Protein-phospholipid Interactions Research Articles

Location of two sulfhydryl groups in the rhodopsin molecule by use of the spin label technique

Sulfhydryl groups of membrane-bound rhodopsin are studied with the spin label technique by using five maleimide derivative probes of different lengths. Two sulfhydryl groups are titrated per molecule of rhodopsin. These groups are located in protected but probably different environments, less then 12 Å away from the aqueous phase. A distance of about 37 Å is measured between the two groups. These results are consistent with a model in which the two groups would be located close by the external surface of the protein but embedded within the membrane layer, the strong immobilization of the label molecules resulting from phospholipid-protein interactions.

Phospholipid-protein interactions in the Ca2+-adenosine triphosphatase of sarcoplasmic reticulum.

Ca2+-adenosine triphosphatase from sarcoplasmic reticulum has been delipidated by gel filtration through a Sephadex G-200 column equilibrated with buffer containing cholate. The delipidated Ca2+-adenosine triphosphatase had negligible adenosine triphosphatase activity, but up to 50% of the ATPase activity was restored when the delipidated enzyme was recombined with phosphilipids. It was shown with the delipidated preparation that the phosphorylation of the enzyme by either ATP or Pi was entirely dependent on phospholipids. Among the purified phospholipids, phosphatidylcholine reactivated the adenosine triphosphatase activity better than phosphatidylethanolamine. Vesicles capable of translocating Ca2+ were reconstituted from delipidated Ca2+-adenosine triphosphatase and phosphatidylethanolamine, but not with phosphatidylcholine alone. We conclude that the firmly bound phospholipids which are purified together with the adenosine triphosphatase protein are not essential for the pump since they can be substituted by phosphatidylethanolamine isolated from soybeans.

Role of gamma-carboxyglutamic acid. An unusual protein transition required for the calcium-dependent binding of prothrombin to phospholipid.

A first order calcium-dependent transition can be monitored by a decrease in the intrinsic fluorescence of the isolated "pro" (Fragment1) region of prothrombin. The maximum fluorescence change is -40% for Fragment 1, and only about -6% for whole prothrombin. The most remarkable features of this transition are its rate and activation energy. The half-life for the transition at 0 degrees is about 100 min, and the temperature dependence shows an activation energy of 21 kcal/mol. The rate constant for the forward reaction is zero order in calcium and is not affected by the presence of phospholipid membranes. The equilibrium for the transition, however, is affected by phospholipid. At 30 degrees, [Ca]eq (the calcium concentration where half of the protein has undergone the transition) is 0.4 mM and the Hill coefficient is 2.6. Under the same conditions but in the presence of phospholipid [Ca]eq is 0.24 mM and the Hill coefficient is about 4.5. The transition is triggered by binding 3 or 4 calcium ions. The rate of Fragment 1 binding to phospholipid vesicles was tested using gel filtration techniques at 0 degrees. The rate constants, activation energy, and [Ca]eq values for this process were shown to correspond to the properties of the fluorescence change. The rate constants, activation energy, and Hill coefficients for binding of whole prothrombin to phospholipid correspond to the same parameters for Fragment 1 but the [Ca]eq values are lower. At 0 degrees, the [Ca]eq is 0.19 mM for the prothrombin transition and 0.1 mM for the transition in the presence of phospholipid. These results demonstrate that Fragment 1 and prothrombin undergo a transition when exposed to calcium ions which necessarily precedes protein-phospholipid interactions. In addition to its role in determining the correct protein structure, calcium plays a second role in prothrombin-phos-pholipid interaction which is in the actual formation of the protein-phospholipid bond. The [Ca]eq for binding protein (after its transition) to phospholipid is about 0.06 mM.

Interaction of short chain and long chain fatty acid phosphoglycerides and bile salts with prothrombin.

An equimolar mixture of phosphatidylserine and (dioleoyl) phosphatidyl-ethanolamine could substitute for brain cephalin preparations in the single stage prothrombin assay. However, no clot promoting activity was observed on the addition of any of the individual long chain fatty acid-containing phospholipids. Short chain fatty acid-containing phospholipids, such as diheptanoylphosphatidylcholine, diheptanoylphosphatidylethanolamine, diheptanoylphosphatidic acid, and dihexanoylphosphatidylcholine, or dihexanoylphosphatidylethanolamine were inhibitory under all conditions studied. Similar effects of these two general classes of phospholipids were observed in a two-stage thrombin generation system, in which a mixture of bovine Factor Xa, Factor Va, and Ca2+ were interacted with prothrombin. In the presence of 25 mM Ca2+, dioleoylphosphatidic acid or brain phosphatidylserine alone, and with other long chain phospholipids, formed complexes with bovine plasma prothrombin. On the other hand, dioleoyl-, diheptanoyl- or dihexanoylphosphatidylcholine under comparable conditions showed no binding to prothrombin. There appeared to be a small degree of binding of diheptanoylphosphatidic acid to prothrombin, but it was insufficient to cause any significant change in apparent molecular weight of prothrombin. A mixture of prothrombin, Factor V, diheptanoylphosphatidic acid/diheptanoylphosphatidylcholine and Ca2+ eluted in the void volume of Sephadex G-200, but showed a much reduced coagulant activity. Though a net negative charge on the phospholipid surface is required for phospholipid-protein interactions, this does not necessarily promote coagulant activity. Bile acids and bile salts, such as cholic acid, deoxycholic acid, taurocholic acid, glycocholic acid, lithocholic acid and dehydrocholic acid, exerted varying levels of stimulation on the prothrombin assay and thrombin generation system, but were not as effective as the phospholipids. Interestingly, no interaction of these bile acids or salts with prothrombin was noted in the presence of Ca2+. The results of these experiments suggest that negatively charged micelles per se are not sufficient for binding alone and that other chemical and physical characteristics of phospholipids are of prime importance.

Possible relationship between membrane proteins and phospholipid asymmetry in the human erythrocyte membrane

After incubation of human erythrocytes at 37 °C in the absence of glucose (A) for 24 h, (B) for 4 h with 8 mM hexanol or (C) for 3 h with SH reagents, phosphatidylethanolamine becomes partly susceptible to hydrolysis by phospholipase A 2 from Naja naja. The presence of glucose during the pretreatments suppresses this effect, except in the case of SH reagents that inhibit glycolysis. After incubation with tetrathionate, up to 45% of the phosphatidylethanolamine is degraded by the enzyme, an amount considerably in excess of the 20% attacked in fresh erythrocytes. Pancreatic phospholipase A 2, an enzyme unable to hydrolyse the phospholipids of intact erythrocytes, partially degrades phosphatidylcholine and phosphatidylethanolamine of erythrocytes pretreated with hexanol or SH reagents. Reagents capable of oxidizing SH groups to disulfides (tetrathionate, o- iodosobenzoate and hydroquinone) even render susceptible to pancreatic phospholipase A 2 phosphatidylserine, a phospholipid supposed to be entirely located in the inner lipid layer of the membrane. Alkylating or acylating SH reagents have no such effect. It is postulated that disulfide bond formation between membrane protein SH groups leads to an alteration in protein-phospholipid interactions and consequently induces a reorientation of phospholipids between the inner and the outer membrane lipid layer.

A phospholipase A 2 with anticoagulant activity II. Inhibition of the phospholipid activity in coagulation

An anticoagulant factor with phospholipase A 2 activity has been isolated from Vipera berus venom. Phospholipase activity was studied on platelet phospholipid and on brain cephalin. The venom factor showed a potent anti-coagulant activity: 1 μg impaired the clotting of 1 ml of citrated recalcified platelet-poor plasma. The anticoagulant inhibited clotting by antagonism to phospholipid. The antagonism constant ( K an = 6.8 · 10 −9 M) demonstrated the high affinity of the inhibitor for phospholipid. As with other phospholipases A 2, the venom factor was thermoresistant but very sensitive to photo-oxidation. Both activities (anticoagulant activity and phospholipase activity) were not markedly dissociated by either denaturation or neutralization processes. Slightly different curves of photo-oxidative inactivation of both activities suggested the presence, on the molecule, of two very close sites responsible for phospholipase and anticoagulant activities. The inhibitor effect on coagulation was independent of the hydrolysis process. In fact, lysoderivatives and fatty acids, resulting from complete hydrolysis with the venom factor, were as active as the native phospholipids. Moreover phospholipase A 2 from other viperidae venom, which did not have anticoagulant activity, produced similarly active lysoderivatives. This showed that the cleavage of the β-acyl bond does not interfere with the activity of phospholipid. A possible mechanism of clotting inhibition by the venom factor was proposed. Owing to its high affinity for phospholipid, the inhibitor would complex phospholipid at its protein binding site impairing the normal arrangement of coagulation protein factors and, consequently, their activation. The positive charges of the inhibitor (p I = 9.2) could bind with phosphoryl or carboxyl groups of phospholipid, making them unavailable for protein binding. The complex formation involves a loss of dissociating capacity of the enzyme towards its substrate. This required an additional interaction of the inhibitor with a coagulation protein factor. The inhibitor could be removed from the complex by specific antibodies, permitting recovery of normal phospholipid-protein interaction. The role of calcium in the complex has not yet been elucidated. This venom factor affords a useful tool for investigating the phospholipid-clotting protein interaction.

Incorporation of fatty acids containing photosensitive groups into phospholipids of Escherichia coli.

A series of new fatty acids containing photosensitive groups at different positions on the paraffin chains supported the growth of an auxotroph of E. coli requiring unsaturated fatty acids. The derivatives were 6-, 9-, 11-, and 12-azidostearic acids, 12-azido-oleic acid, 16-azidopalmitelaidic acid, and 12-(4-azido-2-nitrophenoxy)-stearic and -oleic acids. Analyses of the phospholipids from cultures grown in the presence of the first six compounds showed that these derivatives accounted for 16-43% of the total fatty acids. Further analysis of phospholipids from cultures grown with 12-azido-oleic acid, 11-azidostearic acid, or 16-azidopalmitelaidic acid indicated that the azido fatty acids were at the 2-position of the glycerol moieties. The incorporation of these fatty acid derivatives offers a new approach to the study of membrane structure and, in particular, phospholipid-protein interactions by photolysis-induced crosslinking of the fatty acids to the structures in their immediate vicinity.

Resolution and reconstitution of ion-transport systems.

1. Oxidative phosphorylation was reconstituted with a mitochondrial proton pump (oligomycin-sensitive ATPase) and segments of the oxidation chain (cytochrome oxidase or DPNH-Q1 reductase). A proton pump of bacteriorhodopsin substituted for the respiratory chain components, giving rise to light-induced ATP formation. 2. Since oxidative phosphorylation has thus become a special case of the problem of ion translocation in general, we have investigated and reconsituted other pumps. The reconstituted Ca++ pump of sarcoplasmic reticulum consists of two factors, the Ca++-dependent ATPase and a heat-stable coupling factor. 3. Other information obtained from reconstitution experiments include the role of asymmetry in organized membranes and the specificity of protein-phospholipid interaction. 4. Purified preparations of Ca++-ATPase catalyze the formation of ATP from Pi and ADP in a stepwise reaction stoichiometric with the enzyme and dependent on Ca++.

A new approach to the study of phospholipid-protein interactions in biological membranes. Synthesis of fatty acids and phospholipids containing photosensitive groups.

In a general approach to the study, in vivo and in vitro, of the interactions between phospholipids and proteins in biological membranes, a variety of fatty acids containing photosensitive groups in different positions in the alkyl chains has been synthesized. The fatty acids synthesized include: 16-azidopalmitelaidic acid, 12-azidooleic acid, 6-, 9-, 11-, and 12-azidostearic acid, 12-omicron-(ethyl-2-diazomalonyl)stearic and -oleic acids, 12-omicron-(4-azido-2-nitrophenyl)stearic and -oleic acids, and 12-oxo-10-octadecenoic acid. Some of the above synthetic fatty acids were also prepared in the radioactively labeled form. For in vitro studies, many of the above fatty acids were used to acylate the 2 position in the preparation of a number of mixed acylphosphatidylcholines and mixed acylphosphatidylethanolamines. On sonication, the synthetic phospholipids formed sealed vesicles. Intermolecular cross-linking of the fatty acyl chains in phospholipids was demonstrated on photolysis of the vesicles.

Experimental alteration of phospholipid-protein interactions within the human erythrocyte membrane. Dependence on glycolytic metabolism

Phosphatidylethanolamine in freshly drawn human erythrocytes is trinitrophenylated by 2,4,6-trinitrobenzene sulfonic acid only slowly and to a maximum of 32%. After different preincubation procedures at 37°C in saline media in the absence of glucose (24 h without additive, 1–5 h with 8 mM hexanol or 1–4 h with the SH reagent, 5 mM tetrathionate) the rate of subsequent trinitrophenylation of phosphatidylethanolamine, in the absence of the additives, is greatly enhanced and the amount of phospholipid reacting increased. Glucose or inosine prevent these effects, inhibitors of glycolysis abolish this protection. The results indicate that in fresh as well as in glycolysing incubated erythrocytes phosphatidylethanolamine in the outer layer of the membrane lipid is shielded by a protein. Conformational changes of this protein induced by metabolic starvation and perturbing agents expose the phospholipid head group to 2, 4, 6-trinitrobenzene sulfonic acid. In addition, a “flip-flop” of phosphatidylethanolamine from the inner to the outer layer may also contribute to the effects observed.

Role of cholesterol in membranes effects on phospholipid-protein interactions, membrane permeability and enzymatic activity

The effect of cholesterol on the interaction of proteins with phospholipid membranes was studied using three independent techniques: effects on vesicle permeability, monolayer expansion, and phospholipid-dependent (Na + + K +)-ATPase activity. The proteins studied were: cytochrome c, albumin, hemoglobin, lysozyme, myelin basic protein, myelin proteolipid apoprotein, and the polypeptide gramicidin A. The results were as follows: 1. 1. All the proteins in this study produced a large increase in the permeability of phospholipid vesicles to Na +. When cholesterol was mixed with phospholipid in equimolar proportions, most proteins produced only a comparatively small increase in permeability. This inhibitory effect of cholesterol on the permeability of phospholipid-protein membranes, was 38-fold for cytochrome c and 10-fold for hemoglobin. The only protein that was not affected by cholesterol was the myelin proteolipid apoprotein. Experiments with cytochrome c indicated that the above effects were unlikely to be due to inhibition of its binding to phosphatidylserine vesicles. 2. 2. The presence of cholesterol in phosphatidylserine monolayers inhibits the area expansion produced by the addition of cytochrome c to the bulk phase. The inhibition of monolayer expansion by cholesterol was shown to be considerably larger than that obtained by equivalent dilution of the surface charges of phosphatidylserine by phosphatidylcholine. 3. 3. The presence of cholesterol inhibits the ability of phospholipids to activate a delipidated preparation of (Na + + K +)-ATPase. The degree of inhibition produced by nearly equimolar mixtures of cholesterol with brain phosphatidylserine and dioleylphosphoatidylglycerol was approximately 60% while in mixtures with dipalmitoylphosphatidylglycerol, it was more than 90%. 4. 4. The biological significance of these data are discussed in relation to the possible effects of increased cholesterol levels in cell membranes during the process of aging and the development of atherosclerosis.

Phospholipid-protein interactions: Membrane permeability correlated with monolayer “penetration”

The effectiveness of various soluble proteins in increasing the permeability of phospholipid membranes was found to be directly correlated with their ability to “penetrate” monomolecular films of the same phospholipids. We propose that the increase in permeability depends on hydrophobic associations of the protein with the phospholipid which are facilitated by initial electrostatic binding.

Phospholipid-protein interactions: Membrane permeability correlated with monolayer “penetration”

The effectiveness of various soluble proteins in increasing the permeability of phospholipid membranes was found to be directly correlated with their ability to “penetrate” monomolecular films of the same phospholipids. We propose that the increase in permeability depends on hydrophobic associations of the protein with the phospholipid which are facilitated by initial electrostatic binding.

Phospholipid-protein interaction as a determinant for the substrate specificity of mitochondrial nicotinamide-adenine-dinucleotide (phosphate) transhydrogenase

Treatment of nicotinamide-adenine-dinucleotide (phosphate) transhydrogenase (NADPH 2:NAD oxidoreductase, EC 1.6.1.1.) prepared from rat-heart mitochondria with purified phospholipase A (phosphatide acyl-hydrolase, EC 3.1.1.4) or phospholipid solvents results in a change in the substrate specificity of the enzyme. It is postulated that phospholipid-protein interaction determines the substrate specificity of this transhydrogenase system.

Phospholipid-Protein Interactions in the Formation of Prothrombin Activator

SummarySince Ac-globulin (factor V) is involved in the formation of prothrombin activator, its ability to complex with phospholipids was studied. Purified bovine Ac-globulin was complexed to asolectin, there being presumably a fixed number of binding sites on the phospholipid micelle for Ac-globulin. In contrast to the requirement for calcium ions in the formation of complexes between asolectin and autoprothrombin C, calcium ions were not required for complex formation between asolectin and Ac-globulin to occur ; in fact, the presence of calcium prevented complex formation occurring, the degree of inhibition being dependent on the calcium concentration. By treating isolated, pre-formed aso- lectin-Ac-globulin complexes with calcium chloride solutions, Ac-globulin could be recovered in a much higher state of purity and essentially free of asolectin.Complete activators were formed by first preparing the asolectin-calcium- autoprothrombin C complex and then reacting the complex with Ac-globulin. A small amount of this product was very effective as an activator of purified prothrombin without further addition of calcium or any other cofactor. If the autoprothrombin C preparation used to prepare the complex was free of traces of prothrombin, the complete activator was stable for several hours at room temperature. Stable preparations of the complete activator were centrifuged, resulting in the sedimentation of most of the activity. Experimental evidence also indicated that activator activity was highest when autoprothrombin C and Ac-globulin were complexed to the same phospholipid micelle, rather than when the two clotting factors were complexed to separate micelles. These data suggested that the in vivo prothrombin activator may be a sedimentable complex composed of a thromboplastic enzyme, calcium, Ac-globulin and phospholipid.