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Originally published In Press as doi:10.1194/jlr.M200395-JLR200 on January 16, 2003

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Journal of Lipid Research, Vol. 44, 742-753, April 2003
Copyright © 2003 by Lipid Research, Inc.

Different modulation of phospholipase A2 activity by saturated and monounsaturated N-acylethanolamines

Giovanna Zolese1,*, Michal Wozniak§, Paolo Mariani{dagger}, Letizia Saturni§, Enrico Bertoli* and Annarina Ambrosini*

* Istituto di Biochimica, Università di Ancona, 60100 Ancona, Italy
{dagger} Istituto di Scienze Fisiche and INFM, Università di Ancona, 60100 Ancona, Italy
§ Department of General Chemistry,, University of Gdansk, Gdansk, Poland

Published, JLR Papers in Press, January 16, 2003. DOI 10.1194/jlr.M200395-JLR200

1 To whom correspondence should be addressed. e-mail zolese{at}popcsi.unian.it


   ABSTRACT
TOP
 ABSTRACT
INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
The physiological functions of N-acylethanolamines (NAEs) are poorly understood, although many functions were suggested for these naturally occurring membrane components of plants and animals. The binding with cannabinoid receptors CB1 and CB2 was demonstrated for some NAEs, such as anandamide. However, the chemical nature of these molecules suggests that some of their biological effects on biomembranes could be related, at least partially, to physical interactions with the lipid bilayer. The present work studies the effect of saturated and monounsaturated NAEs on phospholipase A2 (PLA2) activity, which is dependent on lipid bilayer features. The present study, performed by 2-dimethylamino-(6-lauroyl)-naphthalene (Laurdan) fluorescence, demonstrates that the acyl chain length and the presence of a single double bond are crucial for the enzymatic activity modulation by NAEs. In fact, saturated NAEs with 10 carbon atoms don't affect the PLA2 activity, while NAEs with 12 and 16 carbon atoms largely activate the enzyme. On the other hand, an acyl chain length of 18 carbon atoms, with or without the presence of a double bond, only slightly affects the enzymatic activity.

A structural model for NAE-lipid interactions is proposed in order to explain the differences in PLA2 activity modulation by these fatty acid derivatives.

Abbreviations: DPPC, dipalmitoylphosphatidylcholine; GP, generalized polarization; Laurdan, 2-dimethylamino-(6-lauroyl)-naphthalene; LUV, large unilamellar vesicle; Lyso-PC, lysophosphatidylcholine; MLV, multilamellar vesicle; NAE, N-acylethanolamine; NCEA, N-caprinoylethanolamine; NOEA, N-oleoylethanolamine; NPEA, N-palmitoylethanolamine; NSEA, N-stearoylethanolamine; PL, phospholipid; PLA2, phospholipase A2; Prodan, 6-propionyl-2-(dimethylamino)-naphthalene; 3wGP, three-wavelengths GP

Supplementary key words PLA2 activity • 2-dimethylamino-(6-lauroyl)-naphthalene • 6-propionyl-2-(dimethylamino)-naphthalene • X-rays • dipalmitoylphosphatidylcholine • large unilamellar vesicle


   INTRODUCTION
TOP
 ABSTRACT
 INTRODUCTION
MATERIALS AND METHODS
 RESULTS
 DISCUSSION
 REFERENCES
 
N-acylethanolamines (NAEs) are naturally occurring molecules in plants and animals (1). They are usually present in very small amounts in many mammalian tissues and cells (2) and in plasma (3), where they are thought to be bound to serum proteins (3), likely to albumin (4, 5).

Moreover, NAE content is dramatically increased in stress conditions (1), such as in myocardium-infarcted areas (1), glutamate-induced neuronal cytotoxicity (2), cadmium chloride-administered rat testis (6), etc. A great interest in these compounds developed with the discovery that some of these molecules ("endocannabinoids") are endogenous ligands of cannabinoid receptors CB1 and CB2 (7 and references cited therein). An important implication of this discovery is the possibility existing for the development of new drugs with applications similar to those suggested for cannabis (7), such as, for example, alleviation of inflammation and pain, and stimulation of appetite in cancer patients. The endocannabinoids are derivatives of PUFA's ethanolamides (among these, the arachidonic derivative, C20:4, anandamide, is the most known) or the 2-arachydonoyl-glycerol (2-AG). These compounds activate CB1 and CB2 receptors and exhibit many pharmacological actions similar to those exerted by marijuana (7). However, despite the great effort carried out in studies on endocannabinoids, their physiological roles remain elusive. Furthermore, other compounds similar to anandamide, but characterized by saturated or monounsaturated acyl chains, are physiologically present in a large variety of tissues (8). These molecules are thought to be cannabinoid-receptor inactive (7, 8). Some recent works suggested that these NAEs could prolong and enhance anandamide biological activity by preventing its degradation ("entourage effect") (9, 10). This raises the possibility that these compounds could be used therapeutically to affect anandamide actions. Moreover, other roles could be possible for these NAEs: for example, N-palmitoylethanolamine (C16:0; NPEA), a shorter and fully saturated analog of anandamide, is known to be accumulated during inflammation and presents some anti-inflammatory (7) and antinociceptive effects (11). NPEA is now engaged in clinical studies on the treatment of chronic lumbosciatalgia and multiple sclerosis (12). Moreover, a neuroprotective role for NAEs in general and for NPEA in particular has been suggested (13).

It was speculated that NAEs could have cytoprotective functions (13) and membrane stabilizing effects (1), and can protect against lipid peroxidation (1), inhibit the acrosome reaction of sea urchin spermatozoa (14), and might act as lipid mediators to modulate PLD{alpha} activity in plants (15).

The chemical nature of NAEs and some experimental evidence suggest that some of their biological effects could be related, at least partially, to physical interactions with the lipidic part of the membrane (1, 16, 17). In fact, previous studies on liposomes demonstrated that phospholipid (PL) physico-chemical properties, such as fluidity and phase transitions (16, 17), are affected by NAEs with different acyl chain lengths and unsaturation. Many important physiological membrane functions [such as enzymatic activities, membrane permeability, hormonal response, etc. (18–20)] are modulated by physico-chemical properties of the lipid bilayer. For example, many physiological processes involve changes in the amount of water bound to lipid bilayer (20), which affects, in particular, the membrane interfacial polarity. A possible role for surface polarity is the binding of substances to membrane: for example, membrane surface properties modulation affects the binding of protein kinase C to the membrane as well as its activity in the membrane-bound state (20).

Moreover, it was demonstrated (21) that the products of phospholipase A2 (PLA2) hydrolysis affect membrane interfacial polarity, modulating the enzymatic activity.

The aim of this paper is to elucidate the interaction of saturated and monounsatured NAEs with lipid membranes and to study their effects on PLA2 activity, which is largely dependent on (22–24) structural, physico-chemical, and dynamic properties of the lipid bilayer. PLA2 activity studies are of general interest because of the increasing evidence of the important roles played by this enzyme in hormone signal transduction, immune events, allergic reactions, ischemia, spermatozoa acrosomal reaction, and in the development of an inflammatory response in the cell, etc. Furthermore, an enhanced local concentration of active PLA2 was demonstrated in inflammatory tissues (25).

The use of steady-state fluorescence and X-ray diffraction will allow us to characterize NAEs localization in the bilayer and their effects on physico-chemical and structural membrane properties.

For the first time, the effect due to NAEs differing for acyl chain length and unsaturation will be characterized on one-component liposomes of dipalmitoylphosphatidylcholine (DPPC) chosen to compare results obtained with recent papers on PLA2 activity modulation by lipid surface (21, 24, 26–28). In fact, many details (e.g., the temperature dependence of the enzymatic rate) were cleared in DPPC large unilamellar vesicles (LUVs) so that these vesicles are used as a fairly simple model system for investigating the delicate interplay between the PLA2 activity and the physicochemical features of lipid bilayers (29, 30). The study of the NAE-DPPC liposomes interaction with PLA2 could have a practical application for the possible use of these vesicles as drug microcarrier systems. In fact, the possible therapeutic use of NAEs is linked to their vehiculation to tissues (1, 4).


   MATERIALS AND METHODS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
RESULTS
 DISCUSSION
 REFERENCES
 
The fluorescent probes 2-dimethylamino-(6-lauroyl)-naphthalene (Laurdan) and 6-propionyl-2-(dimethylamino)-naphthalene (Prodan) were purchased from Molecular Probes (Eugene, OR). DPPC was obtained from Avanti Polar (St. Louis, MO). NAEs were synthesized as previously described (17): N-caprinoylethanolamine (NCEA; with 10 carbon atoms in the chain, C-10), N-lauroylethanolamine (C12), NPEA (C16), N-stearoylethanolamine (NSEA; C18), and N-oleoylethanolamine (NOEA; C18:1). Porcine pancreas PLA2 (Boheringer, Germany) was dialyzed in water, lyophilized, and stored at -20°C. Enzyme was tested for purity by electrophoresis.

DPPC, Laurdan, and Prodan were dissolved in the proper solvent and stored at -20°C. NAE was dissolved in ethanol immediately before use. Buffer used was 10 mM Hepes, 0.12 M NaCl, 5 mM KCl, and 0.1 mM EDTA buffer (pH 7.4).

Fluorescence measurements
For fluorescence measurements, DPPC LUVs, with or without desired NAE, were prepared by extrusion through 0.1 µm polycarbonate filters 25 times at 55°C using the LiposoFast apparatus. The NAE-DPPC molar ratio was R = 0.1 and, in some experiments, R = 0.01. When present, the fluorescent probes were added to DPPC up to a final probe-lipid Pi molar ratio of 1:1,000 before the preparation of vesicle. Unlabeled LUVs were used to subtract background contribution to emission. DPPC concentration was assessed by phosphate assay (31). LUVs were diluted to a final concentration of 0.3 mM lipid Pi. PLA2 concentration (0.5 µM in the sample) was assessed by measuring the absorbance at 280 nm according to Ludescher et al. (32).

Hydrolysis was triggered by 5 mM Ca2+ injection in the cuvette containing liposomes and the enzyme at a final volume of 2 ml. Steady-state fluorescence measurements were performed on a Perkin-Elmer LS50 B. Both Laurdan and Prodan spectral features are largely sensitive to the polarity and to the molecular dynamics of solvent dipoles in probe microenvironment (33–35). Since, these parameters are very different in the gel and in the liquid-crystalline phases of PLs, these probes were demonstrated to be able to monitor PL phase changes (33–35).

Laurdan and Prodan emission spectra were obtained, exciting at 340 nm and 359 nm, respectively. Generalized polarization (GP) for Laurdan is related to membrane polarity changes and was calculated from emission spectra by the following equation, according to Parasassi et al. (33, 34):

where IB and IR were the intensities at 440 nm and 490 nm.

A decrease of GP value represents an increase of bilayer polarity.

Prodan three-wavelengths GP (3wGP) and the probe partition coefficient (Cp) between the bilayer and the buffer were calculated according to the method introduced by Krasnowska et al. (35).

For these calculations, Prodan emission in buffer was subtracted from the spectrum obtained in membranes. Both spectra were recorded at 38.7°C. Intensities at 420 nm, 480 nm, and 530 nm were measured both in the original spectra and in those obtained from the subtraction of intensity arising from the probe in buffer. 3wGP represents GP values only related to Prodan emission from membranes, with the same significance as Laurdan GP.

For the Cp calculation (35), we used the fluorescence intensities measured on Prodan emission spectrum in membrane at 420 nm (I1) at 480 nm (I2) and 530 nm (I3). The ratio I2W/I3W = k32 = 2.8 (35) was between the emission intensities at 480 nm and 530 nm, respectively, of Prodan in water. The lifetimes for Prodan in the gel-phase membrane and in water were {tau}M =3.98 ns and {tau}W= 1.41 ns, respectively (35). Briefly, Cp was calculated by the formula:

where RM = RF {tau}W/{tau}M and RF = (I2 k32 - I3)/I3.

X-ray diffraction
Samples for X-ray diffraction experiments were prepared mixing aliquots of NAEs and DPPC chloroform solutions at the molar ratio R = 0.1. The solvent was removed by a gentle nitrogen flux. Samples were kept under vacuum for several hours. The lipid film was rehydrated at 55°C in the same buffer used for fluorescence measurements using a lipid-to-buffer weight ratio of 1:3. The obtained samples were used for X-ray diffraction experiments performed using a powder diffractometer on a Rigaku Denki RV300 rotating anode generator. The samples were located between two mylar windows in a circular hole. Diffraction data were collected at the constant temperature of 38.7°C ± 0.1°C controlled by a circulation thermostat. In each experiment, a number of sharp reflections were observed and their spacings were measured following the usual procedure (36, 37). The high-angle profiles, centered in the (3–5 Å)-1 region of the scattering curves, were analyzed as reported (37) to derive the nature of the lipid short-range conformation. Low-angle diffraction profiles were indexed using equations that define the spacing of reflections for the different symmetry systems usually observed in lipid phases (lamellar, hexagonal, or 3-dimensional cubic lattices). The indexing problem was easy to solve because in no case were extra peaks observed, which can be ascribed to the presence of unknown phases or to crystalline structures. In the present experimental conditions, only a series of low-angle Bragg reflections indexed according to the lamellar one-dimensional symmetry (spacing ratios 1:2:3, etc.) was observed. From the symmetry, the dimension of the unit cell, which in this case corresponds to the total thickness of the lipid bilayer and of a water layer, was derived. In the following, a indicates the unit cell dimension, i.e., the distance between the midplane of two opposing lipid bilayers.


   RESULTS
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
DISCUSSION
 REFERENCES
 
Effects of NAEs on DPPC phase behavior
The spectral characteristics of Laurdan, which is particularly sensitive to lipid phase changes, were used for these studies. Laurdan features are due to the extreme sensitivity to polarity and molecular dynamics of solvent dipoles in its environment, which arise from solvent dipolar relaxation processes (33, 34).

Laurdan emission in the bilayer is strictly dependent from the packing of lipid chains. In a lipid phase below the main phase transition gel phase ({tau}M) the emission maximum is near 440 nm, while above the {tau}M (liquid-crystalline phase) this maximum is shifted to 490 nm (33). (Figure 1 shows Laurdan emission spectra recorded in DPPC LUVs at different temperatures.) The steady-state fluorescence parameter GP quantitatively relates the relative fluorescence intensities at 440 nm and 490 nm, and characteristic GP values were shown for gel and liquid crystalline phases (34). These values were demonstrated to be independent from PL headgroups and from pH changes (in the range 4 < pH < 10). However, the observation of an intermediate GP value is not a final proof of the coexistence of separated PL phase domains, but could indicate homogenous lipid membranes with intermediate polarity between gel and liquid-crystalline phases (38). The GP values measured in PLs undergoing phase changes have been related to the number and motional freedom of water molecules in the probe microenvironment (33, 34).



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  		Fig. 1. Steady-state fluorescence emission spectra of 2-dimethylamino-(6-lauroyl)-naphthalene (Laurdan) in dipalmitoylphosphatidylcholine (DPPC) large unilamellar vesicles (LUVs). Samples were suspended in 10 mM Hepes, 0.12 M NaCl, 5 mM KCl, and 0.1 mM EDTA buffer (pH 7.4). Spectra were recorded at {lambda}ex = 340 nm. Curves: a, 30°C; b, 35°C; c, 38°C; d, 39.3°C; e, 40.2°C; f, 40.6°C; g, 40.8°C; h, 41.3°C; i, 42.4°C; l, 47.4°C; m, 50°C.

 
For a characterization of NAEs-DPPC (R = 0.1) interaction, the thermotropic behavior of Laurdan GP (calculated on emission spectra obtained by {lambda}ex 340 nm), was measured (Fig. 2A, B) . Previous studies demonstrated that DPPC LUVs and multilamellar vesicles (MLVs) have nearly identical phase behavior, with similar pretransition and a main transition (Tm) temperatures (39, 40). The gel-to-liquid-crystalline phase transition of DPPC LUVs is known to be highly cooperative (28 and references cited therein). Laurdan GP values (Fig. 2) show a large decrease at the main phase transition of DPPC LUV (Tm), while it is very hard to assess the presence of the pretransition by the small slope changes shown by GP curves although occurring at a temperature (~36.5°C), which is similar to previously reported data (39). For this reason, only the gel-to-liquid-crystalline phase transition will be discussed in all samples. As reported in Table 1, NOEA (C18:1) with a double bond in its acyl chain decreases the DPPC Tm (from 41°C to 39.4°C). In contrast, the other NAEs increase or decrease the lipid Tm with a pattern dependant on the length of their saturated acyl chains. The NAEs tested do not affect significantly the width of the gel-to-liquid-crystalline phase transition (Fig. 2), suggesting no lateral phase separation in lipid domains with different lipid compositions within the bilayer. However, the possible formation of small-scale clusters enriched with specific constituents cannot be excluded. Only NOEA slightly broadens the Tm range, suggesting the presence of domains of different lipid composition. Below and above the main lipid phase transition, Laurdan GP values are not largely modified, compared with pure DPPC, by the presence of NCEA (not shown), NPEA, and NSEA (R = 0.1 for all samples) (Fig. 2A). On the contrary, NOEA-DPPC LUVs (R = 0.1) show lower GP values compared with DPPC at almost each temperature tested (Fig. 2B). This effect is more evident below Tm. A similar but less evident decrease is shown also by NLEA containing LUVs (R = 0.1) (Fig. 2B).




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  		Fig. 2. Effect of N-acylethanolamines (NAEs) on Laurdan generalized polarization (GP) measured in DPPC LUVs as a function of temperature. Samples were suspended in 10 mM Hepes, 0.12 M NaCl, 5 mM KCl, and 0.1 mM EDTA buffer (pH 7.4). GP was calculated from Laurdan emission intensities at 440 nm and 490 nm as described in Materials and Methods ({lambda}ex = 340 nm). A: Diamond, DPPC LUVs; square, N-palmitoylethanolamine (NPEA)-DPPC LUVs; triangle, N-stearoylethanolamine (NSEA)-DPPC LUVs. B: Triangle, DPPC LUVs; asterisk, NLEA-DPPC LUVs; circle, N-oleoylethanolamine (NOEA)-DPPC LUVs.

 

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  		TABLE 1. Effect of N-acylethanolamines on gel-to-liquid-crystalline phase transition temperature

 
Effects of NAEs on PLA2 activity
Previous studies demonstrated an enhanced PLA2 activity at the PL gel-to-liquid-crystalline phase transition, where the lipid packing shows maximum heterogeneity (41). Moreover, the possible use of DPPC liposomes as NAEs delivery systems implicates their use at physiological temperatures. For these reasons, activity studies were performed at the temperature of 38.7°C, below the DPPC Tm (in LUV 41°C).

At this temperature, all samples tested (R = 0.1) are in the gel phase (Fig. 2), as confirmed by X-ray data (see the following section). In the case of NOEA (R = 0.1), the temperature 38.7°C may well be in the range of phase transition.

Normalized Laurdan emission spectra (Fig. 3) indicate that, at 38.7°C, NAEs (R = 0.1) modify the bilayer polarity at the glycerol level, in a manner dependent on the chain length and unsaturation. In fact, the spectrum is not significantly affected by NCEA (not shown); instead, it is enlarged and increased in intensity at longer wavelengths by NLEA and NOEA, (Fig. 3B) while it is narrowed and decreased in the red region (around 490 nm) by NPEA and NSEA (Fig. 3A). GP values were calculated from these spectra and are shown in Table 2, upper panel. It is evident that NLEA and, to a larger extent, NOEA, induce a decrease of GP values (Table 2, upper panel). These results indicate an increased polarity in these samples. On the contrary, NPEA and NSEA have an opposite effect on the polarity of the probe microenvironment, as indicated by the increased values of GP. Almost no changes are induced by NCEA on Laurdan GP.




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  		Fig. 3. Normalized steady-state fluorescence emission spectra of Laurdan in DPPC LUVs: effect of NAEs. Samples were suspended in 10 mM Hepes, 0.12 M NaCl, 5 mM KCl, and 0.1 mM EDTA buffer (pH 7.4). Spectra were recorded at {lambda}ex = 340 nm. T = 38.7°C. A: curves: a, NPEA-DPPC LUVs; b, NSEA-DPPC LUVs; c, DPPC LUVs. B: Curves: c, DPPC LUVs; d, NLEA-DPPC LUVs; e, NOEA-DPPC LUVs.

 

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  		TABLE 2. Laurdan GP values in liposomes containing NAEs

 
Table 2 also reports GP values calculated from spectra recorded on the same samples after incubation with 0.5 µM PLA2 in the absence of Ca2+. No significant GP changes were measured after enzyme incubation. These results indicate that before Ca2+ addition, the enzyme does not significantly perturb the Laurdan microenvironment, either in the absence or in the presence of NAEs (R = 0.1).

The fluorescence features of Laurdan and Prodan were used to study PLA2 activity. Laurdan and Prodan are similar probes that differ only in the length of their hydrophobic tail (12 C and 3 C atoms, respectively). The use of these probes to study the physico-chemical changes occurring in the bilayer during the PLA2-induced hydrolysis was introduced and characterized in previous works (21, 24, 26). In the presence of zwitterionic PL vesicles, PLA2 activity is characterized by a latency period (identified as {tau} or lag time) of slow rate, followed by an abrupt increase of the hydrolytic rate. The length of lag time is dependent on various factors, such as structure, composition, and bilayer dynamics. Moreover, the abrupt increase of hydrolysis rate is believed to be dependent on the formation of threshold concentrations of the reaction products. Laurdan and Prodan spectral changes in DPPC bilayers were correlated with the presence of the hydrolytic products (21, 24, 26) [palmitic acid and lysophosphatidylcholine (Lyso-PC)].

The contemporary use of these probes is particularly useful because they locate in two different environments within the bilayer, since Prodan is located in the region of polar headgroups, while Laurdan is at the hydrophilic-hydrophobic interface (34) (at the glycerol backbone).

Laurdan is tightly anchored by its long lauric-acid tail to a single site in the bilayer (42) both in gel and liquid-crystalline phases, while Prodan molecules were suggested to be distributed in the bilayer among different environments, depending on the physical state of the membrane (43).

The time course of PLA2 hydrolytic activity on DPPC LUVs was derived by measuring the ratio between the intensities at 490 nm and 440 nm (R/B) on both Laurdan and Prodan fluorescence emission spectra (Fig. 4) . In our experimental conditions, PLA2 activity toward DPPC LUVs presents a lag time of 740 ± 102 s when it is measured using the Laurdan R/B. A similar lag time was obtained by measuring the time course of Prodan R/B (Fig. 4) (800 ± 105 s).



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  		Fig. 4. Time courses of 6-propionyl-2-(dimethylamino)-naphthalene (Prodan) and Laurdan fluorescence during DPPC LUV hydrolysis. The reaction was triggered by the addition of 5 mM CaCl2 to the sample containing DPPC and phospholipase A2 (PLA2). T = 38.7°C. The intensities at 490 nm and 440 nm were measured for both probes on steady-state fluorescence emission spectra ({lambda}ex = 359 nm for Prodan and {lambda}ex = 340 nm for Laurdan). A ratio of 490 nm to 440 nm (R-B) is reported for both Prodan (triangle) and Laurdan (x).

 
Previous data (26), performed by Asp-49 PLA2 from Agkistrodom piscivorus piscivorus venom on DPPC LUVs, demonstrated that the abrupt increase of Prodan R-B ratio, which occurs during the hydrolysis, is coincident with the raise of the enzymatic activity at time {tau}. This PLA2 activation is due to an increased enzyme-interface interaction at this time. In that work (26), the lag time measured by Laurdan fluorescence was not coincident with that obtained by Prodan. This different behavior was explained by the higher sensitivity of Prodan emission spectra to the presence of Lyso-PC, which disrupts the bilayer in the region of PL headgroups (26). On the contrary, it was shown that Laurdan fluorescence is less useful for probing PLA2 membrane degradation occurring during the hydrolysis because it is more sensitive to the stabilizing effect of fatty acids than to Lyso-PC (26) accumulation. However, the comparison with other data performed on DPPC LUVs by the same group (21) suggests that, in our experimental conditions (pH 7.4 and 5 mM Ca2+), the fatty acids formed during DPPC hydrolysis are continuously extracted from the bilayer.

It was in fact suggested that Ca2+ concentrations >10 µM decrease the amount of charged palmitic acid (pK = 7.5–8.5 in the bilayer) in the membrane (21). Therefore, it is hypothesized that, in our conditions, both Prodan and Laurdan give similar results because the fatty acid content in the membrane is lower than Lyso-PC content. In fact, it was demonstrated that increasing concentrations of Lyso-PC affect the fluorescence of both probes in a similar way (26). In our samples, similar lag times were obtained either by Laurdan R-B ratio or GP calculated by the same emission wavelengths (unpublished observations). For this reason, the following experiments will show lag times measured by using Laurdan GP data. Figure 5A and B report the time courses of vesicle hydrolysis in DPPC LUVs in the presence and in the absence of the desired NAE. It is evident that NAEs (R = 0.1) modify PLA2 lag times (correspondent to the latency period) in a way that is dependent on the acyl chain length and unsaturation. Lag times (Table 3, upper panel) are largely decreased by NLEA and NPEA (-38.8% and -44.7% compared with pure DPPC, respectively), are almost unchanged by NCEA (not shown), and are only slightly modified by NSEA and NOEA (-16.5% and +14.1%, respectively). Control experiments revealed that NAEs, which are added as exogenous molecules to DPPC LUVs, modify the time courses of vesicle hydrolysis in a way similar to that reported in Fig. 5 (not shown).




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  		Fig. 5. Time courses of Laurdan fluorescence during hydrolysis of DPPC LUVs: effect of NAEs. Reaction conditions were identical to those in Fig. 4. Laurdan fluorescence is expressed as GP, which was calculated from emission intensities at 440 nm and 490 nm, as described in Materials and Methods ({lambda}ex = 340 nm). T = 38.7°C. A: Square, DPPC LUVs; asterisk, NLEA-DPPC LUVs; triangle, NPEA-DPPC LUVs. B: Square, DPPC LUVs; x, NSEA-DPPC LUVs; diamond, NOEA-DPPC LUVs.

 

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  		TABLE 3. PLA2 lag time measurements

 
To evaluate the effect of lower concentrations of NAEs on PLA2 activity, further experiments were performed using an NAE-DPPC (R = 0.01). At this NAEs concentration, Laurdan GP values are significantly modified only by NOEA (Table 2, lower panel). Lag times (Table 3, lower panel) are decreased by NLEA and, to a minor extent, by NPEA (-26% and -14%, respectively), are unmodified by NCEA, and are increased by NSAE and NOEA (+23% and +24%, respectively).

Prodan steady-state fluorescence
The fluorescent probe was used to elucidate the possible different modulation of PL water content and dynamics by NSEA and NPEA. As is Laurdan, Prodan fluorescence is sensitive to the dipolar relaxation phenomenon (35). However, in contrast to Laurdan, Prodan's shorter acyl residue locates the probe near the bilayer surface and affects its affinity for the membrane and different lipid phases (35). It is known that Prodan partitioning in the bilayer is dependent on the membrane order degree (35) since, for its short hydrophobic chain, it is squeezed out from a well-packed lipid phase to the bulk water. Similarly to Laurdan, this probe is sensitive to the content and dynamic of water molecules in the bilayer (35). Moreover, the recently introduced Prodan 3wGP parameter is similar to Laurdan GP and is sensitive to PL phase state. In particular, Prodan 3wGP was demonstrated to detect structural changes involving the PL headgroup region (35). Prodan spectra present a quite relevant intensity in the red wavelengths region, with the presence of an additional band at ~520 nm. This band is due to Prodan molecules in the aqueous phase (35). According to Krasnowska et al. (35), the GP was calculated measuring the emission intensity at three wavelengths (3wGP) chosen to maximize the separation of the contributions from the different Prodan environments. The 3wGP method allows the measurement of the probe Cp between the bilayer and water (35). Results are reported in Table 4. Both the blue shift of the Prodan emission spectrum (data not shown) and the 3wGP increase (from 0.309 ± 0.025 in DPPC to 0.405 ± 0.018 in NPEA/DPPC, and 0.374 ± 0.022 in NSEA/DPPC) indicate that NPEA and NSEA cause a lower bilayer polarity in the microenvironment sensed by Prodan. Moreover, the NAEs-induced decrease of Cp indicates an increased exposure of Prodan to bulk water (Table 4). All these effects are more evident for NPEA.


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  		TABLE 4. 6-propionyl-2-(dimethylamino)-naphthalene three-wavelengths GP and partition coefficients of the probe between phospholipids (PLs) and buffer

 
X-ray diffraction data
X-ray diffraction data were acquired in DPPC MLVs prepared in the absence or in the presence of NAEs (NAE-DPPC molar ratio, R = 0.1).

Three different lamellar phases, namely the Lß', Pß', and L{alpha} phases, are observed as a function of temperature for the fully hydrated DPPC MLV system. The phase transitions from Lß' to Pß' and from Pß' to L{alpha} are usually indicated as pretransition and main transition, respectively. These lamellar structures are characterized by a distorted pseudo-hexagonal, hexagonal, and liquid-like arrangement of the hydrocarbon chains inside the layer, respectively (44, 45). Moreover, in the Pß' phase, the lipid bilayers are distorted by a periodic ripple so that the structure presents a 2-dimensional lattice (46).

Measurements were performed at the fixed temperature 38.7°C. The main purpose was to determine if the presence of NAEs can induce the transition from the lamellar to a different lipid phase. Moreover, X-ray experiments were used to assess the possible presence of phase coexistence, even if the method can underline only the large multiphase domains. The low-angle (Fig. 6) and high-angle (Fig. 7) diffraction patterns have been considered separately. The low-angle diffraction patterns observed in all samples (Fig. 6) are characterized by three and/or four sharp peaks; their spacing clearly indicates a one-dimensional lamellar lattice (1:2:3:4, etc.). Therefore, information about the interlamellar repeat distances can be derived. However, in our experimental conditions (excess buffer), it is not possible to separate the contributions of the lipid and water layers to the measured periodicity (47). The calculated unit cell dimensions (Table 5) are: 71.7 ± 0.5 Å for DPPC, 76.8 ± 0.5 Å for NLEA-DPPC, 72.5 ± 0.5 Å for NSEA-DPPC, and 70.1 ± 0.5 Å for NPEA-DPPC. Note that the unit cell corresponds to the distance between the mid-plane of two opposing lipid bilayers. The absence of other peaks in the spectra gives clear evidence that each sample is characterized by a single pure phase.



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  		Fig. 6. X-ray low-angle diffraction profiles obtained for fully hydrated DPPC multilamellar vesicles (MLVs) prepared in the absence and in the presence of NAEs. NAE-DPPC molar ratio was R = 0.1. Data were acquired at 38.7°C . The Bragg peaks are indexed according to the Miller indexes h00 of the reflection. The corresponding unit cells are indicated.

 


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  		Fig. 7. X-ray high-angle diffraction profiles obtained for fully hydrated DPPC MLVs prepared in the absence and in the presence of NAEs. NAE-DPPC molar ratio was R = 0.1. Data were acquired at 38.7°C . The peak positions are reported, indicating the corresponding interchain distances, as calculated by using the Bragg law. The peak centered at 2{theta} = 18° is due to the sample-holder mylar windows.

 

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  		TABLE 5. X-ray diffraction data in liposomes containing NAEs

 
The high-angle region gives information on the PL hydrocarbon tails: in particular, two peaks characterize the Lß' phase, while a single peak is observed in the Pß' phase. The high-angle diffraction data reported in Fig. 7 demonstrate that DPPC, NLEA-DPPC, and NSEA-DPPC samples are in the Pß' phase, while NPEA-DPPC is in the Lß' phase. Either the reflections centered at about 4.2 Å (DPPC, NLEA-DPPC, and NSEA-DPPC) or those centered at 4.2 Å and 4.1 Å (NPEA-DPPC) (Table 5) give the reason why there are no changes with respect to the typical spectra observed in the same phases for pure DPPC, close to the pretransition temperature.


   DISCUSSION
TOP
 ABSTRACT
 INTRODUCTION
 MATERIALS AND METHODS
 RESULTS
 DISCUSSION
REFERENCES
 
PLA2 is an interfacially activated soluble enzyme whose activity occurs in two distinct steps: 1) the enzyme binds to the vesicle surface, becoming activated; 2) the activated enzyme binds a molecule of substrate PL at the catalytic site, generating the hydrolysis products. Secreted PLA2 requires Ca2+ as cofactor.

Although the molecular mechanism of this interfacial activation is not clarified, it appears that PLA2 activity is affected by bilayer physical properties. In fact, exogenous molecules or other factors that modify the physical properties of the bilayer are known to affect the enzymatic activity. For example, it was shown that several lipophilic compounds reduce the rate of lipid hydrolysis by affecting the bilayer physical properties in a way that promotes desorption of bound enzymes (48). Moreover, factors that allow an easier access of the enzyme to substrate shift the reaction equilibrium toward the products. These factors can be various; for example, the high curvature of the bilayer or the presence of cone-shaped lipids, such as cardiolipin or DAG (49 and references cited therein).

In general, the PLA2 hydrolitic activity is significantly affected by the presence of lipid lateral segregation into domains and by other bilayer defects, which can be in the form of holes in the bilayer (50, 51), or line defects occurring in bilayers undergoing density fluctuations near the main-phase Tm (41). In fact, the increase of PLA2 activity near the lipid Tm was related to the contemporary increase of the heterogeneous lateral bilayer structure (41) due to thermal density fluctuations. Since NAEs modify physico-chemical bilayer properties, the effect of these hydrophobic molecules on the PLA2 activity was examined.

Pig pancreas PLA2 (classified as type I of the enzymatic family) (52) was chosen for this study because it is recognized to represent a good model for in vitro studies of phospholipase activity (53, 54). Moreover, studies on this protein are interesting because the presence of type I PLA2 proteins was demonstrated in various tissues, including kidney, small intestine, spleen, lung, and stomach (55). A pancreatic type I PLA2 was recently identified in the human epidermis (56). Moreover, the presence of a PLA2, which is revealed by immunostaining with antibodies raised against the porcine pancreatic enzyme, was detected in the acrosomal region of human spermatozoa (57).

Each sample tested (R = 0.1) did not show evident phase separation within the bilayer, as evidenced by X-ray diffraction experiments and Laurdan GP thermotropic behavior. Only NOEA shows a slightly larger width of the main transition, which is shifted to low temperatures. This width could reflect a lipid bilayer heterogenity, likely due to the presence of unsaturation, which decreases the acyl chain packing. Lipid Tm changes, measured by Laurdan, indicating that saturated NAEs (characterized by a long acyl chain, an amide linkage, and the free hydroxyl group of ethanolamine) affect DPPC Tm following a similar pattern to alcohols. In fact, it is known that short-chain n-alkanols decrease the temperature of the PL main transition, while molecules with a carbon chain longer than 10–12 carbon atoms (cut-off length) increase it (58, 59). Laurdan data suggest that, at 38.7°C, NLEA- and NOEA-containing samples will be closer to lipid main-phase transition if compared with pure DPPC LUVs and LUVs containing longer NAEs. It is known that PLA2 activity toward pure DPPC LUVs displays a maximum (corresponding to a minimum of lag time) at the gel-to-fluid phase Tm. This effect was related to the thermal density fluctuations inducing the dynamic formation of small-scale gel-fluid lipid domains, and hence microheterogeneity (28, 41).

An increasing degree of microheterogeneity is expected for a bilayer that is approaching the Tm. Due to the effect of NLEA and NOEA on the gel-to-liquid-crystalline temperature transition, a NLEA- and NOEA-induced increase of PLA2 activity was expected, with respect to the control, as a consequence of an increased degree of lateral heterogeneity. An opposite effect was expected for NPEA and NSEA. As predicted, the presence of NLEA increases the PLA2 activity (i.e., it induces a lag time decrease), while NCEA does not modify it. However, the effect of the other NAEs on lag time was unexpected. The slight inhibitory effect of NOEA (measured for both NOEA concentrations used) could be related to the large decrease of lipid chain packing caused by the presence of unsaturation at Carbon 9 and suggested by the low Laurdan GP values measured at temperatures below those of the main lipid phase transition. This membrane disorganization is close to the membrane interfacial region, and it could avoid the optimal enzyme-lipid interaction. In fact, in pancreatic enzymes, this interaction requires that flexible regions in the free enzyme, such as the N-terminus and the surface loop, become more ordered in the complexed form (60). This effect could be prevented by the membrane disorganization induced by NOEA. Another possibility is that PLA2 could dig into the membrane, while a superficial localization is necessary for a good hydrolytic activity (60).

In order to explain the results obtained with saturated NAEs, we propose that the cause of NAEs' effect on PLA2 activity can be their localization in the bilayer, which could be determined by the particular structural properties of these molecules; i.e., the presence of two groups that can form hydrogen bonds (hydroxyl and amide groups) and the long acyl chain. We hypothesize that the chain length can affect the partition coefficient of the NAE within the membrane and/or the localization along the normal to the bilayer plane. The reduced perturbation of DPPC bilayer by NCEA could be a consequence of a low-partition coefficient due to relatively low energy gain by van der Waals interactions with lipid acyl chains, which can be overlaid by the hydrogen bond formation with solvent molecules and/or PL headgroups. The additional two carbons of the NLEA acyl chain could be critical for its incorporation within the bilayer. However, the DPPC Tm lowering suggests an NLEA-induced bilayer disorganization due to an improper fit of the hydrocarbon chain into the membrane matrix. This effect could create voids in the hydrocarbon core, as previously suggested for 1-alkanols with similar carbon chain length (61). The NPEA- and NSEA-induced increase of DPPC Tm could be explained, assuming that these molecules enhance the lipid-lipid interactions and participate in them, similarly to alcohols with the same number of carbon atoms (61). However, in spite of NPEA's and NSEA's similar action on lipid Tm and on the polarity of the Laurdan microenvironment, their PLA2 activity modulation is different. We suggest that NLEA and NPEA can be linked to PL headgroups by two hydrogen bonds: the nitrogen atom of the amide could interact with the carbonyl group of the sn-2 acyl chain, while the hydroxyl group of NAE could be hydrogen bonded with the phosphate group. The long, unfolded chain of NSEA (18:0) could avoid a proper localization of the amide and hydroxyl groups in the bilayer by shifting them in a too-high position to form hydrogen bonds with acceptor groups in DPPC. The model proposed could explain the increased activity by NLEA and NPEA, which can introduce a surface heterogeneity by creating a large void in the headgroup region and permitting a PLA2 greater access to the sn-2 bonds of the substrate. This hypothesis is in line with the behavior suggested by Henshaw et al. (21) for protonated palmitic acid, which forms hydrogen bonds with the PL phosphate group, descending deeper in the bilayer than the ionized form. On the contrary, the low effect of NSEA on PLA2 activity could be due to its higher position in the bilayer. In this case, the slight decrease of lag time could be due to a decreased formation of voids in the headgroup region. However, results obtained with NSEA/DPPC R = 0.01 vesicles could suggest another possible mechanism for NSEA action. In fact, Laurdan GP temperature dependence curves (Fig. 2) cannot exclude the possible formation of small-scale clusters of different lipid compositions for each sample studied. The presence of such clusters could affect bilayer heterogeneity, which appears to be important to promoting the Ca2+-dependent Step 2 (21 and references cited therein). The possible presence of these clusters could be supported by results obtained with the lower NAE concentration (R = 0.01). In fact, the opposite effect shown by the two concentrations of NSEA (R = 0.1 and R = 0.01) on PLA2 activity could be explained by hypothesizing the contemporary action of two different effects. In the case of R = 0.1, the activating effect, due to the heterogeneity introduced in the bilayer by these small clusters, could counteract the inhibiting action due to the NSEA-induced enhanced lipid-lipid interaction. In the case of R = 0.01, the cluster formation could be negligible, and the inhibiting action of NSEA could prevail. In fact, it is known that an increased cohesion of the lipid bilayer is detrimental to the lytic action of the enzyme (49). The hypothesis of different NPEA and NSEA interactions with DPPC headgroups is supported by Prodan results, which indicate an NAEs-induced reduced polarity in the probe microenvironment of these samples. This effect is likely due to an increased PL ordering, which is suggested by 3wGP values (in the order NPEA>>NSEA>DPPC) and by the Prodan repartition Cp. Cp values indicate that probe exposure to bulk water increases in the order DPPC<NSEA<NPEA. This ordering effect is in line with the hypothesis of different NAE-PC interactions for NPEA and NSEA, with the possible formation of strong hydrogen bonds NPEA-PC and of weaker interactions NSEA-PC at the headgroup level. Also, X-ray data confirm the hypothesis of different NAE interactions at the lipid polar headgroups. In fact, at 38.7°C, NPEA-containing DPPC MLV samples are in Lß' phase, while NSEA-containing DPPC liposomes are in Pß', indicating an NPEA-induced shift of Lß'- Pß' lipid transition (pretransition) to higher temperatures. These results confirm the hypothesis of strong interactions affecting the pretransition in the lipid-polar headgroup region. Furthermore, the presence of the Pß' phase in NLEA-DPPC samples is not surprising, because, as previously discussed, NLEA's shorter acyl chain, compared with the palmitoyl chains of the phosphatidylcholine, likely induces bilayer core disorganization and, as a consequence, a decrease of phase transitions temperatures. Moreover, the calculated cell dimensions indicate that the DPPC interlamellar repeat distance is largely increased by NLEA, while only a small effect is induced by NSEA. NLEA-induced results could be indicative of a phase change (i.e., a change in the lipid molecular organization) or an increase of the water thickness between the lipid bilayers. Since the high-angle profile of DPPC-NLEA excludes a phase change, it is deduced that the water thickness is increased, in agreement with a possible increased hydration of PL headgroups. A possible increase of PL headgroups hydration, without phase change, can be deduced also by data reported for DPPC-NPEA. In fact, the calculated cell dimensions for the Lß' phase in this sample are significantly different from the values usually reported for pure DPPC in the same phase (about 64 Å) (47). These measurements suggested an NPEA- and NLEA-induced increase of interlamellar water layer likely correlated with an increased hydration of PC headgroups, in agreement with the hypothesis of the presence of void spaces in this bilayer region.

The possible different localization of NAEs in the bilayer could affect the PLA2 adsorption at the membrane surface, also in the absence of Ca2+. However, data reported in Table 2 indicate that membrane physico-chemical properties probed by Laurdan are unaffected by the presence of the enzyme in the absence of the cation. These results could suggest that NAEs modulate the enzymatic activity without increasing the number of adsorbed enzyme molecules in Step 1. Recent works proposed that membrane surface defects could modify the PLA2 enzymatic activity by altering the equilibrium of Ca2+-dependent Step 2 (26) and by allowing the PLA2 active site greater access to the sn-2 bond of phosphatidylcholine (21). Moreover, an action at Step 2 was suggested for unionized palmitic acid (21). The comparison with our data could indicate that NAEs affect the enzymatic activity at Step 2. However, it is likely to be excluded that NAEs could increase the ability of the substrate molecule to leave the plane of the bilayer, which is a possible mechanism of Step 2 (21, 26). This hypothesis is suggested because the PLA2 activity is almost unmodified by the presence of NOEA, although this unsaturated molecule largely disorganizes the membrane structural matrix in the gel phase, suggesting an increased possibility of lipid substrate leaving the membrane.

Conclusions
The NAEs' lipophilicity provides opportunity to examine the interaction of these molecules with lipids, which could have important pharmacological actions. Fluorescence and X-ray techniques help to understand NAE's interaction with a PC liposome, providing information about structural and physico-chemical changes induced by these molecules on the bilayer. The NAE concentrations used in this work are not normally present in membranes (2), although the NAE-DPPC molar ratio R = 0.01 could be reached in some conditions. In fact, in ischemic canine heart, NAEs can be accumulated to a level of 400–500 nmol/g tissue (2), which corresponds, assuming 20 µmol lipid phosphorous/g tissue (2), to a NAE-PL molar ratio of 0.020:0.025. Moreover, the possible preferential NAE localization in specific membrane domains with different lipid compositions could give rise to microenvironments with different structural and physico-chemical features, as suggested by our present data and a previous work, which demonstrated the formation of a cubic phase when NAEs interact with egg phosphatidylethanolamine (16). For this reason, data on NAE's effects on PLA2 activity could support the hypothesis that some physiological functions of NAEs could be performed through the modulation of structural and physico-chemical membrane features (1, 16, 17).

Moreover, our results could be useful for projecting a proper system for delivering pharmacologically active NAEs to tissues. In fact, small-sized liposomes with a rigid bilayer are considered suitable carrier systems for target drug delivery to tumors and inflammatory foci (62), and recent studies suggested that their use for a possible targeted transport to pathological tissues (e.g., in inflammation) could be linked to their degradation by membrane active and destabilization enzymes, such as PLA2 (63), whose activity is often locally increased in such tissues (64, 65).


   ACKNOWLEDGMENTS
 
This work was supported by Ministero Italiano dell'Università e della Ricerca Scientifica (MURST) to G.Z. and Komitet Badañ Naukowych grant 6905A06220 to M.W.

Manuscript received October 3, 2002 and in revised form December 20, 2002.


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