Thermotropic and dynamic characterization of interactions of acylated alpha-bungarotoxin with phospholipid bilayer membranes

pH-dependent interactions of indinavir and lipids in nanoparticles and their ability to entrap a solute

We have investigated the ability of lipid-indinavir particles composed of 3-to-1 lipid-drug molar ratio to encapsulate an aqueous marker calcein and anti-HIV drug (3)H-phosphonylmethoxypropyl-adenine (PMPA). Even at a high density of indinavir associated to lipid-indinavir nanoparticles, they form an enclosed lipid membrane that allows encapsulation of calcein and PMPA in an aqueous compartment. At neutral pH, practically all indinavir was incorporated into lipid bilayer and lipid associated indinavir can be dissociated with half-maximum pH recorded between 5.2 and 5.5. pH-Dependent release of indinavir did not influence calcein release significantly. However, pH-dependent release of indinavir affected PMPA release. By lowering pH, PMPA release was enhanced in the presence of indinavir in the lipid bilayer. Collectively, these data indicate that indinavir incorporated in lipid particles provides (1) stable bilayers capable of encapsulating other hydrophilic drugs, (2) ability to dissociate indinavir (which is acid stable) from lipid membranes, by lowering the pH, and (3) enabling enhancement in pH-dependent release of aqueous contents. However, the degree of pH-dependent release could be related to the charge and size of an aqueous molecule.

Phases and phase transitions of the phosphatidylcholines

LIPIDAT (http://www.lipidat.chemistry.ohio-state.edu) is an Internet accessible, computerized relational database providing access to the wealth of information scattered throughout the literature concerning synthetic and biologically derived polar lipid polymorphic and mesomorphic phase behavior and molecular structures. Here, a review of the data subset referring to phosphatidylcholines is presented together with an analysis of these data. This subset represents ca. 60% of all LIPIDAT records. It includes data collected over a 43-year period and consists of 12,208 records obtained from 1573 articles in 106 different journals. An analysis of the data in the subset identifies trends in phosphatidylcholine phase behavior reflecting changes in lipid chain length, unsaturation (number, isomeric type and position of double bonds), asymmetry and branching, type of chain-glycerol linkage (ester, ether, amide), position of chain attachment to the glycerol backbone (1,2- vs. 1,3-) and head group modification. Also included is a summary of the data concerning the effect of pressure, pH, stereochemical purity, and different additives such as salts, saccharides, amino acids and alcohols, on phosphatidylcholine phase behavior. Information on the phase behavior of biologically derived phosphatidylcholines is also presented. This review includes 651 references.

Lipid phase separations induced by the association of cholera toxin to phospholipid membranes containing ganglioside GM1

The interactions of cholera toxin and their isolated binding and active subunits with phospholipid bilayers containing the toxin receptor ganglioside GM1 have been studied by using high-sensitivity differential scanning calorimetry and steady-state and time-resolved fluorescence and phosphorescence spectroscopy. The results of this investigation indicate that cholera toxin associates with phospholipid bilayers containing ganglioside GM1, independent of the physical state of the membrane. In the absence of Ca2+, calorimetric scans of intact cholera toxin bound to dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles containing ganglioside GM1 result in a broadening of the lipid phase transition peak and a slight decrease (less than 5%) in the transition enthalpy. In the presence of Ca2+ concentrations sufficient to cause ganglioside phase separation, the association of the intact toxin to the membrane results in a significant decrease of enthalpy change for the lipid transition, indicating that under these conditions the toxin molecule perturbs the hydrophobic core of the bilayer. Calorimetric scans using isolated binding subunits lacking the hydrophobic toxic subunit did not exhibit a decrease in the phospholipid transition enthalpy even in the presence of Ca2+, indicating that the binding subunits per se do not perturb the hydrophobic core of the bilayer. On the other hand, the hydrophobic A1 subunit by itself was able to reduce the phospholipid transition enthalpy when reconstituted into DPPC vesicles. These calorimetric observations were confirmed by fluorescence experiments using pyrene phospholipids.(ABSTRACT TRUNCATED AT 250 WORDS)

Preparation and characterization of heat-sensitive immunoliposomes

Immunoliposomes able to bind specifically to target cells and to release their encapsulated contents upon brief heating were prepared. Monoclonal anti-H2Kk was covalently derivatized with palmitic acid by the method of Huang, A. et al. (Huang, A., Tsao, Y.S., Kennel, S.J. and Huang, L. (1982) Biochim. Biophys. Acta 716, 140-150). The palmitoyl antibody was injected at a controlled rate into a suspension of fused unilamellar dipalmitoylphosphatidylcholine liposomes maintained at a constant temperature. The final protein-to-lipid ratio of the resultant liposomes with incorporated antibody (immunoliposomes) was dependent upon the rate of antibody injection and the lipid concentration. Injection of palmitoyl antibody into a liposome suspension containing 50 mM carboxyfluorescein at 41 degrees C resulted in simultaneous antibody incorporation and entrapment of dye. Immunoliposomes were able to release the entrapped carboxyfluorescein upon heating. The release of dye at temperatures between the pre- and main-transition temperatures of DPPC was abolished by the addition of calf serum (5%). Furthermore, the presence of serum resulted in an increase in the temperature of the maximal release rate and also in the rate of release at that temperature. Retention of antigen-binding capacity was demonstrated by the ability of the immunoliposomes to bind specifically to the target cells. Rapid release of entrapped carboxyfluorescein from immunoliposomes bound to target cells at 4 degrees C was achieved upon brief exposure (less than 3 min) at 41 degrees C. These heat-sensitive immunoliposomes may be useful in enhancing drug delivery to target cells.

alpha-Bungarotoxin immobilized and oriented on a lipid bilayer vesicle surface

We have developed a new method to assess the binding site on alpha-bungarotoxin (alpha-BGT) for the acetylcholine receptor. It involves the covalent attachment of a palmitic acid chain to the toxin molecule, generating monopalmitoyl-alpha-bungarotoxin (PBGT) which is then immobilized on the surface of a lipid vesicle by a process of spontaneous insertion via the acyl chain into preformed unilamellar vesicles (approximately 800 A in diameter). PBGT itself is able to bind specifically to Triton X-100 solubilized acetylcholine receptors with an association constant, KA, of 5.56 X 10(6) M-1 which is approximately 20-fold lower in affinity than native alpha-BGT. Vesicle-associated PBGT binds to acetylcholine receptor enriched microsac membrane vesicles in aqueous buffer with a KA for both lipid and protein of 4.26 X 10(7) M-1. The putative site of acylation on the PBGT molecule is determined by extensive cleavage of a reduced and carboxymethylated PBGT with thermolysin. An acylated fragment is purified by hydrophobic column chromatography and identified by high-pressure liquid chromatography methods from the known primary sequence of the native toxin as a decapeptide including residues Thr47-Glu56 [C. Y. Lee convention used; see Mebs, D., Narita, K., Iwanaga, S., Samejuma, Y., & Lee, C. Y. (1971) Biochem. Biophys. Res. Commun. 44, 711-716]. Sequential hydrolysis of the fragment from the carboxy terminus with carboxypeptidase Y indicates that Lys51 is the sole site of acylation.(ABSTRACT TRUNCATED AT 250 WORDS)