Physical properties of phosphatidylcholine vesicles containing small amount of sodium cholate and consideration on the initial stage of vesicle solubilization

Cholesterol determines and limits rHDL formation from human plasma apolipoprotein A-II and phospholipid membranes

Apolipoprotein (apo) A-II, the second most abundant protein after apo A-I of human plasma high-density lipoproteins (HDL), is the most lipophilic of the exchangeable apolipoproteins. The rate of microsolubilization of dimyristoylphosphatidylcholine (DMPC) membranes by apo A-I to give rHDL increases as the level of membrane free cholesterol (FC) increases up to 20 mol % when the level of reaction decreases to nil. Given its greater lipophilicity, we tested the hypothesis that apo A-II and its reduced and carboxymethylated monomer (rcm apo A-II) would form rHDL at a membrane FC content of >20 mol %. According to turbidimetric titrations, the DMPC/apo A-II stoichiometry is 65/1 (moles to moles). At this stoichiometry, apo A-II forms rHDL from DMPC and FC. Contrary to our hypothesis, apo A-II, like apo A-I, reacts poorly with DMPC containing ≥20 mol % FC. The rate of formation of rHDL from rcm apo A-II and DMPC at all FC mole percentages is faster than that of apo A-II but nil at 20 mol % FC. In parallel reactions, monomeric and dimeric apo A-II form large FC-rich rHDL coexisting with smaller FC-poor rHDL; increasing the FC mole percentage increases the number and size of FC-rich rHDL. On the basis of the compositions of coexisting large and small rHDL, the free energy of transfer of FC from the smallest to the largest particle is approximately −1.2 kJ. On the basis of our data, we propose a model in which apo A-I and apo A-II bind to DMPC via surface defects that disappear at 20 mol % FC. These data suggest apo A-II-containing HDL formed intrahepatically are likely cholesterol-rich compared to the smaller intracellular lipid-poor apo A-I HDL.

[World constructed by self-organization of some amphiphils--with a focus on vesicle formation--]

The world constructed by self-organization of some amphiphils was discussed on the basis of micelle formation, vesicle formation, and oriented-nano-wire formation. First, the micelle formation of a both water- and oil- soluble surfactant, Aerosol OT, was discussed. Solution states of micelles and monomer were discussed on the basis of thermodinamic and NMR spectroscopic analyses of micelle formation. Next, micelle-vesicle transition was discussed. It was proposed that the phospholipid LUV formation by removing detergents and destruction by adding detergents occurred via 4 stages. The 4 stage model instead of the 3 stage model could not only elucidate the complicated phenomena observed during micelle-vesicle transition, but predicted the size and properties of the vesicles formed by detergent removal from mixed micelles. Next, the vesicle formation of a fatty acid with a single hydrophobic chain different from phospholipid, which has two hydorophobic chains, was discussed. The vesicle formation was strongly affected by the presence of preformed vesicles and the size was biased on the preformed vesicles. It was shown there exist two pass ways in the process of micelle-vesicle transition by pH jump. One is fission of the preformed vesicles after transfer of monomers from newly added oleate micelles and the other is transition from the mixed micelles after partial solubilization by the oreate micelles. Then, the vesicle formation of HCO-10, which has 3 hydrophobic chains, the mixed vesicle formation of phosphatidylethanolamine and lysophosphtidylcholine, which can not form vesicles, and the phospholipid vesicle formation and destruction by removing and adding PEG-lipid, were discussed. Lastly, oriented nano wire formation of mulamyldipeptid-conjugated lipids with ca 5 nm of diameter was discussed.

[Mechanism of micelle-vesicle transformation and control of vesicular sizes and properties]

The mechanism of vesicle-to-micelle or micelle-to-vesicle transition was studied in order to control sizes and fluidities of vesicles during periods of preparation. Dependence of particle sizes measured by quasi-elastic light scattering, turbidities, fluidity parameters monitored by ESR spectroscopy, and morphological changes of mixed aggregates of egg yolk phosphatidylcholine (EPC) and a detergent (octylglucoside (OG) or sodium cholate (Na-chol)) on detergent concentration provided a model of vesicle destruction. It possessed three phase transition points, and proceeded in a stepwise fashion: vesicles, small particles containing large amounts of detergents (SUV(*)), intermediate structures, and mixed micelles. Vesicle formation on removal of detergents from micelles proceeded oppositely. Micelle-vesicle transition mechanism was common to all detergents examined. The feature of the mechanism was the presence of SUV(*). Next, SUV(*) was prepared by adding appropriate amount of a detergent to small unilamellar vesicles obtained by sonication. Time-dependent size growth of the SUV(*) was remarkable in the case of OG-containing SUV(*), but was insignificant in the case of Na-chol-containing SUV(*), suggesting the size determining step to be the stage of the SUV(*). The tendency to produce large or small vesicles from micelles was related to the absence or presence, respectively, of a net charge in the detergent molecule. The fluidities of EPC micelles containing small amounts of a detergent possessing a steroidal structure (e.g., Na-chol or CHAPS) were significantly smaller than the corresponding values of a detergent without a steroidal structure (e.g., OG), suggesting a method of control of orderliness of hydrocarbon chains in EPC vesicles.

Process of destruction of large unilamellar vesicles by a zwitterionic detergent, CHAPS: partition behavior between membrane and water phases

The process of vesicle destruction by zwitterionic detergent, 3-[(3-cholamidopropyl)dimethylammonio]-1-propanesulfonate (CHAPS), was examined to clarify the vesicle-micelle transition mechanism. The physicochemical properties including turbidity, apparent particle size, Cl(-) permeability, electron spin resonance (ESR) spectroscopic parameters, and freeze-fracture electron microscopy were investigated. The concentration of CHAPS was analyzed using HPLC to determine the partition coefficient during the solubilization process. The data obtained revealed that maximum turbidity and apparent particle size were found at the effective ratio (R(e)) of 0.21 and 0.49, respectively. With a further increase in CHAPS concentration, turbidity and particle size abruptly decreased, suggesting the formation of mixed micelles. The partition coefficient changed throughout the solubilization process. In the presence of low concentrations of CHAPS, CHAPS partitioned into vesicles without destruction of membrane bilayers. When the R(e)<0.04, the partition coefficient was independent of the detergent concentration with value of 24 M(-1). At R(e) greater than 0.05, the membrane barrier abruptly decreased. At 0.04</=R(e)<0.21, the gradual increase in the partition coefficient accounted for the occurrence of larger vesicles. In range of 0.21</=R(e)<0.52, the abrupt increase in the partition behavior was possibly attributed to the structural change of mixed vesicles to mixed micelles. Furthermore, the ESR results showed that the incorporation of CHAPS into vesicles led to an increase in membrane fluidity near the polar head, and a decrease near the end of the acyl chain. ESR spectra of 5-doxylstearic acid in CHAPS-containing micelles were anisotropic, indicating that the steroidal structure of CHAPS was responsible for the micelles possessing an orderly arrangement of hydrocarbon chains.

Aggregation phenomena on the ternary ionic-nonionic surfactant system: didodecyldimethylammonium bromide/octyl-beta-D-glucopyranoside/water. Mixed microaggregates, vesicles, and micelles

The formation of a variety of mixed colloidal aggregates has been investigated on a ternary ionic-nonionic system constituted by (i) a double-chain cationic surfactant with a 12-carbon atom hydrophobic tail, didodecyldimethylammonium bromide (di-C(12)DMAB), (ii) a nonionic single-chain surfactant, octyl-beta-D-glucopyranoside (OBG), and (iii) water. The study has been carried out by means of conductivity, zeta-potential, transmission electron microscopy (TEM), and cryogenic transmission electron microscopy (cryo-TEM) experiments on the highly diluted, very diluted, and moderately diluted regions. The formation of mixed microaggregates, prior to the appearance of mixed vesicles, has been undoubtly confirmed by conductivity, TEM, and zeta-potential results. The concentrations at which these mixed colloidal aggregates form, i.e., the mixed critical microaggregate concentration (CAC), the mixed critical vesicle concentration (CVC), and the mixed critical micelle concentration (CMC), have been determined from conductivity data, while the zeta-potential experiments allow for the characterization of the aggregate/solution interface. The shape and size of the microaggregates and vesicles have been evaluated from TEM and cryo-TEM micrographs, respectively. All of the experimental evidence has been also analyzed in terms of the theoretical packing parameter, P.

Enhancement of apparent substrate selectivity of proteinase K encapsulated in liposomes through a cholate-induced alteration of the bilayer permeability

Proteinase K-containing liposomes with highly selective membrane permeability properties were prepared. The selectivity obtained was with respect to the two substrate molecules added to the external aqueous phase of the liposomes: acetyl-L-Ala-Ala-Ala-p-nitroanilide (Ac-AAA-pNA) and succinyl-L-Ala-Ala-Ala-p-nitroanilide (Suc-AAA-pNA). The liposome-forming lipid used was POPC (1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine) and modulation of the membrane permeability was achieved using the detergent cholate. Proteinase K-containing mixed liposomes (PKCL) were prepared by adding cholate to preformed proteinase K-containing POPC liposomes (PKL) at a defined effective cholate/POPC molar ratio in the liposomal bilayer membrane R(e). Proteinase K was kept inside PKCL with a negligible amount of leakage into the bulk aqueous phase at R(e) < or = 0.30. At higher R(e), leakage of proteinase K was pronounced, even under conditions where POPC/cholate mixed liposomes seemed to be still intact (0.30 < R(e) < or = 0.39). At R(e) < or = 0.30, the reactivity of proteinase K in the PKCL measured with the externally added substrate Ac-AAA-pNA increased with increasing R(e), while the reactivity measured with Suc-AAA-pNA remained low, regardless of the R(e) value. This showed that externally added Ac-AAA-pNA molecules permeated the liposomal membrane more easily than Suc-AAA-pNA by modulating the membrane with cholate. Consequently, Ac-AAA-pNA was hydrolyzed in PKCL with considerably higher apparent substrate selectivity in comparison with the cases of proteinase K in PKL and free proteinase K (without liposomal encapsulation). The results obtained clearly demonstrate that the prepared PKCL can be utilized as a kind of nano-scaled bioreactor system which can take up a particular target substrate with high apparent substrate selectively from the external phase of the liposomes. Inside the liposomes, the target substrate is then converted into the corresponding products.