Showing papers by "Dick Hoekstra published in 1994"

Evidence for the existence of lipid-diffusion barriers in the equatorial segment of human spermatozoa

Abstract: Liposomes consisting of negatively charged phospholipids interact almost exclusively with the equatorial segment (ES) of human spermatozoa provided the cells have undergone the acrosome reaction (AR) [Arts, Kuiken, Jager and Hoekstra (1993) Eur. J. Biochem. 217, 1001-1009]. Using fluorescently tagged liposomes, this interaction can be observed by fluorescence microscopy, showing either a diffuse fluorescence in the ES region (pattern ESd, presumably reflecting membrane-incorporated lipids as a result of fusion) or a punctate fluorescence (pattern ESp, representing adhering liposomes). These distribution patterns remain unchanged during prolonged incubation, up to 40 min. Not only do these observations suggest the existence of fairly specific liposomal binding sites, associated with the ES region, but also that a barrier to lipid lateral diffusion seems to exist in the ES membrane. Using liposomes that contain fluorescent lipid analogues in either both leaflets or in the inner leaflet only, we demonstrate that this putative barrier entails both membrane leaflets. Treatment with EDTA caused fluorescence to spread from the ES towards other membrane domains. Since only spermatozoa displaying pattern ESd were affected by the chelator, the randomization was not caused by EDTA-induced fusion activity. Therefore, this observation provides further evidence that in spermatozoa displaying pattern ESd the fluorescent lipid analogues were incorporated in the ES membrane as a result of fusion. Furthermore, these experiments support the view of the existence of a transmembranous block to lipid lateral diffusion in the ES, the stability of which may be governed by bivalent cations.

Differential scanning microcalorimetric study of sodium Di-n-dodecylphosphate vesicles in aqueous solution

Abstract: The scans recorded by differential scanning calorimetry are reported for aqueous solutions containing vesicles formed by sodium di-n-dodecylphosphate (DDP). The gel-to-liquid–crystal transition occurs near 35 °C, the melting temperature Tm. The dependences of heat capacity on temperature near Tm are analysed to yield related enthalpies of transition and patch numbers which record the number of DDP monomers which melt co-operatively. The dependences of the enthalpy of transition and patch number offer indications of vesicle size and the tightness of packing of monomers within the vesicles.

Efficiency of alkyl chain packing in dialkyl phosphate vesicles in aqueous-solutions

Abstract: The temperature T-m characterizing the gel to liquid crystalline transition in vesicles formed from dialkyl phosphates increases with an increase in alkyl chain length for phosphates with two identical alkyl groups. The temperature T-m also depends on the counter cation. Calorimetric data are interpreted in terms of cooperative melting in domains characterized by patch numbers. The scans recorded by differential scanning microcalorimetry show two or more extrema where the alkyl chain lengths differ.

Enthalpies of interaction between dimethyldioctadecylammonium bromide vesicles in aqueous-solution and either dipicolinate or sulfate anions

Abstract: Injection of small aliquots of dipicolinate anions (sodium salt) into an aqueous solution containing dimethyl-dioctadecylammonium bromide (DOAB) vesicles is endothermic at 50 °C, becoming first more and then less endothermic. The injection process is effectively athermal for solutions containing more than equimolar amounts of DOAB and dipicolinate anions. A similar pattern is observed when small aliquots of sodium sulfate(aq) are injected into DOAB(aq). The overall patterns of enthalpy changes are attributed to the vesicle–dianion interaction which is exothermic and head-group dehydration with bromide ion displacement which is endothermic. Neverthless, a complexity emerges if the solutions include a buffer which turns out to play a less than passive role. This conclusion is supported by differential scanning microcalorimetry for DOAB(aq) in the presence and absence of HEPES buffer.

Reconstitution of proteolipid protein - some properties and its role in interlamellar attachment

Abstract: Proteolipid apoprotein (PLP) isolated from human brain was reconstituted in dioleoylphosphatidylcholine vesicles by dialysis from 2-chloroethanol, using a dialysis buffer of pH 5.0. Under these conditions, and in contrast with dialysis carried out at pH 7.4, well-defined unilamellar vesicles containing the protein were formed. As judged by electron microscopy and quasi-elastic light scattering, the size of the vesicles was determined by the initial protein/lipid ratio used for reconstitution. When the vesicles were incubated in a buffer at neutral pH, aggregation of the vesicles was observed, but their structure remained intact. Asymmetric aggregation occurred when the reconstituted vesicles were incubated with large unilamellar vesicles (LUVs) devoid of protein. This aggregation was accompanied by loss of membrane integrity, as revealed by extensive leakage of the LUVs, and by membrane lipid dilution, indicative of the occurrence of membrane fusion. Destabilization of the vesicles depended on the presence of negatively charged phosphatidylserine in the membrane of the LUVs. Similar effects, but to a lesser extent, were seen when the LUVs contained sulphatide, a negatively charged lipid prominently present in myelin. DM 20, a natural mutant of PLP, appeared to be far less potent in causing membrane lipid dilution than PLP. This could suggest that a distinct protein sequence of PLP, which is absent from DM 20, may be involved in triggering the observed membrane destabilization. Temperature-dependent experiments indicate that this sequence in PLP displays dynamic properties, its exposure being affected by conformational criteria. Exposure of this particular domain, in conjunction with its affinity for negatively charged lipid, could be related to a perturbation of the integrity of the myelin sheath, as will be discussed.

Gel to liquid-crystal transitions for vesicles in aqueous-solutions prepared using mixtures of sodium dialkylphosphates (r(1)o)(r(2)o)po2(-)na+ and(r(3)o)(2)po2(-)na+ where r(1)=c10h21, r(2)=c14h29 or c18h37 and r(3)=c12h25, c14h29, c16h33 or c18h37

Abstract: The scans recorded by differential scanning microcalorimetry (DSC) for aqueous solutions containing vesicles prepared from mixtures of two sodium dialkylphosphates are complicated where the first surfactant anion (R1O)(R2O)PO2–Na+ has alkyl groups R1 and R2 with different chain lengths, and where the second surfactant anion (R3O)2PO2– has two alkyl groups R3 with the same chain length. For mixed solutions where R1= C10H21, R2= C14H29 or C18H37 and R3= C12H25, C14H29, C16H33 or C18H37, the DSC traces can be understood in geometric terms. Where the chains can be assembled into bilayers with modest mismatch, the DSC traces show well resolved features. With increase in mismatch, the complexities of the DSC traces are consistent with the presence of bilayers in which the domains differ in composition. Poor chain packing and consequent weak intravesicular van der Waals forces between the alkyl chains favour low temperatures for gel to liquid crystal transitions.

Phase transitions in the bilayers of vesicles formed from binary mixtures of symmetric di-n-alkylphosphates in aqueous solutions

Abstract: Vesicles in aqueous solutions were prepared from binary equimolar mixtures of di-n-alkyl-phosphates (sodium and potassium), (R1O)2PO2–M+ and (R2O)2PO2–M+. When the number of carbon atoms in R1 and R2 differs by two and when R1 or R2= C12H25, C14H29, C16H33 and C18H37 the membranes undergo well defined gel to liquid crystal transitions at characteristic temperatures Tm. The recorded Tms are intermediate between the melting temperatures for vesicles prepared from the respective single di-n-alkylphosphates. Furthermore, the extrema recorded by differential scanning microcalorimetry show that the vesicle membrane is made up of domains that differ in composition. For those vesicles produced from di-n-alkylphosphates where the number of carbon atoms in R1 and R2 differs by more than two the plots recorded by the scanning microcalorimeter are complex. The scans show many extrema, suggesting that the bilayers are formed from many domains having different compositions. In all cases, the scan patterns are essentially repeated through several heat–cool–heat…cycles. The temperatures Tm are increased relative to those of the component surfactants when K+ and Na+ salts are mixed, showing that the counter cations play an important role in determining the thermotropic properties of the vesicles reflecting the importance of electrical interactions in determining the packing within the bilayers.

Sodium di-n-dodecyl phosphate vesicles in aqueous-solution - effects of ethanol, propanol, and tetrahydrofuran on the gel to liquid-phase transition

Abstract: For aqueous solutions containing vesicles formed by sodium di-n-dodecyl phosphate, the gel to liquid-crystal transition occurs near 35 degrees C, the temperature T-m. When ethanol is added, T-m decreases, but the scan shows evidence of several transitions as more alcohol is added. The effect of added THF and propanol is similar but more dramatic. The scans are similarly complicated if the aqueous solutions are prepared using a solution of DDP in ethanol. However, vesicle ethanol interactions are endothermic although the shift in T-m points to direct binding of alcohol into the vesicle.

Sodium di-n-dodecylphosphate vesicles in aqueous solution: Effects of added ethanol, and Ca2+ and Na+ ions on the gel—liquid phase transition

Abstract: For aqueous solutions containing vesicles formed by sodium di- n -dodecylphosphate, the gel—liquid-crystal transition occurs near 35°C at the temperature T m . When ethanol is added, T m decreases. When sodium chloride is added, T m shifts to higher temperatures whereas very complex scans with several transitions are recorded when calcium chloride is added. The complexities are attributed to the binding of Ca 2+ ions by phosphate groups at the vesicle surface.