Hydrolytic action of phospholipase A2 in monolayers in the phase transition region: direct observation of enzyme domain formation using fluorescence microscopy

Effect of the lipid hydrolysis products on the phospholipase A2 action towards lipid monolayer

The effect of lauric acid (LA) and lysolauroyllecithin (LLL) on the hydrolysis of lipid in monolayer by phospholipase A2 from Bee venom was studied. It was found that LLL inhibits phospholipase action under both high (39 mN/m) and low (25 mN/m) surface pressure. On the other hand, LA inhibits phospholipase action under the low surface pressure (15 mN/m or 25 mN/m), but increases enzyme activity under high surface pressure (39 mN/m). This activating effect can be suppressed by high ionic strength of the aqueous subphase. It is suggested that an increase of the negative surface charge of the lipid monolayer, followed by an increase of the local concentrations of the positively charged enzyme and calcium near the monolayer is a coupling factor between fatty acid accumulation and phospholipase activation. Such an autocatalytic process can only occur when the substrate is organised into monolayer, bilayer or micelles, therefore it can be considered as a reason for the substrate activation and induction time before lipid hydrolysis.

Regulation by gangliosides and sulfatides of phospholipase A2 activity against dipalmitoyl- and dilauroylphosphatidylcholine in small unilamellar bilayer vesicles and mixed monolayers

The modulation by gangliosides GM1 and GD1a, and sulfatide (Sulf) of the activity of porcine pancreatic phospholipase A2 was studied with small unilamellar vesicles of dipalmitoylphosphatidylcholine (L-dpPC) and lipid monolayers of dilauroylphosphatidylcholine (L-dlPC). The presence of Sulf always led to an increase of the maximum rate of the enzymatic reaction, irrespective on whether the vesicles were above, in the range of, or below the bilayer transition temperature. Sulf did not modify the latency period for the reaction that is observed at the bilayer transition temperature. Gangliosides inhibited the maximum rate of enzymatic activity bilayer vesicles in the gel phase but the effect was complex. When the reaction was carried out at a temperature within the range of the bilayer phase transition, the gangliosides inhibited the maximal rate of the reaction in proportion to their content in the bilayer. However, at the same time the latency period observed with vesicles of pure phospholipid at this temperature was shortened in proportion to the mole fraction of gangliosides in the bilayer. At temperatures above the bilayer phase transition, gangliosides stimulated the activity of PLA2. Preincubation of the enzyme with Sulf or gangliosides did not affect the activity against bilayer vesicles of pure substrate. These glycosphingolipids did not modify the rate or extent of desorption of the enzyme from the interface, nor the pre-catalytic steps for the interfacial activation of PLA2, or the enzyme affinity for the phospholipid substrate. Also, the activity of the enzyme was not altered irreversibly by glycosphingolipids. Our results indicate that Sulf and gangliosides modulate the catalytic activity of PLA2 at the interface itself, beyond the initial steps of enzyme adsorption and activation, probably through modifications of the intermolecular organization and surface electrostatics of the phospholipid substrate.

Tryptophan fluorescence study on the interaction of pulmonary surfactant protein A with phospholipid vesicles

The fluorescence characteristics of surfactant protein A (SP-A) from porcine and human bronchoalveolar lavage were determined in the presence and absence of lipids. After excitation at either 275 or 295 nm, the fluorescence emission spectrum of both proteins was characterized by two maxima at about 326 and 337 nm, indicating heterogeneity in the emission of the two tryptophan residues of SP-A, and also revealing a partially buried character for these fluorophores. Interaction of both human and porcine SP-A with various phospholipid vesicles resulted in an increase in the fluorescence emission of tryptophan without any shift in the emission wavelength maxima. This change in intrinsic fluorescence was found to be more pronounced in the presence of dipalmitoyl phosphatidylcholine (DPPC) than with dipalmitoyl phosphatidylglycerol (DPPG), DPPC/DPPG (7:3, w/w) and 1-palmitoyl-sn-glycerol-3-phosphocholine (LPC). Intrinsic fluorescence of SP-A was almost completely unaffected in the presence of egg phosphatidylcholine (egg-PC). In addition, we demonstrated a shielding of the tryptophan fluorescence from quenching by acrylamide on interaction of porcine SP-A with DPPC, DPPG or LPC. This shielding was most pronounced in the presence of DPPC. In the case of human SP-A, shielding was only observed on interaction with DPPC. From the intrinsic fluorescence measurements as well as from the quenching experiments, we concluded that the interaction of some phospholipid vesicles with SP-A produces a conformational change on the protein molecule and that the interaction of SP-A with DPPC is stronger than with other phospholipids. This interaction appeared to be independent of Ca2+ ions. Physiological ionic strength was found to be required for the interaction of SP-A with negatively charged vesicles of either DPPG or DPPC/DPPG (7:3, w/w). Intrinsic fluorescence of SP-A was sensitive to the physical state of the DPPC vesicles. The increase in intrinsic fluorescence of SP-A in the presence of DPPC vesicles was much stronger when the vesicles were in the gel state than when they were in the liquid-crystalline state. The effect produced by SP-A on the lipid vesicles was also dependent on temperature. The aggregation of DPPC, DPPC/DPPG (7:3, w/w) or dimyristoyl phosphatidylglycerol (DMPG) was many times higher below the phase-transition temperature of the corresponding phospholipids. These results strongly indicate that the interaction of SP-A with phospholipid vesicles requires the lipids to be in the gel phase.

Conformational changes in oxidized phospholipids and their preferential hydrolysis by phospholipase A2: a monolayer study

Cleavage of oxidized fatty acids by phospholipase A2 has been implicated as the first step in the repair mechanism for oxidative damage to membrane phospholipids. However, the mechanism by which this enzyme preferentially hydrolyzes oxidized fatty acyl chains is poorly understood. Using a lipid monolayer technique, we found that the molecular surface areas of 1-palmitoyl-2-(9/13-hydroperoxylinoleoyl)-phosphatidylcholine (PLPC-OOH) and 1-palmitoyl-2-(9/13-hydroxylinoleoyl)phosphatidylcholine (PLPC-OH) were increased by as much as 50% relative to the parent nonoxidized 1-palmitoyl-2-linoleoylphosphatidylcholine (PLPC). These experimental data directly indicate a drastically changed molecular conformation of oxidized phospholipids in which the hydroperoxy or hydroxy group in the sn-2 fatty acid is close to the lipid-water interface. Phospholipases A2 from porcine pancreas and from bee venom were shown to break down PLPC-OOH and PLPC-OH monolayers much faster than PLPC monolayers. In all cases, the presence of serum albumin in the subphase enhanced monolayer breakdown by extracting hydrolysis products from the monolayer, but monolayer breakdown was always much faster for oxidized than for nonoxidized PLPC. This did not appear to be due to change in the extent of monolayer penetration by phospholipase A2, since enzyme-monolayer interaction studies revealed essentially identical penetration behavior of bee venom phospholipase A2 with PLPC, PLPC-OOH, and PLPC-OH monolayers. We propose that the altered molecular conformation of oxidized phospholipids facilitates access to the sn-2 ester bond, thereby ensuring their preferential hydrolysis in the presence of a phospholipase A2.

Phase separated anionic domains in ternary mixed lipid monolayers at the air-water interface

A series of ternary mixed monolayers containing varying amounts of 1,2-dipalmitoyl-sn-glycero-3-phosphocholine (DPPC) and equimolar additions of 1-palmitoyl-2-hydroxy-sn-glycero-3-phosphocholine (LYSO-PC) and palmitic acid (PA) were studied at the air-water interface. These mixed monolayers were used to model phospholipid biomembrane interfaces resulting from phospholipase A2 (PLA2) hydrolysis. Recent work [D.W. Grainger A. Reichert, H. Ringsdorf and C. Salesse (1989) Biochim. Biophys. Acta. 1023, 365-379] has shown that PLA2 hydrolysis of pure phospholipid monolayers results in formation of large PLA2 domains at the air-water interface. These domains are proposed to result from PLA2 adsorption to phase separated regions in the hydrolyzed monolayer. To elucidate the phase behaviour in these monolayer systems, surface pressure-area isotherms were measured for the ternary mixtures on pure water and buffered subphases. Fluorescence microscopy at the air-water interface was used to image fluorescent probe-doped monolayer mixtures during isothermal compressions. A water-soluble cationic carbocyanine dye was used to probe the interfacial properties of the mixed monolayers. Isotherm data do not provide unambiguous evidence for either phase separation or ideal mixing of monolayer components. Fluorescence microscopy is more revealing, showing that lateral phase separation of microstructures containing palmitic acid occurred only when monolayer subphases contained Ca2+ ions at alkaline pH. At either low pH or on Ca(2+)-free subphases, phase separation was not observed.

Role of lateral phase separation in the modulation of phospholipase A2 activity

Phospholipase A2-catalyzed hydrolysis of phosphatidylcholine large unilamellar vesicles is characterized by a period of slow hydrolysis followed by a rapid increase in the rate of hydrolysis. The temporal relationship between the burst of PLA2 activity and the lateral distribution of substrate and product lipids was examined by simultaneously recording product accumulation and the fluorescence of 1-pyrenyldecanoate, a fatty acid derivative sensitive to lipid distribution and lateral diffusion. The excimer: monomer ratio of the probe changes slowly prior to the burst in activity and then abruptly at the time of the burst. A partial phase diagram for the ternary codispersion of substrate and products (dipalmitoylphosphatidylcholine and 1:1 monopalmitoylphosphatidylcholine/palmitic acid) was constructed by differential scanning calorimetry and suggests gel/gel immiscibility in this system. Thus, the changes in pyrene fluorescence during the time course of hydrolysis appear to be due to lateral phase separation. The critical mole fraction of product both for lateral phase separation in the gel state and for elimination of the lag phase is approximately 0.083. The simultaneous recordings of PLA2 activity and pyrene fluorescence show that the lateral rearrangement of lipids begins prior to and continues during the rapid activation process of PLA2. Two possible effects of lateral phase separation are that concentration of the protein in the product-rich regions promotes putative dimerization or that formation of phase interface regions promotes enzyme activation.

Quenching of fluorescein-conjugated lipids by antibodies. Quantitative recognition and binding of lipid-bound haptens in biomembrane models, formation of two-dimensional protein domains and molecular dynamics simulations

Three model biomembrane systems, monolayers, micelles, and vesicles, have been used to study the influence of chemical and physical variables of hapten presentation at membrane interfaces on antibody binding. Hapten recognition and binding were monitored for the anti-fluorescein monoclonal antibody 4-4-20 generated against the hapten, fluorescein, in these membrane models as a function of fluorescein-conjugated lipid architecture. Specific recognition and binding in this system are conveniently monitored by quenching of fluorescein emission upon penetration of fluorescein into the antibody's active site. Lipid structure was shown to play a large role in affecting antibody quenching. Interestingly, the observed degrees of quenching were nearly independent of the lipid membrane model studied, but directly correlated with the chemical structure of the lipids. In all cases, the antibody recognized and quenched most efficiently a lipid based on dioctadecylamine where fluorescein is attached to the headgroup via a long, flexible hydrophilic spacer. Dipalmitoyl phosphatidylethanolamine containing a fluorescein headgroup demonstrated only partial binding/quenching. Egg phosphatidylethanolamine with a fluorescein headgroup showed no susceptibility to antibody recognition, binding, or quenching. Formation of two-dimensional protein domains upon antibody binding to the fluorescein-lipids in monolayers is also presented. Chemical and physical requirements for these antibody-hapten complexes at membrane surfaces have been discussed in terms of molecular dynamics simulations based on recent crystallographic models for this antibody-hapten complex (Herron et al., 1989. Proteins Struct. Funct. Genet. 5:271-280).

Substrate level modulation of the activity of phospholipase A2 in vitro by 12-O-tetradecanoylphorbol-13-acetate

The action of porcine pancreatic phospholipase A2 towards fluorescent phospholipid analogs is either enhanced or suppressed by 4 beta-12-O-tetradecanoylphorbol-13- acetate (TPA), depending on the chemical structure of the substrate and the concentration of Ca2+. In the presence of nmolar Ca2+ concentrations increasing [TPA] enhanced by approx. 5-fold the rate of hydrolysis of the pyrene-labelled acidic alkyl-acyl phospholipid, 1-octacosanyl-2-[6- (pyrene-1-yl)] hexanoyl-sn-glycero-3-phosphatidylmethanol. Maximal effect was obtained at high TPA/substrate molar ratios approaching 1:2. In the presence of 4 mM CaCl2 maximal activation was reduced to approximately 1.5-fold. With the corresponding phosphatidylcholine derivative as a substrate increasing [TPA] reduced fatty acid release maximally by 90% both at low [Ca2+] as well as in the presence of 4 mM CaCl2. Essentially identical results were obtained using 4 alpha-TPA, a stereoisomer which does not activate protein kinase C.

Spontaneous domain formation of phospholipase A2 at interfaces: fluorescence microscopy of the interaction of phospholipase A2 with mixed monolayers of lecithin, lysolecithin and fatty acid

Fluorescence microscopy has recently been proven to be an ideal tool to investigate the specific interaction of phospholipase A2 with oriented substrate monolayers. Using a dual labeling technique, it could be shown that phospholipase A2 can specifically attack and hydrolyze solid analogous L-alpha-DPPC domains. After a critical extent of monolayer hydrolysis the enzyme itself starts to aggregate forming regular shaped protein domains (Grainger et al. (1990) Biochim. Biophys. Acta 1023, 365-379). In order to confirm that the existence of hydrolysis products in the monolayer is necessary for the observed aggregation of phospholipase A2, mixed monolayers of D- and L-alpha-DPPC, L-alpha-lysoPPC and palmitic acid in different ratios were examined. The phase behavior and the interaction of these films with phospholipase A2 were directly visualized with an epifluorescence microscope. Above a certain critical concentration of lysolecithin and palmitic acid in the monolayer, compression of these mixed films leads to phase separation and formation of mixed domains of unknown composition. Their high negative charge density is evidenced by preferential binding of a cationic dye to these phase-separated areas. Introduction of fluorescence-labeled phospholipase A2 underneath these mixed domains results in rapid binding of the protein to the domains without visible hydrolytic activity, regardless of whether the L-form or the D-form of the DPPC were used. In binary mixtures, only those with DPPC/palmitic acid show formation of phase-separated areas which can be specifically targeted by phospholipase A2 leading to a rapid formation (within 2 min) of protein domains. Experiments with pyrenedecanoic acid containing monolayers give the first direct evidence that acid is located above the enzyme domains. These results show that a locally high negative charge density of the phase-separated domains is one of the prerequisites for the binding of phospholipase A2. In addition, however, small amounts of D- or L-alpha-DPPC headgroups within the domains of the monolayer seem to be necessary for recognition followed by fast binding of the protein to the domains. This is confirmed by experiments with mixed monolayers of diacetylene carboxylic acid and D-alpha-DPPC. The acid--immiscible with lecithin--forms well defined pure acid domains in the monolayer. While the cationic dye can be docked rapidly to these phase separated areas, no preferential enzyme binding and thus no protein domain formation below these acid domains can be induced.

Regulation of fatty acid 18O exchange catalyzed by pancreatic carboxylester lipase. 2. Effects of lateral lipid distribution in mixtures with phosphatidylcholine

The lipase-catalyzed exchange of the carboxyl oxygens of 13,16-cis,cis-docosadienoic acid (DA) was studied in the presence of a nonsubstrate matrix lipid, 1-palmitoyl-2-oleoylphosphatidylcholine. For mixed lipid films at the argon-water interface exposed to pancreatic carboxylester lipase (EC 1.1.1.13), the extent of oxygen exchange showed an abrupt increase as the abundance of DA in the interface was increased from 0.5 to 0.6 mole fraction. This compositional range was independent of the level of enzyme used and of the surface pressure, i.e., lipid packing density, of the film. Concomitant with the transition was a change in the apparent mechanism of exchange from coupled to random sequential. Like the extent of oxygen exchanged, the shift in mechanism was independent of all variables except the lipid composition of the interface. The absence of any chemical or physical change accompanying the exchange reaction precludes mechanistic explanations based on the generation of reaction products by the enzyme. Instead, the results suggest that the lateral distribution of DA in phosphatidylcholine-DA interfaces regulates the expression of carboxylester lipase activity and its apparent mechanism. Preliminary measurements give an average cluster size of 1825 molecules of DA when its mole fraction is 0.35. As the DA content of the interface reaches 0.5-0.6, there appears to be a lipid head-group based percolative transition in which DA becomes the continuum. Because this transition involves the lateral organization of the lipids themselves, other interfacially active enzymes may be regulated similarly.

Epifluorescence microscopic observation of monolayers of dipalmitoylphosphatidylcholine: dependence of domain size on compression rates

A fluorescence microscopic technique was used to observe phase transitions in monolayers of DPPC. The sizes of the domain structures observed were found to be dependent on the rate of compression of the monolayer. The distribution of domain sizes for different rates of compression were unimodal, but the scatter in the sizes was greater during slow compressions.

Line defects in gel phase lipid monolayers

We investigate various models of the hydrolysis of gel-phase phosphatidylcholine monolayers by phospholipase A2 (Grainger et al. (1989) FEBS Lett. 252, 73-82). We assume that the probability of hydrolysis of a given lipid depends only upon how many of its nearest neighbour lipids have already been hydrolysed. We find that the experimental data are consistent with a model in which line defects exist in the gel phase and that lipids on such defects are more easily hydrolysed than the other gel-phase lipids. Based on this model, we calculate the course of hydrolysis of a gel-phase region possessing line defects, and we suggest how such a structure might be made and the model tested. An experiment, similar to that proposed by us, has been carried out by Grainger et al. (1990) Biochim. Biophys. Acta 1023, 365-379). We also calculate the fractal dimension, df, of the interface created by the hydrolytic process and show that a measurement of df might identify how this process proceeds.

Fluorescence microscopy of phospholipid monolayer phase transitions

Over many years, a detailed picture of the phase transitions in phospholipid monolayers at the air-water interface has been constructed from extensive studies of the force-area, viscoelastic and surface potential properties of phospholipid monolayers, yet the microscopic nature of the transitions has remained obscure. Recent investigations have focused specifically on these aspects. Through the use of fluorescence microscopy, electron diffraction and X-ray scattering experiments, in combination with data obtained by classical methods, a wealth of new information regarding the properties of monolayers undergoing phase transitions has been generated. Direct observation of fluid-solid phase coexistence at the air-water interface has been achieved with fluorescence microscopy and on solid supports with electron microscopy. The fluid-solid coexistence region has been studied most thoroughly to date, but regions of gas-fluid and fluid-fluid phase coexistence have also been detected. Numerous factors govern the properties of the coexistence region: however, the prominent features can be explained in terms of a competition between forces: long-range electrostatic forces and short-range attractive forces. In this review these recent experimental findings and theoretical interpretations are summarized.

On the principles of functional ordering in biological membranes

Integrating the available data on lipid-protein interactions and ordering in lipid mixtures allows to emanate a refined model for the dynamic organization of biomembranes. An important difference to the fluid mosaic model is that a high degree of spatiotemporal order should prevail also in liquid crystalline, "fluid" membranes and membrane domains. The interactions responsible for ordering the membrane lipids and proteins are hydrophobicity, coulombic forces, van der Waals dispersion, hydrogen bonding, hydration forces and steric elastic strain. Specific lipid-lipid and lipid-protein interactions result in a precisely controlled yet highly dynamic architecture of the membrane components, as well as in its selective modulation by the cell and its environment. Different modes of organization of the compositionally and functionally differentiated domains would correspond to different functional states of the membrane. Major regulators of membrane architecture are proposed to be membrane potential controlled by ion channels, intracellular Ca2+, pH, changes in lipid composition due to the action of phospholipase, cell-cell coupling, as well as coupling of the membrane with the cytoskeleton and the extracellular matrix. Membrane architecture is additionally modulated due to the membrane association of ions, lipo- and amphiphilic hormones, metabolites, drugs, lipid-binding peptide hormones and amphitropic proteins. Intermolecular associations in the membrane and in the membrane-cytoskeleton interface are further selectively controlled by specific phosphorylation and dephosphorylation cascades involving both proteins and lipids, and regulated by the extracellular matrix and the binding of growth factors and hormones to their specific receptor tyrosine kinases. A class of proteins coined architectins is proposed, as a notable example the pp60src kinase. The functional role of architectins would be in causing specific changes in the cytoskeleton-membrane interface, leading to specific configurational changes both in the membrane and cytoskeleton architecture and corresponding to (a) distinct metabolic/differentiation states of the cell, and (b) the formation and maintenance of proper three dimensional membrane structures such as neurites and pseudopods.