Facing up to membranes: structure/function relationships in phospholipases

Continuous enteral nutrition attenuates pulmonary edema in rats exposed to 100% oxygen

Adult rats exposed to hyperoxia develop anorexia, weight loss, and a lung injury characterized by pulmonary edema and decreased lung liquid clearance. We hypothesized that maintenance of nutrition during hyperoxia could attenuate hyperoxia-induced pulmonary edema. To test this hypothesis, we enterally fed adult male Sprague-Dawley rats via gastrostomy tubes and exposed them to oxygen (inspired O(2) fraction >0.95) for 64 h. In contrast to controls, enterally fed hyperoxic animals did not lose weight and had smaller pleural effusions and wet-to-dry weight ratios (a measure of lung edema) that were not different from room air controls. Enterally fed rats exposed to hyperoxia had increased levels of mRNA for the Na(+)-K(+)-ATPase alpha(1)- and beta(1)-subunits and glutathione peroxidase. These findings suggest that maintenance of nutrition during an oxidative lung injury reduces lung edema, perhaps by allowing for continued expression and function of protective proteins such as the Na(+)-K(+)-ATPase.

Molecular dynamics study of phospholipase A2 on a membrane surface

The desolvation of lipid molecules in a complex of the enzyme human synovial phospholipase A2 with a lipid membrane is investigated as a mechanism that enhances the overall activity of the enzyme. For this purpose the interaction of the enzyme phospholipase A2 with a dilauryl-phosphatityl-ethanolamin (DLPE) membrane monolayer surface has been studied by means of molecular dynamics simulations. Two enzyme-membrane complexes, a loose and a tight complex, are considered. For comparison, simulations are also carried out for the enzyme in aqueous solution. The conformation, dynamics, and energetics of the three systems are compared, and the interactions between the protein and lipid molecules are analyzed. Free energies of solvation are calculated for the lipid molecules in the enzyme-membrane interface. Along with the calculated dielectric susceptibility at this interface, the results show the desolvation of lipids in a tightly bound, but not in a loosely bound protein-membrane complex. The desolvated lipids are found to interact mainly with hydrophobic protein residues, including Leu-2, Val-3, Ala-18, Leu-19, Phe-24, Val-31, and Phe-70. The results also explain why the turnover rate of phospholipase A2 complexed to a membrane is enhanced after a critical amount of negatively charged reaction product is accumulated.

Changes in the structure of bovine phospholipase A2 upon micelle binding

Phospholipase A2 (PLA2) is a calcium-dependent enzyme which hydrolyses the 2-acyl ester bond of phospholipids. The extracellular PLA2s are activated by as much as 10000-fold on binding to micelles or vesicles of substrate, possibly due to a conformational change induced in the enzyme. We have studied the complex of bovine pancreatic PLA2 with micelles of SDS by ultracentrifugation, equilibrium dialysis, microcalorimetry, fluorescence and n.m.r. spectroscopy. Ultracentrifugation and equilibrium dialysis measurements showed that on average 1.28 (+/- 0.17) PLA2 molecules and 26.4 (+/- 3.1) SDS molecules are involved in the complex and that there is a rapid equilibrium between micellar species containing one or more enzyme monomers. The estimated heat of formation of the complex, measured calorimetrically as the heat released when PLA2 was injected into excess 10 mM SDS, was 162.3 +/- 1.5) kJ/mol [38.8 (+/- 0.35) kcal/mol] of PLA2 added. The fluorescence of the single tryptophan at position 3 in the N-terminal helix of the protein increases when PLA2 binds to SDS micelles, indicating that this part of the protein is in a more hydrophobic environment in the complex. The structural changes in PLA2 on addition of [2H25]SDS were monitored using n.m.r. spectroscopy. The overall structure of the protein is unchanged, but changes in nuclear Overhauser effects (NOEs) were observed for residues in the N-terminal helix, at the active site region and in a lysine-rich region near the C-terminus. The NOE changes at the N-terminus indicate that this portion of the protein molecule adopts a more ordered, helical conformation when bound to a micelle. We suggest that these conformational changes could be the mechanism by which the enzyme becomes activated in the presence of aggregated substrate.

Modeling of substrate and inhibitor binding to phospholipase A2

Molecular graphics and molecular mechanics techniques have been used to study the mode of ligand binding and mechanism of action of the enzyme phospholipase A2. A substrate-enzyme complex was constructed based on the crystal structure of the apoenzyme. The complex was minimized to relieve initial strain, and the structural and energetic features of the resultant complex analyzed in detail, at the molecular and residue level. The minimized complex was then used as a basis for examining the action of the enzyme on modified substrates, binding of inhibitors to the enzyme, and possible reaction intermediate complexes. The model is compatible with the suggested mechanism of hydrolysis and with experimental data about stereoselectivity, efficiency of hydrolysis of modified substrates, and inhibitor potency. In conclusion, the model can be used as a tool in evaluating new ligands as possible substrates and in the rational design of inhibitors, for the therapeutic treatment of diseases such as rheumatoid arthritis, atherosclerosis, and asthma.

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.

Membrane structure, toxins and phospholipase A2 activity

The phospholipid-hydrolyzing enzyme phospholipase A2 (PLA2) (EC 3.1.1.4) exists in several forms which can be located in the cytosol or on cellular membranes. We review briefly cellular regulatory mechanisms involving covalent modification by protein kinase C and the action of Ca2+, cytokines, G proteins and other cellular proteins. The major focus is the role of phospholipid structure on PLA2 activity, including (1) the mechanism of PLA2 action on synthetic phospholipid bilayers, (2) perturbation of synthetic and cellular membranes with lipophilic agents and membrane-interactive peptides and (3) the ability of these agents to activate endogenous PLA2 activity, with emphasis on the venom and plant toxins melittin, cardiotoxin and Pyrularia thionein.

Structures of free and inhibited human secretory phospholipase A2 from inflammatory exudate

Phospholipase A2 (PLA2) participates in a wide range of cellular processes including inflammation and transmembrane signaling. A human nonpancreatic secretory PLA2 (hnps-PLA2) has been identified that is found in high concentrations in the synovial fluid of patients with rheumatoid arthritis and in the plasma of patients with septic shock. This enzyme is secreted from certain cell types in response to the proinflammatory cytokines, tumor necrosis factor or interleukin-1. The crystal structures of the calcium-bound form of this enzyme have been determined at physiological pH both in the presence [2.1 angstrom (A) resolution] and absence (2.2 A resolution) of a transition-state analogue. Although the critical features that suggest the chemistry of catalysis are identical to those inferred from the crystal structures of other extracellular PLA2s, the shape of the hydrophobic channel of hnps-PLA2 is uniquely modulated by substrate binding.

Role of the N-terminal region of phospholipase A2 subunit of beta 1-bungarotoxin in the toxin-Ca2+ complex-formation

beta 1-Bungarotoxin consists of a phospholipase A2 subunit and a non-phospholipase A2 subunit. Modification of beta 1-bungarotoxin with CNBr resulted in cleavage at Met-6 and Met-8 of its phospholipase A2 subunit. Analysis of the fluorescence data of both the toxin-Ca2+ complex at 300-350 nm and the toxin-Tb3+ complex at 450-650 nm showed the existence of two binding sites for both metal ions on the different domains of the toxin molecule. At pH 7.6 the association constants for the high-affinity and low-affinity sites of the toxin-Ca2+ complex were determined to be 2.79 x 10(3) +/- 0.21 x 10(3) M-1 and 0.47 x 10(3) +/- 0.06 x 10(3) M-1 respectively. For the toxin-Tb3+ complex the association constant for the high-affinity site was 2.95 x 10(3) +/- 0.43 x 10(3) M-1 and that for the low-affinity site was 0.11 x 10(3) +/- 0.03 x 10(3) M-1. Removal of the N-terminal octapeptide of the phospholipase A2 subunit from the toxin molecule caused disintegration of the low-affinity site but did not disrupt the high-affinity site. This might accompany a change in the configuration around His-48 of the phospholipase A2 subunit. Between pH 6 and 8 the binding of metal ions to the high-affinity site increased but that to the low-affinity site did not change with increasing pH. The neurotoxicity and enzymic activity of the toxin were lost on removal of the low-affinity site.

Interfacial catalysis: the mechanism of phospholipase A2

A chemical description of the action of phospholipase A2 (PLA2) can now be inferred with confidence from three high-resolution x-ray crystal structures. The first is the structure of the PLA2 from the venom of the Chinese cobra (Naja naja atra) in a complex with a phosphonate transition-state analogue. This enzyme is typical of a large, well-studied homologous family of PLA2S. The second is a similar complex with the evolutionarily distant bee-venom PLA2. The third structure is the uninhibited PLA2 from Chinese cobra venom. Despite the different molecular architectures of the cobra and bee-venom PLA2s, the transition-state analogue interacts in a nearly identical way with the catalytic machinery of both enzymes. The disposition of the fatty-acid side chains suggests a common access route of the substrate from its position in the lipid aggregate to its productive interaction with the active site. Comparison of the cobra-venom complex with the uninhibited enzyme indicates that optimal binding and catalysis at the lipid-water interface is due to facilitated substrate diffusion from the interfacial binding surface to the catalytic site rather than an allosteric change in the enzyme's structure. However, a second bound calcium ion changes its position upon the binding of the transition-state analogue, suggesting a mechanism for augmenting the critical electrophile.

Crystal structure of cobra-venom phospholipase A2 in a complex with a transition-state analogue

The crystal structure of a complex between a phosphonate transition-state analogue and the phospholipase A2 (PLA2) from Naja naja atra venom has been solved and refined to a resolution of 2.0 angstroms. The identical stereochemistry of the two complexes that comprise the crystal's asymmetric unit indicates both the manner in which the transition state is stabilized and how the hydrophobic fatty acyl chains of the substrate are accommodated by the enzyme during interfacial catalysis. The critical features that suggest the chemistry of binding and catalysis are the same as those seen in the crystal structure of a similar complex formed with the evolutionarily distant bee-venom PLA2.

Oxygen radicals in lung pathology

Pulmonary tissue can be damaged in different ways, for instance by xenobiotics (paraquat, butylated hydroxytoluene, bleomycin), during inflammation, ischemia reperfusion, or exposure to mineral dust or to normobaric pure oxygen levels. Reactive oxygen species are partly responsible for the observed pulmonary tissue damage. Several mechanisms leading to toxicity are described in this review. The reactive oxygen species induce bronchoconstriction, elevate mucus secretion, and cause microvascular leakage, which leads to edema formation. Reactive oxygen species even induce an autonomic imbalance between muscarinic receptor-mediated contraction and the beta-adrenergic-mediated relaxation of the pulmonary smooth muscle. Vitamin E and selenium have a regulatory role in this balance between these two receptor responses. The autonomic imbalance might be involved in the development of bronchial hyperresponsiveness, occurring in lung inflammation. Finally, several antioxidants are discussed which may be beneficial as therapeutics in several lung diseases.

Mineral dust exposure and free radical-mediated lung damage

Chronic exposure to several types of mineral dust particles induces an inflammatory reaction in the lung. Dust particles activate alveolar macrophages and prime leukocytes (neutrophils, eosinophils, and basophils), leading to an enhanced release of reactive oxygen species. Sometimes mineral dust particles also contain radicals. Reactive oxygen species (superoxide anion radical, hydrogen peroxide, hydroxyl radical, and singlet oxygen) may lead to tissue damage. These are able to break DNA strands, to destroy proteins, and to induce the process of lipid peroxidation. The effects of oxygen radicals on the beta-adrenergic and muscarinic receptor response of the guinea pig and rat tracheal strip are described. The beta-adrenergic receptor response appeared to be more susceptible to oxidative stress than the muscarinic receptor response. This may lead to an autonomic imbalance on exposure to oxygen radicals. The lipid peroxidation product 4-hydroxy-2,3-trans-nonenal diminished the beta-adrenergic responsiveness in guinea pig tracheal preparations. Histologic examinations indicated that at low concentrations of cumene hydroperoxide (10(-4) M) the epithelial layer of rat trachea was already destroyed, whereas no effect on the muscarinic response was found. Oxygen radical-mediated damage in lung tissue may lead to lung emphysema, hyperresponsiveness, and hypersensitivity. Pharmacotherapeutic interventions that prevent initiation or propagation of these free radical reactions may have a beneficial effect in mineral dust-associated lung disease.

Studies on the status of lysine residues in phospholipase A2 from Naja naja atra (Taiwan cobra) snake venom

Phospholipase A2 (PLA2) from Naja naja atra (Taiwan cobra) snake venom was subjected to lysine modification with trinitrobenzene sulphonic acid (TNBS), and two major trinitrophenylated (TNP) derivatives, TNP-1 and TNP-2, were separated by h.p.l.c. TNP-1 contained only one TNP group on Lys-6 and showed a marked decrease in enzymic activity, but still retained 45% of the lethal toxicity. Both Lys-6 and Lys-65 were modified in TNP-2, and modification of Lys-65 caused a further reduction of the lethal toxicity to 12.6%. However, the antigenicity of both TNP-1 and TNP-2 remained unchanged. The reactivity of Lys-6 and Lys-65 toward TNBS was greatly enhanced by Ca2+ and dihexanoyl-lecithin, suggesting that the two Lys residues are not directly involved in the binding of Ca2+ and substrate. The modified derivatives retained their affinity for Ca2+, indicating that Lys-6 and Lys-65 did not participate in the Ca2+ binding. The TNP derivatives could be regenerated with hydrazine hydrochloride. The biological activities of the regenerated PLA2 are almost the same as those of native PLA2. These results indicate that Lys-6 and Lys-65 are important for the biological activities of PLA2, and incorporation of a bulky TNP group on Lys-6 and Lys-65 might give rise to a distortion of the active conformation of PLA2.