Comparative study of the cytolytic activity of myotoxic phospholipases A2 on mouse endothelial (tEnd) and skeletal muscle (C2C12) cells in vitro

A rapid in vitro cytolytic effect of some myotoxic phospholipases A2 (PLA2s) isolated from the venoms of Viperidae snakes has been previously described. This study was undertaken to investigate if cytolytic activity is a common property of the myotoxic proteins from this group. Murine endothelial cells (tEnd) and skeletal muscle myotubes (C2C12) were utilized as targets. The release of lactic dehydrogenase was quantified as a measure of cell damage, 3 h after exposure of cells to the different PLA2s, including representatives from the genera Bothrops, Agkistrodon, Trimeresurus, Crotalus (family Viperidae), and Notechis (family Elapidae). All of the group II myotoxic PLA2s tested displayed rapid cytolytic activity when tested in the micromolar range of concentrations (8-32 microM). In contrast, the group I myotoxic PLA2 notexin was devoid of this activity. Aspartate-49 and lysine-49 PLA2 group II variants showed a comparable cytolytic effect. Skeletal muscle myotubes, obtained after fusion and differentiation of C2C12 myoblasts, were significantly more susceptible to the cytolytic action of myotoxins than endothelial cells, previously reported to be more susceptible than undifferentiated myoblasts under the same assay conditions. Cytolytic activity appears to be a common characteristic of group II myotoxic PLA2s of the Viperidae. Bee venom PLA2, a group III enzyme of known myotoxicity, also displayed cytotoxic activity on C2C12 myotubes, being devoid of activity on endothelial cells. These results suggest that in vitro differentiated skeletal muscle myotubes may represent a suitable model target for the study of myotoxic PLA2s of the structural group II found in snake venoms.

Action of human group IIa secreted phospholipase A2 on cell membranes. Vesicle but not heparinoid binding determines rate of fatty acid release by exogenously added enzyme

Human group IIa phospholipase A2 (hIIa-PLA2) is a highly basic protein that is secreted from a number of cells during inflammation and may play a role in arachidonate liberation and in destruction of invading bacteria. It has been proposed that rodent group IIa PLA2 is anchored to cell surfaces via attachment to heparan sulfate proteoglycan and that this interaction facilitates lipolysis. hIIa-PLA2 contains 13 lysines, 2 histidines, and 10 arginines that fall into 10 clusters. A panel of 26 hIIa-PLA2 mutants were prepared in which 1-4 basic residues in each cluster were changed to glutamate or aspartate (charge reversal). A detailed analysis of the affinities of these mutants for anionic vesicles and for heparin and heparan sulfate in vitro and of the specific activities of these proteins for hydrolysis of vesicles in vitro and of living cell membranes reveal the following trends: 1) the affinity of hIIa-PLA2 for heparin and heparan sulfate is modulated not by a highly localized site of basic residues but by diffuse sites that partially overlap with the interfacial binding site. In contrast, only those residues on the interfacial binding site of hIIa-PLA2 are involved in binding to membranes; 2) the relative ability of these mutants to hydrolyze cellular phospholipids when enzymes were added exogenously to CHO-K1, NIH-3T3, and RAW 264.7 cells correlates with their relative in vitro affinity for vesicles and not with their affinity for heparin and heparan sulfate. 3) The rates of exogenous hIIa-PLA2-catalyzed fatty acid release from wild type CHO-K1 cells and two mutant lines, one lacking glycosaminoglycan and one lacking heparan sulfate, were similar. Thus basic residues that modulate interfacial binding are important for plasma membrane fatty acid release by exogenously added hIIa-PLA2. Binding of hIIa-PLA2 to cell surface heparan sulfate does not modulate plasma membrane phospholipid hydrolysis by exogenously added hIIa-PLA2.

Membrane penetration of cytosolic phospholipase A2 is necessary for its interfacial catalysis and arachidonate specificity

To determine the mechanism of calcium-dependent membrane binding of cytosolic phospholipase A2 (cPLA2), we measured the interactions of cPLA2 with phospholipid monolayers and polymerizable mixed liposomes containing various phospholipids. In the presence of calcium, cPLA2 showed much higher penetrating power than secretory human pancreatic PLA2 toward anionic and electrically neutral phospholipid monolayers. cPLA2 also showed ca. 30-fold higher binding affinity for nonpolymerized 2, 3-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-1-phosphoglycerol (D-BLPG) liposomes than for polymerized ones where the membrane penetration of protein is significantly restricted. Consistent with this difference in membrane binding affinity, cPLA2 showed 20-fold higher activity toward fluorogenic substrates, 1-O-(1-pyrenedecyl)-2-arachidonoyl-sn-glycero-3-phosphocholine, inserted in nonpolymerized D-BLPG liposomes than the same substrate in polymerized D-BLPG liposomes. Furthermore, cPLA2 showed much higher sn-2 acyl group specificity (arachidonate specificity) and headgroup specificity in nonpolymerized D-BLPG liposomes than in polymerized D-BLPG liposomes. Finally, diacylglycerols, such as 1, 2-dioleoyl-sn-glycerol, selectively enhanced the membrane penetration, hydrophobic membrane binding, and interfacial enzyme activity of cPLA2. Taken together, these results indicate the following: (1) calcium not only brings cPLA2 to the membrane surface but also induces its membrane penetration. (2) This unique calcium-dependent membrane penetration of cPLA2 is necessary for its interfacial binding and substrate specificity. (3) Diacylglycerols might work as a cellular activator of cPLA2 by enhancing its membrane penetration and hydrophobic membrane binding.

Desmoplastic cerebral astrocytoma of infancy: a case report

We present a case of desmoplastic cerebral astrocytoma of infancy (DCAI) in a 9-month-old boy including immunohistochemical and proliferative activity studies. It was mainly composed of glial fibrillary acidic protein (GFAP)-positive astrocytes and desmoplastic stroma. Studies with Ki-67 and synthetic phase fraction disclosed a low proliferative activity. Flow cytometric study revealed diploidy pattern. These findings suggest a positive correlation with the favorable prognosis.

Roles of individual domains of annexin I in its vesicle binding and vesicle aggregation: a comprehensive mutagenesis study

To understand the mechanism by which annexin I induces membrane aggregation, a comprehensive mutagenesis of all six Ca2+-binding sites was performed. When the cap residues of type II Ca2+-binding sites were systematically mutated to Ala, a type II site in domain II was shown to be essential for Ca2+-dependent vesicle binding of annexin I. Domain II was not, however, directly involved in vesicle aggregation. Instead, type II sites in domains III and IV, respectively, and type III sites in domains I and IV were involved in vesicle aggregation. When all type II sites were deactivated, three type III sites provided residual vesicle binding and aggregating activities. Their contributions to these activities in the presence of type II sites were, however, relatively insignificant. To further investigate the role of each domain harboring a type II site, a set of mutants containing only a specific type II site(s) were generated and their activities measured. These measurements again underscored the importance of domain II in vesicle binding of annexin I and the involvement of domains III and IV in vesicle aggregation. The roles of individual domains in vesicle binding and aggregation can be accounted for by the conformational change of membrane-bound annexin I involving modular rotation of domains (I/IV) following the initial membrane adsorption of domains (II/III). In conjunction with mutagenesis studies on other annexins, these results show that individual domains of annexins, although structurally homologous, have distinct functions and that different annexins might interact with membranes via different domains.

Mutagenesis of the C2 domain of protein kinase C-alpha. Differential roles of Ca2+ ligands and membrane binding residues

The C2 domains of conventional protein kinase C (PKC) have been implicated in their Ca2+-dependent membrane binding. The C2 domain of PKC-alpha contains several Ca2+ ligands that bind multiple Ca2+ ions and other putative membrane binding residues. To understand the roles of individual Ca2+ ligands and protein-bound Ca2+ ions in the membrane binding and activation of PKC-alpha, we mutated five putative Ca2+ ligands (D187N, D193N, D246N, D248N, and D254N) and measured the effects of mutations on vesicle binding, enzyme activity, and monolayer penetration of PKC-alpha. Altered properties of these mutants indicate that individual Ca2+ ions and their ligands have different roles in the membrane binding and activation of PKC-alpha. The binding of Ca2+ to Asp187, Asp193, and Asp246 of PKC-alpha is important for the initial binding of protein to membrane surfaces. On the other hand, the binding of another Ca2+ to Asp187, Asp246, Asp248, and Asp254 induces the conformational change of PKC-alpha, which in turn triggers its membrane penetration and activation. Among these Ca2+ ligands, Asp246 was shown to be most essential for both membrane binding and activation of PKC-alpha, presumably due to its coordination to multiple Ca2+ ions. Furthermore, to identify the residues in the C2 domain that are involved in membrane binding of PKC-alpha, we mutated four putative membrane binding residues (Trp245, Trp247, Arg249, and Arg252). Membrane binding and enzymatic properties of two double-site mutants (W245A/W247A and R249A/R252A) indicate that Arg249 and Arg252 are involved in electrostatic interactions of PKC-alpha with anionic membranes, whereas Trp245 and Trp247 participate in its penetration into membranes and resulting hydrophobic interactions. Taken together, these studies provide the first experimental evidence for the role of C2 domain of conventional PKC as a membrane docking unit as well as a module that triggers conformational changes to activate the protein.

Structure based design: novel spirocyclic ethers as nonpeptidal P2-ligands for HIV protease inhibitors

A series of novel spirocyclic ethers were designed to function as nonpeptidal P2-ligands for HIV-1 protease inhibitors. Incorporation of designed ligands in the (R)-(hydroxyethylamino)sulfonamide isostere afforded potent HIV protease inhibitors.

Bacterial expression and characterization of human secretory class V phospholipase A2

Mammalian secretory class V phospholipase A2 (PLA2) is a newly discovered PLA2 that is implicated in eicosanoid formation in inflammatory cells. As a first step towards understanding the structure, function and regulation of this PLA2, we constructed a bacterial expression vector for human secretory class V PLA2 (hV-PLA2), over-expressed and purified the protein, and determined its physical and kinetic properties. When compared with human class IIa enzyme (hIIa-PLA2), hV-PLA2 has several distinct properties. First, hV-PLA2 can catalyse the hydrolysis of phosphatidylcholine more effectively than hIIa-PLA2 by two orders of magnitude. Secondly, hV-PLA2 has much higher binding affinity and activity for compactly packed phosphatidylcholine bilayers than hIIa-PLA2. Finally, hV-PLA2 has much reduced thermal stability compared with hIIa-PLA2. These data suggest that hV-PLA2 is better suited than hIIa-PLA2 for acting on the outer cellular membrane and liberating arachidonic acid from membrane phospholipids. Also, the unusually low thermal stability of hV-PLA2 might contribute to tighter regulation of its activities in extracellular media.

Differential membrane-binding and activation mechanisms of protein kinase C-alpha and -epsilon

To elucidate the mechanisms of membrane binding and activation of conventional and novel protein kinase C (PKC), we measured the interactions of rat PKC-alpha and -epsilon with phospholipid monolayers and vesicles of various compositions. Besides the established difference in calcium requirement, the two isoforms showed major differences in their membrane-binding and activation mechanisms. For PKC-alpha, diacylglycerol (DG) specifically enhanced the binding of PKC-alpha to phosphatidylserine (PS)-containing vesicles by 2 orders of magnitude, allowing PKC-alpha high specificity for PS. Also, PKC-alpha could penetrate into the phospholipid monolayer with a packing density comparable to that of the cell membrane only in the presence of Ca2+ and PS. When compared to PKC-alpha, PKC-epsilon had lower binding affinity for PS-containing vesicles both in the presence and in the absence of DG. As a result, PKC-epsilon did not show pronounced specificity for PS. Also, PKC-epsilon showed reduced penetration into PS-containing monolayers, which was comparable to the Ca2+-independent penetration of PKC-alpha into the same monolayers. Taken together, these results suggest the following: (1) The role of Ca2+ in the membrane binding of PKC-alpha is to expose a specific PS-binding site. (2) Once bound to membrane surfaces, PS specifically induces the partial membrane penetration of PKC-alpha that allows its optimal interactions with DG, hence the enhanced membrane binding and activation. (3) PKC-epsilon, due to the lack of Ca2+ binding, cannot specifically interact with PS and DG, which implies the presence of other physiological activator(s) for this isoform.

Potent HIV protease inhibitors incorporating high-affinity P2-ligands and (R)-(hydroxyethylamino)sulfonamide isostere

Design and synthesis of a series of very potent nonpeptide HIV protease inhibitors are described. The inhibitors are derived from novel high affinity P2-ligands and (R)-(hydroxyethylamino)sulfonamide isostere.

Mapping the interfacial binding surface of human secretory group IIa phospholipase A2

Human secretory group IIa phospholipase A2 (hIIa-PLA2) contains a large number of prominent cationic patches on its molecular surface and has exceptionally high affinity for anionic surfaces, including anionic membranes. To identify the cationic amino acid residues that support binding of hIIa-PLA2 to anionic membranes, we have performed extensive site-directed mutagenesis of this protein and measured vesicle binding and interfacial kinetic properties of the mutants using polymerized liposomes and nonpolymerized anionic vesicles. Unlike other secretory PLA2s, which have a few cationic residues that support binding of enzyme to anionic membranes, interfacial binding of hIIa-PLA2 is driven in part by electrostatic interactions involving a number of cationic residues forming patches on the putative interfacial binding surface. Among these residues, the amino-terminal patch composed of Arg-7, Lys-10, and Lys-16 makes the most significant contribution to interfacial adsorption, and this is supplemented by contributions from other patches, most notably Lys-74/Lys-87/Arg-92 and Lys-124/Arg-127. For these mutants, complete vesicle binding occurs in the presence of high vesicle concentrations, and under these conditions the mutants display specific activities comparable to that of wild-type enzyme. These studies indicate that electrostatic interactions between surface lysine and arginine residues and the interface contribute to interfacial binding of hIIa-PLA2 to anionic vesicles and that cationic residues closest to the opening of the active-site slot make the most important interactions with the membrane. However, because the wild type binds extremely tightly to anionic vesicles, it was not possible to exactly determine what fraction of the total interfacial binding energy is due to electrostatics.

A phospholipase A2 kinetic and binding assay using phospholipid-coated hydrophobic beads

A novel kinetic and membrane-binding assay for phospholipase A2 (PLA2) has been developed utilizing phospholipid-coated hydrophobic styrene-divinylbenzene beads (5.2 +/- 0.3 microm diameter). Phospholipids formed a stable monolayer film on styrene-divinylbenzene beads with average surface packing density of (1.3 +/- 0.2) x 10(-2) molecule/A2. Secretory PLA2 readily hydrolyzed 1-palmitoyl-2-[3H]-oleoyl-sn-glycero-3-phosphoglycerol coated on styrene-divinylbenzene beads which could be easily monitored by measuring the radioactivity of fatty acid released to solution in the presence of bovine serum albumin. For human cytosolic PLA2 with high specificity for sn-2 arachidonyl group, styrene-divinylbenzene beads coated with 1-stearoyl-2-[14C]-arachidonyl-sn-glycero-3-phosphocholine and dioleoylglycerol (7:3, mol/mol) were used as substrate. PLA2 activity was linearly proportional to the enzyme concentration in the range from 1 to 150 nM for human class II secretory PLA2 and from 1 to 20 nM for cytosolic PLA2; the specific activity was 1.6 and 1.7 micromol/min/mg, respectively. Finally, styrene-divinylbenzene beads coated with polymerized 1,2-bis[12-(lipoyloxy) dodecanoyl]-sn-glycero-3-phosphoglycerol were used to measure the membrane binding affinity of PLA2, which in conjunction with kinetic data provides important insights into how PLA2 interacts with membranes.

Cloning, expression, and regulation of rabbit cyclooxygenase-2 in renal medullary interstitial cells

Prostaglandin synthesis requires cyclooxygenase-1 (COX1) or -2 (COX2), which mediate the conversion of arachidonate to prostaglandin H2. COX1 is the predominant constitutive isoform, whereas COX2 expression is typically low. In the present studies we cloned rabbit COX2 and determined its distribution in unstimulated tissues. Screening rabbit eye and uterine libraries yielded two cDNAs containing identical inserts with a 1,812-nucleotide open-reading frame. This encoded a 604-amino acid polypeptide, 90% identical to human, rat, and mouse COX2. Expression of the rabbit COX2 in HEK-293 cells enhanced prostanoid synthesis. Constitutive COX2 mRNA expression was highest in kidney and urinary bladder. COX2 expression was primarily in renal outer medullary interstitial cells with cortical expression in macula densa. In cultured medullary interstitial cells, COX2 mRNA predominated, with little COX1 expression. Interstitial cell COX2 mRNA but not COX1 was induced by phorbol ester and epidermal growth factor but suppressed by dexamethasone. Phorbol ester also upregulated immunoreactive COX2. Constitutive COX2 in these tissues has important implications for side effects of COX2-selective inhibitors.

Bacterial expression and characterization of human pancreatic phospholipase A2

Mammalian pancreatic phospholipases A2 (PLA2) have recently been implicated in cell surface receptor-mediated inflammation. As a first step toward understanding how human pancreatic PLA2 (hp-PLA2) interacts with membranes and other biological targets including cell-surface receptors, we constructed its bacterial expression vector which can be used for the mutagenesis and protein over-expression. The expression vector (pSH-hp) was constructed using a synthetic hp-PLA2 gene whose transcription is controlled by T7 promoter. hp-PLA2 was expressed as a mature protein in high concentration in Escherichia coli cells and formed inclusion body. The solubilization of inclusion body protein followed by the refolding and purification produced ca. 5 mg of pure protein from one liter of growth medium. Kinetic studies of recombinant human, bovine and porcine pancreatic PLA2s using polymerized mixed liposomes and micelles as substrates showed that despite their highly homologous structures these mammalian pancreatic PLA2s have distinct phospholipid head group specificity and different activity toward various lipid substrates.

Structural aspects of interfacial adsorption. A crystallographic and site-directed mutagenesis study of the phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus

Recent genetic and structural studies have shed considerable light on the mechanism by which secretory phospholipases A2 interact with substrate aggregates. Electrostatic forces play an essential role in optimizing interfacial catalysis. Efficient and productive adsorption of the Class I bovine pancreatic phospholipase A2 to anionic interfaces is dependent upon the presence of two nonconserved lysine residues at sequence positions 56 and 116, implying that critical components of the adsorption surface differ among enzyme species (Dua, R., Wu, S.-K., and Cho, W. (1995) J. Biol. Chem. 270, 263-268). In an effort to further characterize the protein residues involved in interfacial catalysis, we have determined the high resolution (1.7 A) x-ray structure of the Class II Asp-49 phospholipase A2 from the venom of Agkistrodon piscivorus piscivorus. Correlation of the three-dimensional coordinates with kinetic data derived from site-directed mutations near the amino terminus (E6R, K7E, K10E, K11E, and K16E) and the active site (K54E and K69Y) defines much of the interface topography. Lysine residues at sequence positions 7 and 10 mediate the adsorption of A. p. piscivorus phospholipase A2 to anionic interfaces but play little role in the enzyme's interaction with electrically neutral surfaces or in substrate binding. Compared to the native enzyme, the mutant proteins K7E and K10E demonstrate comparable (20-fold) decreases in affinity and catalysis on polymerized mixed liposomes of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine and 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol, while the double mutant, K7E/K10E, shows a more dramatic 500-fold decrease in catalysis and interfacial adsorption. The calculated contributions of Lys-7 and Lys-10 to the free energy of binding of A. p. piscivorus phospholipase A2 to anionic liposomes (-1.8 kcal/mol at 25 degrees C per lysine) are additive (i.e. -3.7 kcal/mol) and together represent nearly half of the total binding energy. Although both lysine side chains lie exposed at the edge of the proposed interfacial adsorption surface, they are geographically remote from the corresponding interfacial determinants for the bovine enzyme. Our results confirm that interfacial adsorption is largely driven by electrostatic forces and demonstrate that the arrangement of the critical charges (e.g. lysines) is species-specific. This variability in the topography of the adsorption surface suggests a corresponding flexibility in the orientation of the active enzyme at the substrate interface.

High specificity of human secretory class II phospholipase A2 for phosphatidic acid

Lysophosphatidic acid (LPA) is a potent lipid second messenger which stimulates platelet aggregation, cell proliferation and smooth-muscle contraction. The phospholipase A2 (PLA2)-catalysed hydrolysis of phosphatidic acid (PA) is thought to be a primary synthetic route for LPA. Of the multiple forms of PLA2 present in human tissues, human secretory class-II PLA2 (hs-PLA2) has been implicated in the production of LPA from platelets and whole blood cells challenged with inflammatory stimuli. To explore further the possibility that hs-PLA2 is involved in the production of LPA, we rigorously measured the phospholipid head group specificity of hs-PLA2 by a novel PLA2 kinetic system using polymerized mixed liposomes. Kinetic analysis of recombinant hs-PLA2 demonstrates that hs-PLA2 strongly prefers PA as substrate over other phospholipids found in the mammalian plasma membrane including phosphatidylserine (PS), phosphatidylcholine (PC) and phosphatidylethanolamine (PE). The order of preference is PA >> PE approximately PS > PC. To identify amino acid residues of hs-PLA2 that are involved in its unique substrate specificity, we mutated two residues, Glu-56 and Lys-69, which were shown to interact with the phospholipid head group in the X-ray-crystallographic structure of the hs-PLA2-transition-state-analogue complex. The K69Y mutant showed selective inactivation toward PA whereas the E56K mutant displayed a most pronounced inactivation to PE. Thus it appears that Lys-69 is at least partially involved in the PA specificity of hs-PLA2 and Glu-56 in the distinction between PE and PC. In conjunction with a recent cell study [Fourcade, Simon, Viode, Rugani, Leballe, Ragab, Fournie, Sarda and Chap (1995) Cell 80, 919-927], these studies suggest that hs-PLA2 can rapidly hydrolyse PA molecules exposed to the outer layer of cell-derived microvesicles and thereby produce LPA.

Mutagenesis of residues adjacent to transmembrane prolines alters D1 dopamine receptor binding and signal transduction

Proline is highly conserved in the presumed transmembrane alpha-helices of seven-transmembrane helix-containing, G protein coupled receptors. Unique properties of this imino acid have led to speculations of structural and perhaps dynamic importance for seven-transmembrane helix-containing receptor function. To avoid potentially deleterious consequences of proline-directed mutagenesis, substitutions were made in the X residue of X-Pro peptide bonds (where X is the residue on the amino-terminal side of proline), which may influence static geometries and potential agonist-induced conformational changes at the X-Pro peptide bond. In the fifth helix, Ile205 was substituted with either an alanine (I205A) or a tyrosine (I205Y). Similarly, in the sixth helix, Leu286 was substituted with either an alanine (L286A) or a tyrosine (L286Y). Mutant I205A demonstrated subtle changes in D1 pharmacology and signal transduction. The I205Y and L286Y mutations produced comparatively drastic impairments in both binding and signal transduction. Remarkably, the L286A mutation resulted in constitutive activity characterized by elevated basal signal transduction and increased agonist potencies. In addition, (R)-(+)-SCH23390, a classical antagonist at the wild-type D1 receptor, behaved as a partial agonist at L286A. This is the first report of a constitutively active receptor resulting from this point mutation and the first report of a constitutively active mutant dopamine receptor. These results are discussed in terms of binding pocket geometry and potential mechanisms of signal transduction.

Roles of surface hydrophobic residues in the interfacial catalysis of bovine pancreatic phospholipase A2

The interfacial binding is a unique and important step in the phospholipase A2 (PLA2) catalyzed hydrolysis of phospholipids which is distinct from the binding of a substrate to the active site. To assess the roles of surface hydrophobic residues of PLA2 in these processes, we selectively mutated Leu-19 and Leu-20 of bovine pancreatic PLA2 to charged (L19K and L20K), uncharged polar (L19S and L20S), and amphiphilic (L19W and L20W) groups and measured their kinetic and binding properties using various phospholipid aggregates, including micelles, monolayers, and polymerized mixed liposomes. The mutations of Leu-19 and Leu-20 did not significantly change either the tertiary structure or the thermodynamic stability of bovine pancreatic PLA2. Toward monomeric 1,2-dihexanoyl-sn-glycero-3-phosphocholine, all Leu-20 mutants (L20S, L20W, and L20K) showed activities comparable to that of wild type whereas the substitution of Leu-19 with less hydrophobic side chains (L19S and L19K) reduced the activity to 70% and 50%. Toward zwitterionic 1,2-dioctanoyl-sn-glycero-3-phosphocholine (diC8PC) micelles, L20S and L20K mutants showed only 30% and 35% of the wild-type activity, respectively, whereas L20W was about twice as active as wild type. Also, L19S and L19K showed 75% and 15% of the wild-type activity, respectively. Toward anionic Trition X-100/sodium deoxycholate/diC8PC (4:2:1) mixed micelles, L20W and L20K were 2.6 times and twice more active than wild type. To determine the sn-2 acyl group selectivity of wild type and mutants, polymerized mixed liposomes were used which contained 1,2-bis[12-(lipoyloxy)-dodecanoyl]-sn-glycero-3-phosphoglycerol and 1 mol % of either 1-2[12-(1-pyrenebutanoyloxy)dodecanoyl]-2-hexanoyl-sn-glycero-3-++ +phosphocholine or 1-[12-(1-pyrenebutanoyloxy)dodecanoyl]-2-dodecanoyl-sn-glycero-3-+ ++phosphocholine. These measurements showed that Leu-19 was involved in the substrate binding and the sn-2 acyl group selectivity of bovine pancreatic PLA2 and that Leu-20 made a direct contact with the surface of phospholipid aggregates. The binding affinities of mutants to micelles, polymerized liposomes, and monolayers were well consistent with their kinetic behaviors, supporting the notion that the altered activities of Leu-19 mutants and Leu-20 mutants were due to the change in their substrate binding and interfacial binding, respectively. Finally, the L20W mutant represents the first example of protein engineering of PLA2 which results in a significant increase in interfacial binding to densely packed neutral monolayers and bilayers.

Interactions of annexin V with phospholipid monolayers

To understand the mechanism of annexin V-membrane interactions, we measured the interaction of human recombinant annexin V with phospholipid monolayers with differing head group and acyl group structures. Annexin V interacted with anionic phospholipid monolayers via non-specific electrostatic interactions, which was highly dependent on the surface pressure of monolayer with a sharp maximum. The unique surface pressure dependence of the annexin V-monolayer binding is strikingly similar to that observed for the binding of Ca2+ to anionic phospholipid monolayers, which indicates that the annexin V-bound Ca2+ binds two phospholipids at the membrane surface and that factors governing the Ca(2+)-phospholipid complex formation regulate the overall annexin V-Ca(2+)-membrane interactions.

Hydrophobic residues of the D2 dopamine receptor are important for binding and signal transduction

Dopamine receptors belong to the seven transmembrane helix-containing, G protein-coupled receptor superfamily. Mutagenesis studies suggest that dopamine and its analogues interact with aspartate-114 in helix 3 and two helix 5 serines (194 and 197) of the D2 receptor. In addition to these amino acids, hydrophobic residues within the receptor core may be important not only for binding but also for receptor activation. Described is a site-directed mutagenesis investigation into the roles of these hydrophobic residues in the long isoform of the human D2 receptor. Replacement of helix 6 phenylalanines (389 or 390) with alanines resulted in disrupted binding to several agonists and antagonists and impaired inhibition of adenylyl cyclase activity. Replacement of the helix 5 phenylalanine-198 with an alanine selectively disrupted [3H]N-0437 binding, whereas the affinities for other agonists and antagonists remained unchanged. This mutant remained functionally intact when stimulated with dopamine or bromocriptine. Replacement of the helix 7 phenylalanine-411 or the helix 6 leucine-387 with alanines produced receptors that bound agonists well but were unable to inhibit adenylyl cyclase. Based on these data, two conserved helix 6 phenylalanines (389 and 390) appear to be crucial for ligand binding, and phenylalanine-411 in helix 7 and leucine-387 in helix 6 may be important for propagating conformational changes from the agonist binding site(s) to G protein coupling domain(s) of the D2 receptor.

Taiwanese love styles and their association with self-esteem and relationship quality

This study is an examination of how Lee's (1973, 1977, 1988) love styles are related to Taiwanese attitudes toward romantic relationships. Traditional literature was explored to ascertain Chinese beliefs about love and customs surrounding courtships and marriage. Taiwanese students at the University of Texas completed the Love Attitudes Scale (C. Hendrick & S. Hendrick, 1990), a measure of Lee's love styles. Six factors were extracted, using a principle components analysis, that were similar in many ways to the love styles derived from American samples, but that reflected Chinese beliefs and attitudes. The relationships of Taiwanese love styles to self-esteem and relationship quality (i.e., satisfaction, the probability of breaking up, and the number of conflicts experienced in a month) suggested the significance of Chinese traditions as well as current social changes in Taiwan.

Isolation of small polarized bile duct units

Fragments of small interlobular bile ducts averaging 20 microns in diameter can be isolated from rat liver. These isolated bile duct units form luminal spaces that are impermeant to dextran-40 and expand in size when cultured in 10 microM forskolin for 24-48 hr. Secretion is Cl- and HCO3- dependent and is stimulated by forskolin > dibutyryl cAMP > secretion but not by dideoxyforskolin, as assessed by video imaging techniques. Secretin stimulates Cl-/HCO3- exchange activity, and intraluminal pH increases after forskolin administration. These studies establish that small polarized physiologically intact interlobular bile ducts can be isolated from rat liver. These isolated bile duct units should be useful preparations for assessing the transport properties of small bile duct segments, which are the primary site of injury in cholestatic liver disorders, known as "vanishing bile duct syndromes."

Highly efficient immobilization of phospholipase A2 and its biomedical applications

A new method for the immobilization of phospholipase A2 (PLA2) has been developed to enhance the activity retention of immobilized PLA2. When PLA2 from the venom of Agkistrodon piscivorus piscivorus was pretreated with 4-nitro-3-octanoyl-oxybenzoic acid to acylate epsilon-amino groups of two lysines (Lys-7 and Lys-10) and the resulting acylated enzyme was covalently coupled onto carbonyldiimidazole-activated cross-linked agarose beads, the immobilized acylated enzyme showed high retention of activity toward various aggregated phospholipids. Toward densely packed phospholipid bilayers, such as large unilamellar vesicles of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine, the immobilized acylated A. p. piscivorus PLA2 was 25-fold more active than the soluble A. p. piscivorus PLA2. The general applicability of our immobilization protocol was demonstrated by the high retention of activity achieved for the immobilized acylated PLA2 from the venom of Naja naja naja. In particular, full activity retention of the immobilized acylated A. p. piscivorus PLA2 toward phospholipids on the surface of human low density lipoproteins suggests its potential usefulness in a newly developed PLA2-based therapy for hypercholesterolemia.

A structure-function study of bovine pancreatic phospholipase A2 using polymerized mixed liposomes

A new combinatorial approach that includes the genetic variation of protein structure and the chemical modification of phospholipid structure in polymerized mixed liposomes was used to delineate the structure-function relationships in the interfacial catalysis of bovine pancreatic phospholipase A2 (PLA2). Based on previous structural and mutational studies, several bovine PLA2 mutants were generated in which a positive charge of putatively important lysyl side chains was reversed (K10E, K53E, K56E, and K116E) or neutralized (K56Q and K116Q). Kinetic parameters of bovine wild type and mutant PLA2s determined using polymerized mixed liposomes consisting of 1-hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphoethanolamine (or -phosphoglycerol) and 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol showed that Lys-53 is involved specifically in the interaction with a substrate bound in the active site. Also, these results showed that Lys-10 and Lys-116 are involved in the interaction of bovine PLA2 with anionic interfaces but not in the interaction with the active site-bound substrate. In particular, Lys-116 makes more significant contribution than Lys-10 by approximately 1.0 kcal/mol to the binding to anionic interfaces. Most importantly, Lys-56 was shown to participate in the interaction with both the active site-bound substrate and anionic interfaces. These findings establish Lys-56 and Lys-116 as essential residues for the binding of bovine pancreatic PLA2 to anionic interfaces. Lastly, our structure-function analysis based on the use of polymerized mixed liposomes was further supported by equilibrium binding measurements of these proteins using 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol polymerized liposomes and by kinetic analyses using monomeric substrates, 1,2-dihexanoyl-sn-glycero-3-phosphoethanolamine and -phosphoglycerol.

Effects of specific fatty acid acylation of phospholipase A2 on its interfacial binding and catalysis

Monomeric phospholipase A2 (PLA2) from the venom of Agkistrodon piscivorus piscivorus (App-D49) was treated with 3-acyloxy-4-nitrobenzoic acids to acylate the epsilon-amino groups of two lysines (Lys-7 and Lys-10) in the amino terminal region. Resulting 7,10-diacylated-App-D49s, with acyl groups ranging from lauroyl to palmitoyl, spontaneously aggregated in solution. By contrast, 7,10-dioctanoyl-App-D49 existed as a monomer under the same condition. Kinetic and interfacial binding properties of diacylated enzymes indicated that they catalyzed the hydrolysis at the interface as a monomer. When compared to nonacylated App-D49, diacylated enzymes showed slightly increased activity or decreased activity toward monodispersed 1,2-dibutyryl-sn-glycero-3-phosphocholine, Triton X-100/1,2-dilauroyl-sn-glycero-3-phosphocholine mixed micelles, and small unilamellar vesicles (SUV) of 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine (POPC). Toward densely-packed liquid-crystalline phospholipid bilayers, such as large unilamellar vesicles (LUV) of POPC, however, diacylated enzymes exhibited a large increase in activity, which reacted up to 250-fold for 7,10-dilauroyl-App-D49 ((kcat/Km)app = (1.0 +/- 0.02) x 10(6) M-1 s-1). Measurements of the penetration of individual diacylated enzymes into 2-oleoyl-3-palmitoyl-sn-glycero-1-phosphocholine (i.e., D-POPC) monolayers indicated that the acyl groups enhanced the interfacial binding of protein by interacting with hydrocarbon moieties of phospholipids and that these hydrophobic interactions remained effective even when the phospholipid packing density was high. Furthermore, fluorometric measurements of the binding of diacylated enzymes to polymerized vesicles of 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphocholine showed that the hydrophobic interactions increased the enzymatic activity toward LUV by accelerating the migration of enzyme molecules to vesicle surfaces. The analysis of the kinetic course of POPC LUV hydrolysis showed that diacylated enzymes as a catalyst were superior to nonacylated App-D49 in that they were not only more catalytically efficient but also able to catalyze more turnovers without being trapped in product-containing vesicles. In summary, the acylation of App-D49 by 3-acyloxy-4-nitrobenzoic acids provides a simple and convenient way of converting the enzyme into a highly active form toward densely-packed liquid-crystalline phospholipid bilayers, which might have potential industrial and biomedical applications.

A continuous fluorometric assay for phospholipases using polymerized mixed liposomes

A versatile continuous fluorometric assay for phospholipases A2, C, and D has been developed utilizing polymerized mixed liposomes made of pyrene-containing phospholipids (5 mol%) uniformly inserted in the polymerized liposomes of 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphoglycerol (BLPG) and its derivatives. 1-Hexadecanoyl-2-(1-pyrenedecanoyl)-sn-glycero-3-phosphocholine was used for phospholipase A2 and N-(1-pyrenesulfonyl)-egg phosphatidyl ethanolamine for phospholipases C and D. Fluorescence emission of pyrene moieties in polymerized mixed liposomes was strongly quenched by BLPG molecules and, thus, the hydrolysis of pyrene-containing phospholipids and the subsequent displacement of pyrene moieties from the liposomes resulted in a large increase in fluorescence intensity. All the phospholipases tested selectively and rapidly hydrolyzed the inserted pyrene-containing phospholipids, which were readily monitored by measuring an increase in fluorescence emission at 380 nm. Assay conditions for individual phospholipases were optimized by altering interfacial properties of polymerized liposomes, such as surface charge, and subsequently by changing the chemical structure of hydrolyzable phospholipids. Phospholipase activities were linearly proportional to enzyme concentrations in the range from 0.1 to 50 ng. Specific activity determined for phospholipases from a wide variety of sources ranged from 0.5 to 100 mumol/min/mg. Polymerized mixed liposomes are exceptionally stable against chemical and physical degradation and the assay requires only a small amount of pyrene-containing phospholipids. In addition, the polymerized matrix of BLPG (and its derivatives), due to its inertness to the phospholipase hydrolysis, allows the direct measurement of the equilibrium dissociation constant for a protein-liposome complex.

Inhibition of human secretory class II phospholipase A2 by heparin

By means of kinetic analyses using Triton X-100/deoxycholic acid/dilauroylglycerophosphoethanolamine (4:2:1, molar ratio) mixed micelles we examined the effects of heparin on the activity of several phospholipases A2 (PLA2). Heparin avidly bound cationic PLA2s including human secretory class II PLA2 and thereby inhibited their hydrolysis of phospholipids in the mixed micelles. Initial velocity measurements indicated that heparin behaved as a competitive inhibitor for human secretory class II PLA2 and closely related A.h. blomhoffii PLA2 and A.p. piscivorus PLA2. In particular, heparin showed the highest specificity for human secretory class II PLA2. In the absence of deoxycholic acid in mixed micelles, A.h. blomhoffii PLA2 was also strongly inhibited by heparin. The observed inhibition was not due to the interaction of heparin with the active site of PLA2 because heparin did not inhibit the hydrolysis of monomeric substrates by PLA2s. Both kinetic measurements and fluorescence measurements of PLA2-bound 8-anilino-1-naphthalene sulfonate in the presence of varying amounts of heparin showed that a heparin molecule bound about seven molecules of PLA2. When positive charges of four lysines in the amino-terminal region of A.h. blomhoffii PLA2 were neutralized by limited carbamoylation, heparin neither bound the carbamoylated A.h. blomhoffii PLA2 nor inhibited the hydrolysis of Triton X-100/dilauroylglycerophosphocholine mixed micelles by the carbamoylated A.h. blomhoffii PLA2 that retained 50% activity of native A.h. blomhoffii PLA2. Also, heparin did not inhibit the hydrolysis of mixed micelles by 7,10-bis(octanoyl)ated A.p. piscivorus PLA2 in which two lysines in the amino-terminal alpha-helix are acylated. These results indicate that the inhibition of human secretory class II PLA2 and related cationic PLA2s by heparin originates from the interaction of heparin with cationic residues in the amino-terminal region that forms a part of interfacial binding site. In addition, unique structural features of human secretory class II PLA2, together with its unique mode of interaction with heparin, suggest that this PLA2 might have an additional heparin-binding site. Although the heparin-PLA2 binding diminished as the ionic strength of reaction medium increased, the inhibition of human secretory class II PLA2 by heparin remained significant at the physiological ionic strength. An estimated value of inhibition constant (Ki) was 0.1 microM under physiological conditions, which suggests that a normal pharmaceutical dose of heparin might inhibit human secretory class II PLA2 and regulate its biological effects.

Use of polymerized mixed liposomes to study interactions of phospholipase A2 with membranes

Polymerized liposomes of thiol-based phospholipids, 1,2-bis[12-(lipoyloxy)dodecanoyl]-sn-glycero-3-phosphocholine (BLPC) and -phosphoglycerol (BLPG) were used to study interactions of several phospholipases A2 (PLA2) with membranes. Large liposomes (an average diameter of 100 +/- 10 nm) prepared from BLPC or BLPG were readily hydrolyzed by PLA2. Once polymerized, however, these liposomes were resistant to the PLA2 hydrolysis. When liposomes were prepared from a mixture of 1-hexadecanoyl-2-(1-pyrenyldecanoyl)-sn-glycero-3-phosphocholine (pyrene-PC) (5 mol%) and BLPC, fluorescence measurements of resulting polymerized mixed liposomes showed that the pyrene-PC molecules exist solely as monomers without forming a patch and were selectively hydrolyzed by PLA2. Progress of the hydrolysis can be readily monitored by measuring the change in fluorescence emission at 380 nm in the presence of bovine serum albumin. Rapid and selective hydrolysis of inserted phospholipids in polymerized mixed liposomes supports the notion that facile migration of a phospholipid substrate from membrane to the active site of enzyme is a critical step in the catalysis of PLA2. On the basis of these findings, various combinations of polymerized mixed liposomes were prepared and their hydrolysis by PLA2 measured. When compared to the substrate specificity of PLA2s determined using Triton X-100/phospholipid mixed micelles, results from polymerized mixed liposomes indicate that electrostatic interactions between the interfacial binding site of PLA2 and membrane surfaces play an important role in the determination of substrate specificity of PLA2 and in the regulation of PLA2 activities.(ABSTRACT TRUNCATED AT 250 WORDS)

The chemical basis for interfacial activation of monomeric phospholipases A2. Autocatalytic derivatization of the enzyme by acyl transfer from substrate

A basic monomeric phospholipase A2 from the venom of the American water moccasin, Agkistrodon piscivorus piscivorus, undergoes Ca2+-dependent, autocatalytic acylation during the course of hydrolysis of both model and natural phospholipid substrates. Acylation occurs at 2 lysine residues, Lys-7 and Lys-10, in the NH2-terminal alpha-helical segment of the enzyme, and when both positions are fully derivatized, the stable bisacylphospholipase A2 becomes a dimer in solution. The acylated enzyme is fully activated toward monomolecular layers of lecithins. Similar studies applied to the monomeric phospholipases A2 from porcine pancreas and from the venom of Agkistrodon contortrix contortrix also showed irreversible activation of the enzymes by substrate with the same kinetic consequences and formation of dimers. Acylation thus enables these enzymes to overcome the lag period observed under such conditions with native monomeric phospholipases, a phenomenon referred to as interfacial activation. Activation of the enzyme by acylation potentiates the phospholipase for interfacial recognition via formation of a dimeric enzyme. The naturally occurring phospholipase A2 dimer from Crotalus atrox venom displays no lag in the hydrolysis of lecithin monolayers nor does it undergo substrate level acylation. These facts support our proposal that dimerization concomitant with acylation is responsible for the large rate enhancements seen in the hydrolysis of aggregated phospholipids by monomeric phospholipases. Our findings demonstrate for the first time a chemical mechanism for interfacial activation of and interfacial recognition by phospholipases A2.

A new class of phospholipases A2 with lysine in place of aspartate 49. Functional consequences for calcium and substrate binding

We report here the discovery of a new class of phospholipases A2 in which Asp-49, a residue considered to be an obligate component of the catalytic apparatus, is replaced by a lysine. Asp-49 is invariant among the more than 30 venom and pancreatic phospholipases A2 sequenced to date, and its beta-carboxylate group has been shown to be a ligand for calcium in a binding site which also involves contributions from the peptide carbonyl oxygens of Tyr-28, Gly-30, and Gly-32, the so-called calcium-binding loop. The change of Asp-49 to a lysine, and other substitutions in regions heretofore thought to be invariant, including the calcium-binding loop, suggested that the new phospholipases might differ functionally with respect to calcium and/or substrate binding. Indeed, although the Lys-49 phospholipases A2 show a dependence on calcium similar to that of the Asp-49 enzymes, they may be distinguished by the fact that, in the absence of phospholipid, they do not bind calcium to any measurable extent under conditions where Asp-49 enzymes bind a stoichiometric amount of calcium. Furthermore, in the absence of calcium, they show binding to single bilayer phospholipid vesicles under conditions where Asp-49 phospholipases do not bind at all. These results suggest a reversed order of addition of calcium and substrate in the formation of the ternary catalytic complex in the Lys-49 phospholipases A2. Although the mechanistic implications of these structural and functional alterations are not defined at present, it is clear that Asp-49 is not essential for phospholipase A2 catalysis and that it does not participate in the enzyme-calcium-phospholipid catalytic complex.