyl radicals when it is added after initiation of autoxidation in the lipid phase (DBHN) or in the aqueous phase (ABAP). The radical trapping ability of alpha-T X BSA becomes evident if it is mixed with the DLPC for some hours before initiation. This result is interpreted in terms of diffusion of alpha-tocopherol from the bound alpha-T X BSA form to the liposome before it exhibits antioxidant activity.     

The transfer of antioxidants between liposomes

Abstract

A quantitative study is reported on a comparison of antioxidant action of the a-tocopherol model 2,2,5,6,7-pentamethyl-6-hydroxychroman (PMHC), 2,6-di-tert-butyl-4-methoxvphenol (DBHA) and a-tocopherol when these antioxidants are delivered to peroxidizing dilinoleoylphosphatidylcholine (DLPC) bilayers by (a) intermembrane transfer between donor dimyristoylphosphatidylcholine (DMPC) liposomes and acceptor DLPC liposomes, versus (b) the conventional coevaporation methods. The profiles of inhibited oxidation using method (a) were comparable with those of method (b) during inhibited peroxidation of DLPC liposomes, initiated by lipid-soluble azo-bis-dimethylvaleronitrile (ADVN), and quantitative determinations of the rate of chain initiation, R_i, were the same for the two methods. Differences observed, between antioxidant action by methods (a) and (b), when using the water-soluble initiator azo-bis-amidinopropane hydrochloride (ABAP), are attributed to slow diffusion of ABAP through the multilamellar DLPC system and a resultant non-uniform initiation. PMHC underwent facile intermembrane transfer through a barrier of dialysis tubing, from donor DMPC liposomes to acceptor DLPC or DMPC liposomes, based on analytical and quantitative inhibition studies. α-Tocopherol was comparatively slow to undergo intermembrane transfer by direct contact between liposomes and transferred only slightly through a barrier. The slight but measurable solubility of PMHC and DBHA in the aqueous phase supports a pathway of intermembrane transfer involving a water-soluble intermediate.     

Influence of phorbol esters on ionophore-mediated calcium exchange-diffusion in liposomes

The interference of phorbol esters upon the process of A23187-mediated calcium exchange diffusion was examined in multilamellar liposomes formed of different types of lipids and incubated at variable temperatures. Phorbol esters facilitated the process of calcium ionophoresis in liposomes formed of dipalmitoylphosphatidylcholine (DPPC) or dimyristoylphosphatidyl-choline (DMPC) and incubated below transition temperature. The magnitude of this facilitating action was negatively correlated with the tumor-promoting capacity of the phorbol esters. The phorbol esters also facilitated calcium ionophoresis in liposomes formed of a mixture of DPPC and cholesterol, provided that the temperature exceeded 34 degrees C. The magnitude of the latter facilitating action was positively correlated with both the temperature and the tumor-promoting potency of the phorbol esters. Thus, the existence of a parallelism between the biological potency of phorbol esters and their biophysical effect in this artificial system tightly depended on such factors as the lipid composition of the liposomal matrix and the ambient temperature.     

Solubilization of Multilamellar Liposomes in the Presence of Non-ionized Drug

The solubilization of multilamellar liposomes by metoprolol tartrate (MPL) has been studied as a function of pH, [MPL], [dimyristoylphosphatidylcholine (DMPC)], temperature and lipid composition. The solubilization of liposomes at 37° C by 7.3 mM MPL occurred at different rates at different pH values. MPL completely solubilized by 7.2 mM DMPC liposomes after about 17 hat pH 12, but only a partial solubilization occurred at pH 10 and 11. Between pH 7 and 9 no change in turbidity was observed after 1 week. Addition of cholesterol (CHOL) to DMPC (2:1 mol) had very little effect on solubilization after 24 h, however with DMPC:CHOL (5:1 mol) the decrease in turbidity was observed after 24 h, even though solubilization was much less compared with that of DMPC alone. The rate of solubilizaiton was decreased when dipalmitoylphosphatidylcholine liposomes were employed. Addition of dicetylphosphate (DCP) to DMPC liposomes reduced the rate of solubilization significantly. The solubilization of liposomes by 7.3 mM MPL as a function of [DMPC], indicated that the lower the liposome concentration the greater the effect on solubilization. It is concluded that MPL in the non-ionized form has a solubilizing effect on liposomes, and addition of CHOL or DCP to DMPC has a stabilizing effect against solubilization.     

Ethanolamine plasmalogen and cholesterol reduce the total membrane oxidizability measured by the oxygen uptake method

To investigate the effects of ethanolamine plasmalogen, phosphatidylethanolamine, cholesterol, and alpha-tocopherol on the oxidizability of membranes, various large unilamellar vesicles (LUVs) including these lipids and antioxidant were examined for their total membrane oxidizabilities, evaluated as R(p)/R(i)(1/2) value (where R(p) is rate of oxygen consumption and R(i)(1/2) is the square root of rate of chain initiation) by the oxygen uptake method with water-soluble radical initiator and inhibitor. Incorporation of bovine brain ethanolamine plasmalogen (BBEP) into vesicles as well as cholesterol led to lower the total membrane oxidizability dose-dependently. The effect of BBEP was more efficient in the presence of cholesterol in vesicles. On the other hand, diacyl counterpart, egg yolk phosphatidylethanolamine, and a typical radical scavenger, alpha-tocopherol, had no effect on the membrane oxidizability. Alpha-tocopherol only prolonged an induction period dose-dependently in the present oxidizing system, suggesting a novel antioxidant mechanism of ethanolamine plasmalogens besides the action of scavenging radicals.     

Binding of insulin to the external surface of liposomes: Effect of surface curvature,temperature, and lipid composition

The binding of insulin to the external surface of phosphatidylcholine liposomes as a function of the temperature, the surface curvature, and the composition of lipids was studied. The amount of the saturated binding of insulin to liposomes was assessed by gel-filtration chromatography. The binding of insulin to small unilamellar vesicles was highly dependent upon the temperature, favoring low temperatures. As the temperature increased, there was a distinct temperature range where the binding of insulin to small unilamellar vesicles decreased. The temperature ranges for dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC) small unilamellar vesicles were found to be 10–20°C and 21–37°C, respectively. These temperature ranges were quite different from the reported ranges of the gel → liquid crystalline phase transition temperatures ($\text{T}{}_{\mathrm{<mtext>c</mtext>}}$) for DMPC or DPPC small unilamellar vesicles. In contrast to other proteins, the amount of insulin bound to DMPC and DPPC small unilamellar vesicles was negligible at or above the upper limit of the above temperature ranges, and increased steadily to 6–7 μmol of insulin per mmol of phospholipid as the temperature decreased to or below the lower limit of these temperature ranges. On the other hand, the binding of insulin to the large multilamellar liposomes cannot be detected at all temperatures tested. The affinity of insulin to neutral phosphatidylcholine small unilamellar vesicles appeared to be related to the surface curvature of the liposomes, favoring the liposomes with a high surface curvature. Furthermore, the amount of insulin bound to small unilamellar vesicles decreased as the content of the cholesterol increased. The presence of 10% molar fraction of phosphatidic acid did not appear to affect the binding of insulin to small unilamellar vesicles. However, the presence of 5% molar fraction of stearylamine in DPPC small unilamellar vesicles increased the amount of bound insulin as well as the extent of aggregation of liposomes. The results of the present study suggest that the interstitial regions of the acyl chains of phospholipids between the faceted planes of small unilamellar vesicles below $\text{T}{}_{\mathrm{<mtext>c</mtext>}}$ may be responsible for the hydrophobic interaction of insulin and small unilamellar vesicles. The tight binding of insulin to certain small unilamellar liposomes could lead to an overestimation of the true amount of insulin encapsulated in liposomes, if care is not taken to eliminate the bound insulin during the procedure of encapsulating insulin in liposomes.     

Chlorpromazine interaction with glycerophospholipid liposomes studied by magic angle spinning solid state (13)C-NMR and differential scanning calorimetry

Phosphatidylserine (PS) extracted from pig brain and synthetic dipalmitoylphosphatidylcholine (DPPC) and dimyristoylphosphatidylcholine (DMPC) were used to make DPPC/DMPC and DPPC/PS large unilamellar liposomes with a diameter of approximately 1 microm. Chlorpromazine-HCl (CPZ), an amphipathic cationic psychotropic drug of the phenothiazine group, is known to partition into lipid bilayer membranes of liposomes with partition coefficients depending on the acyl chain length and to alter the bilayer structure in a manner depending on the phospholipid headgroups. The effects of adding CPZ to these membranes were studied by differential scanning calorimetry and proton cross polarization solid state magic angle spinning (13)C-nuclear magnetic resonance spectroscopy (CP-MAS-(13)C-NMR). CP-MAS-(13)C-NMR spectra of the DPPC (60%)/DMPC (40%) and the DPPC (54%)/DMPC (36%)/CPZ (10%) liposomes, show that CPZ has low or no interaction with the phospholipids of this neutral and densely packed bilayer. Conversely, the DPPC (54%)/PS (36%)/CPZ (10%) bilayer at 25 degrees C demonstrates interaction of CPZ with the phospholipid headgroups (PS). This CPZ interaction causes about 30% of the acyl chains to enter the gauche conformation with low or no CPZ interdigitation among the acyl chains at this temperature (25 degrees C). The DPPC (54%)/PS (36%)/CPZ (10%) bilayer at a sample temperature of 37 degrees C (T(C)=31.2 degrees C), shows CPZ interdigitation among the phospholipids as deduced from the finding that approximately 30% of the phospholipid acyl chains carbon resonances shift low-field by 5-15 ppm.     

Modification of HT 29 cell response to the vasoactive intestinal peptide (VIP) by membrane fluidization

We used liposomes made with phospholipids of fatty acid chain length ranging from C12:0 to C16:0 to modify the cAMP dependent protein kinase (PK) activity of HT 29 cells induced by VIP or forskolin. Both VIP and forskolin effects were inhibited in dilauroylphosphatidylcholine (DLPC) treated cells. PK activity was slightly lowered when cells were treated by dimyristoylphosphatidylcholine (DMPC) liposomes. However neither VIP nor forskolin-induced PK activities were affected with dipalmitoylphosphatidylcholine (DPPC) liposomes. Furthermore, the binding of [125I]VIP to DLPC treated cells was drastically lowered whereas no change was observed when cells were incubated with DMPC or DPPC liposomes. On the other hand, the interaction of HT 29 cells with DLPC vesicles provoked a decrease in membrane cholesterol content with subsequent increase in membrane fluidity. These findings provide evidence that, in HT 29 cells, the mechanisms of VIP-receptor interaction and of adenylate cyclase activation is lipid dependent and is regulated by membrane fluidity.     

High-sensitivity scanning calorimetric study of mixtures of cholesterol with dimyristoyl- and dipalmitoylphosphatidylcholines

A highly sensitive and stable scanning microcalorimeter is employed in a reinvestigation of the effect of cholesterol on multilamellar suspensions of dimyristoylphosphatidylcholine (DMPC) and dipalmitoylphosphatidylcholine (DPPC). Below 20 mol % cholesterol the DPPC mixtures give heat-capacity curves each of which can be resolved into a narrow and a broad peak, suggesting the coexistence of two immiscible solid phases; above 20 mol % only the broad peak is observed and this disappears at about 50 mol %. The DMPC mixtures show a more complicated behavior; from about 13.5 to 20 mol % cholesterol the observed curves appear to be the sum of three component peaks. As with the DPPC mixtures, only a single broad peak is observed above 20 mol % cholesterol, and this broad peak becomes undetectable above about 50 mol %. These results are discussed.     

螺旋脂质体的研究——Ⅰ.含有心磷脂的两相体系形成螺旋脂质体的条件

用CL(心磷脂)与DMPC(二肉豆蔻酰磷脂酰胆碱)或DPPC(二棕榈酰磷脂酰胆碱)所组成的两组体系制备脂质体,可形成少量管状脂质体.加Ca~(2+)或其它二价阳离子后可形成单股或双股螺旋.对产生这类螺旋脂质体的各种条件进行了研究.