Quantitation of lateral stress in lipid layer containing nonbilayer phase preferring lipids by frequency-domain fluorescence spectroscopy

Polymyxin B-Enriched Exogenous Lung Surfactant: Thermodynamics and Structure

The use of an exogenous pulmonary surfactant (EPS) to deliver other relevant drugs to the lungs is a promising strategy for combined therapy. We evaluated the interaction of polymyxin B (PxB) with a clinically used EPS, the poractant alfa Curosurf (PSUR). The effect of PxB on the protein-free model system (MS) composed of four phospholipids (diC16:0PC/16:0-18:1PC/16:0-18:2PC/16:0-18:1PG) was examined in parallel to distinguish the specificity of the composition of PSUR. We used several experimental techniques (differential scanning calorimetry, small- and wide-angle X-ray scattering, small-angle neutron scattering, fluorescence spectroscopy, and electrophoretic light scattering) to characterize the binding of PxB to both EPS. Electrostatic interactions PxB-EPS are dominant. The results obtained support the concept of cationic PxB molecules lying on the surface of the PSUR bilayer, strengthening the multilamellar structure of PSUR as derived from SAXS and SANS. A protein-free MS mimics a natural EPS well but was found to be less resistant to penetration of PxB into the lipid bilayer. PxB does not affect the gel-to-fluid phase transition temperature, Tm, of PSUR, while Tm increased by ∼+ 2 °C in MS. The decrease of the thickness of the lipid bilayer (dL) of PSUR upon PxB binding is negligible. The hydrophobic tail of the PxB molecule does not penetrate the bilayer as derived from SANS data analysis and changes in lateral pressure monitored by excimer fluorescence at two depths of the hydrophobic region of the bilayer. Changes in dL of protein-free MS show a biphasic dependence on the adsorbed amount of PxB with a minimum close to the point of electroneutrality of the mixture. Our results do not discourage the concept of a combined treatment with PxB-enriched Curosurf. However, the amount of PxB must be carefully assessed (less than 5 wt % relative to the mass of the surfactant) to avoid inversion of the surface charge of the membrane.

Acetylcholinesterase inhibition in electric eel and human donor blood: an in vitro approach to investigate interspecies differences and human variability in toxicodynamics

In chemical risk assessment, default uncertainty factors are used to account for interspecies and interindividual differences, and differences in toxicokinetics and toxicodynamics herein. However, these default factors come with little scientific support. Therefore, our aim was to develop an in vitro method, using acetylcholinesterase (AChE) inhibition as a proof of principle, to assess both interspecies and interindividual differences in toxicodynamics. Electric eel enzyme and human blood of 20 different donors (12 men/8 women) were exposed to eight different compounds (chlorpyrifos, chlorpyrifos-oxon, phosmet, phosmet-oxon, diazinon, diazinon-oxon, pirimicarb, rivastigmine) and inhibition of AChE was measured using the Ellman method. The organophosphate parent compounds, chlorpyrifos, phosmet and diazinon, did not show inhibition of AChE. All other compounds showed concentration-dependent inhibition of AChE, with IC50s in human blood ranging from 0.2-29 µM and IC20s ranging from 0.1-18 µM, indicating that AChE is inhibited at concentrations relevant to the in vivo human situation. The oxon analogues were more potent inhibitors of electric eel AChE compared to human AChE. The opposite was true for carbamates, pointing towards interspecies differences for AChE inhibition. Human interindividual variability was low and ranged from 5-25%, depending on the concentration. This study provides a reliable in vitro method for assessing human variability in AChE toxicodynamics. The data suggest that the default uncertainty factor of ~ 3.16 may overestimate human variability for this toxicity endpoint, implying that specific toxicodynamic-related adjustment factors can support quantitative in vitro to in vivo extrapolations that link kinetic and dynamic data to improve chemical risk assessment.

Catalytic modulation of human cytochromes P450 17A1 and P450 11B2 by phospholipid

Unlike most of the drug-metabolizing cytochrome P450s, microsomal P450 17A1 and mitochondrial P450 11B2 catalyze sequential multi-step reactions in steroid biosynthesis. The membrane phospholipid composition might be one parameter that modulates the efficiency and processivity of specific pathways. Here we systematically examined the effects of physiologically relevant phospholipids on the catalysis of purified P450 17A1, P450 11B2, and P450 11B1 in reconstituted assay systems. Both dioleoylphosphatidylcholine (DOPC, 18:1) and dilauroylphosphatidylcholine (DLPC, 12:0) were found to be very efficient in reconstituting 17-hydroxylase and 1720-lyase reactions of P450 17A1. Phosphatidylethanolamine (PE) specifically enhanced 1720-lyase activity up to 2.4-fold in the presence of phosphatidylcholine. On the other hand, P450 11B2-catalyzed production of aldosterone from 11-deoxycorticosterone was very low and from 18-hydroxycorticosterone nil, implying low processivity. DOPC or cardiolipin, which is exclusively located in the inner mitochondrial membrane, maximized aldosterone yield. In sharp contrast, reconstitution of homologous P450 11B1 with DOPC significantly decreased corticosterone formation without affecting the synthesis of 18-hydroxycorticosterone. The intrinsic fluorescence of P450 17A1 and 11B2 increased in the presence of DOPC, DLPC and PE. Acrylamide quenching studies showed that PE decreased solvent accessibility for tryptophan in P450 17A1, as did 20:4 PC or 18:2 PC for P450 11B2. A moderately positive correlation between the proportion of high-spin substrate-bound species and catalytic activity was only observed in the presence of phosphatidylcholines with low-temperature phase transition. These results demonstrate the potential for phospholipids to regulate the activity of steroidogenic P450 activities and thereby steroid hormone biosynthetic pathways.

Changes in lipid mobility associated with alamethicin incorporation into membranes

The binding state of the antibiotic peptide alamethicin with phospholipid bilayers was investigated in terms of the changes induced in lipid mobility. Fluorescence anisotropy was used for the study. It was found that an increase in peptide concentration induced different changes in lipid mobility above and below a critical peptide concentration. This concentration was also critical for an increase in the cooperative binding of the peptide, as detected by circular dichroism. Above the critical peptide concentration, the mobility of both lipid regions, around the polar head and hydrocarbon chain, became restricted with an increased peptide concentration. Below the critical level, however, an increased peptide concentration induced a "wobbling" of the lipid hydrocarbon chain. These results show that an increase in the cooperative binding of the peptide is accompanied by a change in the dominant configuration of the binding peptide. When the binding peptide increases, the dominant configuration appears to shift from surface association to deep incorporation within the membrane. This shift in configuration means that in the formation of ion-conductive pores, voltage-driven insertion of the peptide is a prominent step below a critical peptide concentration.

Phospholipid acyl chain rotational dynamics are independent of headgroup structure in unilamellar vesicles containing binary mixtures of dioleoyl-phosphatidylcholine and dioleoyl-phosphatidylethanolamine

We have examined relationships between phospholipid headgroup structure and acyl chain dynamics, and their respective roles in modulating the physical properties of biological membranes. Fluorescence lifetime and anisotropy measurements were used to assess structural changes involving the lipid acyl chains in homogeneous populations of small and large unilamellar vesicles containing binary mixtures of dioleoyl-phosphatidylcholine (PC) and dioleoyl-phosphatidylethanolamine (PE) in the liquid-crystalline (Lalpha) phase. These measurements involve three different fluorescent lipid analogs containing diphenylhexatriene (DPH) linked to either a trimethylamine moiety (i.e., TMA-DPH) or the sn-1 position of monostearoyl-phospholipids containing PC or PE headgroups (i.e., DPH-PC and DPH-PE). The average lifetimes, rotational correlation times, and order parameters associated with DPH-PC and DPH-PE are virtually identical, and are not affected by alterations in the PE content of the membrane. These results suggest that the average cross-sectional areas of the phospholipid acyl chains of DOPE and DOPC relative to the membrane normal are similar in these unilamellar vesicles. Since PC headgroups are larger than those of PE, differences in the relative orientation of the phosphocholine and phosphoethanolamine moieties relative to the membrane surface probably function to maintain optimal van der Waals contact interactions between acyl chains. On the other hand, the average lifetime associated with TMA-DPH, whose chromophoric group is near the membrane surface, increases with increasing PE content. The position of TMA-DPH relative to the membrane surface does not change, since the rotational dynamics of TMA-DPH are independent of the PE concentration. Therefore, alterations in the average lifetime of TMA-DPH results from polarity differences near the membrane surface at the level of the glycerol backbone. These results are discussed in terms of how differences in the average conformation of the glycerol backbones or phospholipid headgroups of PE and PC have the potential to regulate membrane function.

Evidence for superlattice arrangements in fluid phosphatidylcholine/phosphatidylethanolamine bilayers

Recently, evidence for cholesterol and phosphatidylcholine (PC) molecules to adapt superlattice arrangements in fluid lipid bilayers has been presented. Whether superlattice arrangements exist in other biologically relevant lipid membranes, such as phosphatidylethanolamine (PE)/PC, is still speculative. In this study, we have examined the physical properties of fluid 1-palmitoyl-2-oleoyl-PC (POPC) and 1-palmitoyl-2-oleoyl-PE (POPE) binary mixtures as a function of the POPE mole fraction (X(PE)) using fluorescence and Fourier transform infrared spectroscopy. At 30 degrees C, i.e., above the Tm of POPE and POPC, deviations, or dips, as well as local data scattering in the excimer-to-monomer fluorescence intensity ratio of intramolecular excimer forming dipyrenylphosphatidylcholine probe in POPE/POPC mixtures were detected at X(PE) approximately 0.04, 0.11, 0.16, 0.26, 0.33, 0.51, 0.66, 0.75, 0.82, 0.91, and 0.94. The above critical values of X(PE) coincide (within +/-0.03) with the critical mole fractions X(HX,PE) or X(R,PE) predicted by a headgroup superlattice model, which assumes that the lipid headgroups form hexagonal or rectangular superlattice, respectively, in the bilayer. Other spectroscopic data, generalized polarization of Laurdan and infrared carbonyl and phosphate stretching frequency, were also collected. Similar agreements between some of the observed critical values of X(PE) from these data and the X(HX,PE) or X(R,PE) values were also found. However, all techniques yielded critical values of X(PE) (e.g., 0.42 and 0.58) that cannot be explained by the present headgroup superlattice model. The effective cross-sectional area of the PE headgroup is smaller than that of the acyl chains. Hence, the relief of "packing frustration" of PE in the presence of PC (larger headgroup than PE) may be one of the major mechanisms in driving the PE and PC components to superlattice-like lateral distributions in the bilayer. We propose that headgroup superlattices may play a significant role in the regulation of membrane lipid compositions in cells.

Detection of membrane packing defects by time-resolved fluorescence depolarization

Packing defects in lipid bilayer play a significant role in the biological activities of cell membranes. Time-resolved fluorescence depolarization has been used to detect and characterize the onset of packing defects in binary mixtures of dilinoleoylphosphatidylethanolamine/1-palmitoyl-2- oleoylphosphatidylcholine (PE/PC). These PE/PC mixtures exhibit mesoscopic packing defect state (D), as well as one-dimensional lambellar liquid crystalline (L alpha) and two-dimensional inverted hexagonal (HII) ordered phases. Based on previous electron microscopic investigations, this D state is characterized by the presence of interlamellar attachments and precursors of HII phase between the lipid layers. Using a rotational diffusion model for rod-shaped fluorophore in a curved matrix, rotational dynamics parameters, second rank order parameter, localized wobbling diffusion, and curvature-dependent rotational diffusion constants of dipyenylhexatriene (DPH)-labeled PC (DPH-PC) in the host PE/PC matrix were recovered from the measured fluorescence depolarization decays of DPH fluorescence. At approximately 60% PE, abrupt increases in these rotational dynamics parameters were observed, reflecting the onset of packing defects in the host PE/PC matrix. We have demonstrated that rotational dynamics parameters are very sensitive in detecting the onset of curvature-associating packing defects in lipid membranes. In addition, the presence of the D state can be characterized by the enhanced wobbling diffusional motion and order packing of lipid molecules, and by the presence of localized curvatures in the lipid layers.

The effects of non-lamellar forming lipids on membrane protein-lipid interactions

The role of lipid polymorphism in the regulation of membrane-associated protein function is examined, based on recent studies which showed that changes in the levels of phosphatidylethanolamine (PE), cholesterol and phospholipid unsaturation, modulate the activity of the key signal transduction enzyme, protein kinase C (PKC). It is shown that effects of membrane compositional changes on PKC activity involve a perturbation of protein-lipid interactions with the head group region rather than with the hydrophobic interior of the bilayer. A key determinant in the perturbation of these interactions is suggested to be an elastic curvature energy, termed curvature stress, which results from the unfavorable packing of non-lamellar forming lipids in a planar bilayer. PKC activity is shown to be a biphasic function of curvature stress, with an optimum value of this parameter corresponding to an optimally active PKC conformation. Thus, it is shown that the maximal activity of conformationally distinct PKC isoforms may require a different optimum value of curvature stress. Furthermore, it is hypothesized that curvature stress may have differing effects on the conformation of membrane-associated PKC activity induced by diacylglycerols, phorbol esters or other activators, based on recent studies showing that these agents induce the formation of disparate active conformers of the enzyme.

Effects of unsaturation and curvature on the transverse distribution of intramolecular dynamics of dipyrenyl lipids

The roles of acyl chain unsaturation and curvature in the excimer formation efficiency (EFE) of site-specific conjugated pyrene molecules in lipid membranes have been investigated by steady-state and time-resolved fluorescence spectroscopy. Six 1-2-(pyrenyl-n-acyl)-phosphatidylcholine (dipy(n)PC) probes, with pyrenyl chains of varying methylene units n from 4 to 14 carbons, were incorporated separately into dioleoylphosphatidylcholine (DOPC) or dioleoylphosphatidylethanolamine (DOPE) lipid membranes at 0.1 mol%. Both the excimer-to-monomer fluorescence intensity ratio and association-to-dissociation rate constant ratio of conjugated pyrenes were used to quantify EFE. At all temperatures (T = 0-30 degrees C) and for n = 4 and 6, the EFE for DOPE was always smaller than EFE for DOPC. At T < 10 degrees C (where DOPE and DOPC are in the liquid crystalline L alpha phase) and for n > 8, the EFE for curvature frustrated DOPE was significantly greater than EFE for nonfrustrated DOPC (control), and the difference increased gradually with n. At T> 18 degrees C (where DOPE is in the inverted hexagonal H(II) phase and DOPC is in the L alpha phase) and for n > 8, EFE for the curvature-relaxed DOPE was again smaller than the EFE for DOPC control. The contributions of splay conformation and internal dynamics of pyrenyl chains to EFE were examined separately using a lattice model. Our results suggest that i) the cis double bonds of the host lipid matrix strongly perturb both the conformation and dynamics of conjugated pyrenes at the specific location around n = 8, and ii) the lateral stress at the upper part (n < 8) of the curvature frustrated bilayer membranes (DOPE) may be significantly relaxed once the membrane surface adopts a favorable negative interfacial curvature.

Detection and characterization of the onset of bilayer packing defects by nanosecond-resolved intramolecular excimer fluorescence spectroscopy

Bilayer packing defects in binary dilinoleoylphosphatidylethanolamine and 1-palmitoyl-2-oleoylphosphatidylcholine (DLPE/POPC) lipid mixtures have been studied by the use of nanosecond-resolved intramolecular excimer fluorescence spectroscopy. Frequency-domain fluorescence intensity decays of dual-chain labelled dipyrenyl lipids of different chain lengths in DLPE/POPC mixtures were acquired at both the monomer (392 nm) and excimer (475 nm) emission channels and at 20 degrees C. On the basis of a new intramolecular excimer formation kinetic model, the extent of aggregation and the rotational mobility, in terms of the equilibrium constant of the monomer to aggregated state and the excimer association rate constant, respectively, of the intralipid pyrenes were calculated from the frequency-domain data. Within the range of 60-100% DLPE where bilayer defects are known to coexist with bilayer and non-bilayer states, a prominent peak in the equilibrium constant and a concomitant dip in the excimer association constant at approximately 80% DLPE were observed. Our nanosecond-resolved fluorescence results suggest that the intramolecular excimer kinetic parameters of dipyrenyl lipids are very sensitive to the onset of bilayer packing defects in lipid membranes. Moreover, the onset of bilayer defect state is characterized by the greater extent of aggregation and more hindered rotational mobility of the acyl chains as compared with the bilayer (0% DLPE) and non-bilayer inverted hexagonal (100% DLPE) states of the lipid membranes.

Intramolecular excimer kinetics of fluorescent dipyrenyl lipids: 1. DMPC/cholesterol membranes

The intramolecular dynamics of the excimer forming dipyrenyl lipids (DipynPC) of different chain lengths (n) in ethanol and in dimyristoylphosphatidycholine (DMPC) membranes was investigated by the use of frequency-domain fluorescence intensity decay technique. Based on a 3-state model, the extent of aggregation and rotational rate of the two intralipid pyrene moieties in the dipyrenyl lipids were estimated from the frequency-domain data. In ethanol (20 degrees C), the rotational rate for DipynPC increased progressively as n was varied from 4 to 12. At the gel (L beta)-to-liquid crystalline (L alpha) phase transition of DMPC (approximately 23 degrees C), the rotational rate increased and aggregation decreased significantly for Dipy10PC, whereas only the rotational rate was changed for Dipy4PC. In the presence of 30 mol% cholesterol, significant increases in both the rotational rate and aggregation were observed for Dipy10PC in both L beta and L alpha phases. However, for the case of Dipy4PC, an increase in the rotational rate but a decrease in the aggregation were noticed only in the L beta phase, and no similar changes were detected in the L alpha phase. Our results indicate differential effects of cholesterol on the conformational dynamics of acyl chains at different depths of the membranes.

Intramolecular excimer kinetics of fluorescent dipyrenyl lipids: 2. DOPE/DOPC membranes

The intramolecular dynamics of the excimer-forming dipyrenyl lipids (DipynPE) of different chain lengths (n) in fully hydrated dioleoylphosphatidylethanolamine (DOPE) and dioleoylphosphatidylcholine (DOPC) binary mixtures was investigated by the use of frequency-domain fluorescence intensity dcay technique. Using a 3-state model (see companion paper), the extent of aggregation and rotational rate of the two covalently attached pyrene moieties in DipynPE were estimated from the frequency-domain data. At 1 degrees C, the rotational rate and aggregation for Dipy4PE and Dipy10PE were insensitive to DOPE% of the lipid bilayer. At 27 degrees C, the rotational rate decreased, whereas the aggregation increased steadily for Dipy10PE as the DOPE% of the bilayer increased from 0 to 80. However, an abrupt increase in the rotational rate and a decrease in the aggregation for Dipy10PE were detected as the DOPE% reached 100, at which point the membranes are in the inverted hexagonal (HII) phase. No similar changes were found for Dipy4PE. These results indicate that the presence of PE with large intrinsic-curvature increases the lateral stress at the region near the center of the bilayer, and that this stress can be relieved as the membranes enter the highly curved HII phase.

Probability of alamethicin conductance states varies with nonlamellar tendency of bilayer phospholipids

With few exceptions, membrane lipids are usually regarded as a kind of filler or passive solvent for membrane proteins. Yet, cells exquisitely control membrane composition. Many phospholipids found in plasma membrane bilayers favor packing into inverted hexagonal bulk phases. It was suggested that the strain of forcing such lipids into a bilayer may affect membrane protein function, such as the operation of transmembrane channels. To investigate this, we have inserted the peptide alamethicin into bilayer membranes composed of lipids of empirically determined inverted hexagonal phase "spontaneous radii" Ro, which will have expectably different degrees of strain when forced into bilayer form. We observe a correlation between measured Ro and the relative probabilities of different conductance states. States of higher conductance are more probable in dioleoylphosphatidylethanolamine, the lipid of highest curvature, 1/Ro, than in dioleoylphosphatidylcholine, the lipid of lowest curvature.