Surface change of biological membranes as a possible regulator of membrane-bound enzymes

Lidocaine turns the surface charge of biological membranes more positive and changes the permeability of blood-brain barrier culture models

The surface charge of brain endothelial cells forming the blood-brain barrier (BBB) is highly negative due to phospholipids in the plasma membrane and the glycocalyx. This negative charge is an important element of the defense systems of the BBB. Lidocaine, a cationic and lipophilic molecule which has anaesthetic and antiarrhytmic properties, exerts its actions by interacting with lipid membranes. Lidocaine when administered intravenously acts on vascular endothelial cells, but its direct effect on brain endothelial cells has not yet been studied. Our aim was to measure the effect of lidocaine on the charge of biological membranes and the barrier function of brain endothelial cells. We used the simplified membrane model, the bacteriorhodopsin (bR) containing purple membrane of Halobacterium salinarum and culture models of the BBB. We found that lidocaine turns the negative surface charge of purple membrane more positive and restores the function of the proton pump bR. Lidocaine also changed the zeta potential of brain endothelial cells in the same way. Short-term lidocaine treatment at a 10 μM therapeutically relevant concentration did not cause major BBB barrier dysfunction, substantial change in cell morphology or P-glycoprotein efflux pump inhibition. Lidocaine treatment decreased the flux of a cationic lipophilic molecule across the cell layer, but had no effect on the penetration of hydrophilic neutral or negatively charged markers. Our observations help to understand the biophysical background of the effect of lidocaine on biological membranes and draws the attention to the interaction of cationic drug molecules at the level of the BBB.

The role of mucus as an invisible cloak to transepithelial drug delivery by nanoparticles

Mucosal administration of drugs and drug delivery systems has gained increasing interest. However, nanoparticles intended to protect and deliver drugs to epithelial surfaces require transport through the surface-lining mucus. Translation from bench to bedside is particularly challenging for mucosal administration since a variety of parameters will influence the specific barrier properties of the mucus including the luminal fluids, the microbiota, the mucus composition and clearance rate, and the condition of the underlying epithelia. Besides, after administration, nanoparticles interact with the mucosal components, forming a biomolecular corona that modulates their behavior and fate after mucosal administration. These interactions are greatly influenced by the nanoparticle properties, and therefore different designs and surface-engineering strategies have been proposed. Overall, it is essential to evaluate these biomolecule-nanoparticle interactions by complementary techniques using complex and relevant mucus barrier matrices.

Effect of surface-potential modulators on the opening of lipid pores in liposomal and mitochondrial inner membranes induced by palmitate and calcium ions

The effect of surface-potential modulators on palmitate/Ca2+-induced formation of lipid pores was studied in liposomal and inner mitochondrial membranes. Pore formation was monitored by sulforhodamine B release from liposomes and swelling of mitochondria. ζ-potential in liposomes was determined from electrophoretic mobility. Replacement of sucrose as the osmotic agent with KCl decreased negative ζ-potential in liposomes and increased resistance of both mitochondria and liposomes to the pore inducers, palmitic acid, and Ca2+. Micromolar Mg2+ also inhibited palmitate/Ca2+-induced permeabilization of liposomes. The rate of palmitate/Ca2+-induced, cyclosporin A-insensitive swelling of mitochondria increased 22% upon increasing pH from 7.0 to 7.8. At below the critical micelle concentration, the cationic detergent cetyltrimethylammonium bromide (10 μM) and the anionic surfactant sodium dodecylsulfate (10-50 μM) made the ζ-potential less and more negative, respectively, and inhibited and stimulated opening of mitochondrial palmitate/Ca2+-induced lipid pores. Taken together, the findings indicate that surface potential regulates palmitate/Ca2+-induced lipid pore opening.

The Secretion and Action of Brush Border Enzymes in the Mammalian Small Intestine

Microvilli are conventionally regarded as an extension of the small intestinal absorptive surface, but they are also, as latterly discovered, a launching pad for brush border digestive enzymes. Recent work has demonstrated that motor elements of the microvillus cytoskeleton operate to displace the apical membrane toward the apex of the microvillus, where it vesiculates and is shed into the periapical space. Catalytically active brush border digestive enzymes remain incorporated within the membranes of these vesicles, which shifts the site of BB digestion from the surface of the enterocyte to the periapical space. This process enables nutrient hydrolysis to occur adjacent to the membrane in a pre-absorptive step. The characterization of BB digestive enzymes is influenced by the way in which these enzymes are anchored to the apical membranes of microvilli, their subsequent shedding in membrane vesicles, and their differing susceptibilities to cleavage from the component membranes. In addition, the presence of active intracellular components of these enzymes complicates their quantitative assay and the elucidation of their dynamics. This review summarizes the ontogeny and regulation of BB digestive enzymes and what is known of their kinetics and their action in the peripheral and axial regions of the small intestinal lumen.

The effect of pH on the electrical capacitance of phosphatidylcholine-phosphatidylserine system in bilayer lipid membrane

This paper reports measurements on the pH dependence of the electrical capacitance of lipid membranes formed by 1:1 phosphatidylcholine-phosphatidylserine mixtures. A theoretical model was developed to describe this dependence, in which the contributions of functional groups (as the active centers of adsorption of the hydrogen and hydroxide ions) to the overall membrane capacitance were assumed to be additive. The proposed model was verified experimentally using electrochemical impedance spectroscopy. The theoretical predictions agreed with the experimental results over the measured pH range. A minimum corresponding to the isoelectric point appeared in both the theoretical equation and the experimental data.

Peculiarities of live cells' interaction with micro- and nanoparticles

Experimental evidence collected more than 20 years ago in different laboratories suggests that the interactions between live biological cells and micro- and nanoparticles depend on their metabolic state. These experiments were conducted by reputable groups, led by prominent leaders such as H. Pohl of the USA, who was the inventor of dielectrophoresis, and B. Derjaguin of the Soviet Union who was the leading author of DLVO theory. The experiments had been mostly conducted with microparticles in the early 1980s. In the early 1990s, Ukrainian researchers showed that the interaction of live cells with gold nanoparticles consisted of an initial reversible step that also depended on cell metabolism. They found indirect evidence that the ion pumps of the cells were responsible for the reversible step. Ion pumps generate a transmembrane potential, a measurable and widely-used characteristic of the cell's energetic state. The transmembrane potential, in turn, strongly affects the zeta-potential, as was experimentally discovered 40 years ago by several independent groups using cell electrophoresis. This relationship should be taken into account when DLVO theory is considered as the basis for describing the interactions between live cells and micro- and nanoparticles. Unfortunately, detail theoretical analysis indicates that such modification would not be sufficient for explaining observed peculiarities mentioned above. That is why distinguished theoreticians such as Pohl, Frohlich, Derjaguin and others have suggested three theoretical models, presumably to explain these experiments. These theoretical models should be considered to be complementary to the well-established concepts developed on this subject in the molecular biology of cells and cell adhesion. This paper is not a revision of the existing models. It is an overview of the old and forgotten experimental data and discussion of the suggested theoretical models. The unusual interaction mechanisms are only specific for live biological cells and serve a dual role: either as a first barrier to protect the cell from potentially damaging, dispersed particulates, or as a means of accumulating useful substances. Both functions are critical for the modern problem of nanotoxicology.

Relevance of fatty acid oxidation in regulation of the outer mitochondrial membrane permeability for ADP

The present study on saponin-treated rat heart muscle fibers has revealed a new function of the fatty acid oxidation system in the regulation of the outer mitochondrial membrane (OMM) permeability for ADP. It is found that oxidation of palmitoyl-CoA+carnitine, palmitoyl-L-carnitine and octanoyl-L-carnitine (alone or in combination with pyruvate+malate) dramatically decreased a very high value of apparent K(m) of oxidative phosphorylation for ADP. Octanoyl-D-carnitine, as well as palmitate, palmitoyl-CoA, and palmitoyl-L-carnitine were not effective in this respect, when their oxidation was prevented by the absence of necessary cofactors or blocked with rotenone. Our data suggest that oxidation, but not transport of fatty acids into mitochondria, induces an increase in the OMM permeability for ADP.

Binding of rat brain hexokinase to recombinant yeast mitochondria: identification of necessary physico-chemical determinants

The association of rat brain hexokinase with heterologous recombinant yeast mitochondria harboring human porin (Yh) is comparable to that with rat liver mitochondria in terms of cation requirements, cooperativity in binding, and the effect of amphipathic compounds. Mg2+, which is required for hexokinase binding to all mitochondria, can be replaced by other cations. The efficiency of hexokinases, however, depends on the valence of hydrophilic cations, or the partition of hydrophobic cations in the membrane, implying that these act by reducing a prohibitive negative surface charge density on the outer membrane rather than fulfilling a specific structural requirement. Macromolecular crowding (using dextran) has dual effects. Dextran added in excess increases hexokinase binding to yeast mitochondria, according to the porin molecule they harbor. This effect, significant with wild-type yeast mitochondria, is only marginal with Yh as well as rat mitochondria. On the other hand, an increase in the number of hexokinase binding sites on mitochondria is also observed. This increase, moderate in wild-type organelles, is more pronounced with Yh. Finally, dextran, which has no effect on the modulation of hexokinase binding by cations, abolishes the inhibitory effect of amphipathic compounds. Thus, while hexokinase binding to mitochondria is predetermined by the porin molecule, the organization of the latter in the membrane plays a critical role as well, indicative that porin must associate with other mitochondrial components to form competent binding sites on the outer membrane.

Acetylcholinesterase activity in chronic renal failure

Twenty healthy subjects and 39 Chronic Renal Failure patients (CRF-patients) maintained on chronic hemodialysis were used in this investigation to study the changes in acetylcholinesterase (AChE) activity of red blood cells (RBCs). The CRF-patients were all undergoing hemodialysis treatment. AChE activity from the CRF-patients was determined before and after dialysis. An additional objective was to study the effect of chronic renal failure on human red blood cell aging. Blood samples were drawn from controls and CRF-patients in tubes containing EDTA or sodium heparin as an anticoagulant. Red blood cells were purified to avoid interference with monocytes, reticulocytes and leukocytes. The purified RBCs were subfractionated into young (y) (1.08-1.09), mid (m) (1.09-1.11) and old (o) (1.11-1.12) percoll density (g/mL) fractions using a discontinous percoll gradient. The mean +/- SD AChE per gram hemoglobin (U/g Hgb) activities in whole blood (WB), purified human red blood cells (PRBCs), young human red blood cells (y-RBCs), mid age human red blood cells (m-RBCs) and old human red blood cells (o-RBCs) in CRF-patients were 31.2+/-3.43, 29.3+/-3.26, 30.4+/-3.91, 25.1+/-5.25, 17.1+/-6.02 in females and 29.8+/-5.39, 28.8+/-5.29, 28.7+/-5.29, 23.7+/-5.39 and 16.0+/-5.60 in males. AChE activity from CRF-patients were higher than that found in the control subjects. The aging of human RBCs in both the controls and CRF-patients showed a progressive reduction in AChE activity. AChE activity of RBCs from female CRF-patients were significantly higher (p < 0.05) than that of the female control subjects. The RBCs isolated from male CRF-patients showed a higher AChE activity than control males, but a significant difference was only observed with the mid-age-cells. These studies further indicate that AChE activity remained insignificantly different in the various density based age subfractions of RBCs of both CRF-patients and controls.

Serine base exchange enzyme activity is modulated by sphingosine and other amphiphilic compounds: possible role of positive charge in increasing the synthesis of phosphatidylserine

It has been found that sphingosine and sphingosylphosphorylcholine (amphiphilic cations) have a stimulatory, and cholesterol 3-sulfate (an amphiphilic anion), an inhibitory, effect on [14C]serine incorporation into phosphatidylserine in glioma C6 and rat liver microsomes. In glioma intact cells sphingosine stimulates phosphatidylserine synthesis in a process independent of protein kinase C, but suppressed by thapsigargin. We suggest that the stimulation of the enzyme occurs by the interaction of amphiphilic cations with the membrane cosubstrate phospholipids, leading to a charge redistribution on their phosphate groups, and hence facilitating the enzyme action. A new hypothesis concerning the mechanism of the serine base exchange reaction is discussed.

Calorimetric and spectroscopic studies of the thermotropic phase behavior of the n-saturated 1,2-diacylphosphatidylglycerols

The polymorphic phase behavior of a homologous series of n-saturated 1,2-diacyl phosphatidylglycerols (PGs) was studied by differential scanning calorimetry and Fourier transform infrared and 31P-nuclear magnetic resonance spectroscopy. When dispersed in aqueous media under physiologically relevant conditions, these compounds exhibit two thermotropic phase transitions that are structurally equivalent to the well-characterized pretransitons and gel/liquid-crystalline phase transitions exhibited by bilayers of the corresponding 1,2-diacyl phosphatidylcholines. Furthermore, when incubated at low temperatures, their gel phases spontaneously transform into one or more solid-like phases that appear to be highly ordered, quasicrystalline bilayers that are probably partially dehydrated. The quasicrystalline structures, which form upon short-term, low-temperature annealing of these lipids, are meta-stable with respect to more stable structures, to which they eventually transform upon prolonged low-temperature incubation. The rates of formation of the quasicrystalline phases of the PGs generally tend to decrease as hydrocarbon chain length increases, and PGs whose hydrocarbon chains contain an odd number of carbon atoms tend to be slower than those of neighboring even-numbered homologs. The calorimetric data also indicate that the quasicrystalline phases of these compounds become progressively less stable relative to both their gel and liquid-crystalline phases as the length of the hydrocarbon chain increases and that they decompose either to the liquid-crystalline phase (short- and medium-chain compounds) or to the normal gel phase (long-chain compounds) upon heating. The spectroscopic data indicate that although there is odd-even alternation in the structures of the quasicrystalline phases formed upon short-term low-temperature incubation of these compounds, the structural features of the stable quasicrystalline phases eventually formed are all similar. Furthermore, the degree of hydration and the nature of hydrogen bonding interactions in the headgroup and interfacial regions of these PG bilayers differ significantly from that observed in all other phospholipid bilayers studied so far. We suggest that many of the properties of PG bilayers can be rationalized by postulating that the glycerol moiety of the polar headgroup is directly involved in shielding the negative charges at the surface of the bilayer by means of hydration-like hydrogen bonding interactions with the phosphate moiety.

Differential effects of spermine on phosphatidylinositol 3-kinase and phosphatidylinositol phosphate 5-kinase

The metabolism of phosphoinositides plays an important role in the signal transduction pathways. We report here that naturally occurring polyamines affect the activities of phosphatidylinositol (PI) 3-kinase and PI 4-phosphate (PIP) 5-kinase differently. While polyamines inhibited the PI 3-kinase activity, they stimulated the activity of PIP 5-kinase in the order of spermine > spermidine > putrescine. Spermine inhibited the PI 3-kinase activity in a concentration-dependent manner with an IC50 of 100 microM. On the other hand, spermine (5 mM) stimulated the activity of PIP 5-kinase 2-3 fold. Kinetic studies of spermine-mediated inhibition of PI 3-kinase revealed that it was noncompetitive with respect to ATP. The effect of Mg2+ and PIP2 concentration on kinase activity was sigmoidal, with spermine inhibiting PI 3-kinase activity at all PIP2 concentrations. While 1 mM calcium stimulated PI 3-kinase activity at submaximal concentrations of Mg2+ (1.25 mM), inhibition was observed at optimal concentration of Mg2+ (2 mM). We propose that spermine may modulate the cellular signal by virtue of its differential effects on phosphoinositide kinases.

The effect of erythropoietin on platelet function in uraemic children on haemodialysis

The effect of 1-year erythropoietin (rHu-EPO) treatment on the bleeding time, platelet aggregation, ATP and thromboxane B2 (TXB2) release, cyclic AMP (cAMP) concentration and platelet surface positive charge were studied in 8 haemodialysed children with chronic uraemia and 8 controls. The pre-dialysis haematocrit (Hct) was 0.21 + 0.01 before and 0.36 + 0.01 following 1 year of rHu-EPO therapy. At the end of this period the pre-dialysis bleeding time became normal (P < 0.05); this was associated with a significant increase in platelet aggregability (P < 0.05), ATP release (P < 0.05) and TXB2 production (P < 0.01), and with a significant decrease in platelet cAMP concentration (P < 0.01). A further increase in platelet aggregation, ATP release and TXB2 production and a decrease in platelet cAMP concentration was observed following bicarbonate haemodialysis (BHD) (P < 0.01). There was a significant positive correlation between platelet aggregation and ATP release (r = 0.78, P < 0.05), as well as platelet aggregation and TXB2 production (r = 0.68, P < 0.05). A significant negative correlation was found between platelet aggregability and cAMP concentration (r = -0.7, P < 0.05). The platelet surface positive charge, which was significantly lower in the patients than in the controls (P < 0.01), did not change during rHu-EPO therapy, nevertheless BHD resulted in a significant increase (P < 0.05), suggesting the surface charge may influence platelet aggregation. In an in vitro and an in vivo study, rHu-EPO and the higher Hct did not increase platelet aggregation directly. Long-term administration of rHu-EPO stimulated complex functional and biochemical changes in the platelets of uraemic patients, which resulted in an improved aggregability.

Effect of fatty acids on energy coupling processes in mitochondria

Long-chain fatty acids are natural uncouplers of oxidative phosphorylation in mitochondria. The protonophoric mechanism of this action is due to transbilayer movement of undissociated fatty acid in one direction and the passage of its anion in the opposite direction. The transfer of the dissociated form of fatty acid can be, at least in some kinds of mitochondrion, facilitated by adenine nucleotide translocase. Apart from dissipating the electrochemical proton gradient, long-chain fatty acids decrease the activity of the respiratory chain by mechanism(s) not fully understood. In intact cells and tissues fatty acids operate mostly as excellent respiratory substrates, providing electrons to the respiratory chain. This function masks their potential uncoupling effect which becomes apparent only under special physiological or pathological conditions characterized by unusual fatty acid accumulation. Short- and medium-chain fatty acids do not have protonophoric properties. Nevertheless, they contribute to energy dissipation because of slow intramitochondrial hydrolysis of their activation products, acyl-AMP and acyl-CoA. Long-chain fatty acids increase permeability of mitochondrial membranes to alkali metal cations. This is due to their ionophoric mechanism of action. Regulatory function of fatty acids with respect to specific cation channels has been postulated for the plasma membrane of muscle cells, but not demonstrated in mitochondria. Under cold stress, cold acclimation and arousal from hibernation the uncoupling effect of fatty acids may contribute to increased thermogenesis, especially in the muscle tissue. In brown adipose tissue, the special thermogenic organ of mammals, long-chain fatty acids promote operation of the unique natural uncoupling protein, thermogenin. As anionic amphiphiles, long-chain fatty acids increase the negative surface charge of biomembranes, thus interfering in their enzymic and transporting functions.

The ionic strength of the intermembrane space of intact mitochondria is not affected by the pH or volume of the intermembrane space

Ionic strength affects the electron transport activity of cytochrome c through its electrostatic interactions with redox partners and membrane lipids. We previously reported (Cortese, J.D., Voglino, A.L. and Hackenbrock, C.R. (1991) J. Cell Biol. 113, 1331-1340) that the ionic strength (I) of the intermembrane space (IMS-I) in isolated, intact condensed mitochondria is similar to the external, bulk I, over a wide range of bulk I. We now consider the possible effects of IMS-pH and IMS-volume, both variable parameters of mitochondrial function in situ, on IMS-I. IMS-pH and IMS-I were measured with pH- and I-sensitive fluorescent probes (highly fluorescent FITC-dextran for IMS-pH and FITC-BSA for IMS-I). These probes were delivered into the IMS of intact mitochondria via probe encapsulation into asolectin vesicles, followed by low pH-induced fusion of the vesicles with the outer membranes of intact mitochondria. IMS-pH was found to be 0.4-0.5 units lower than bulk pH over the pH range 6.0-8.5 for mitochondria with a large IMS-volume separating the two mitochondrial membranes (condensed configuration), and 0-0.2 units lower for mitochondria with a small IMS-volume and membranes closely opposed (orthodox configuration). This small pH difference between IMS-pM and bulk pH did not influence the similarity between IMS-I and bulk I. When the IMS-volume was osmotically decreased, bringing the two mitochondrial membranes in close proximity as in the orthodox configuration, IMS-I followed the bulk I above 10 mM but did not respond to changes in bulk I below 10 mM. The lack of response of the IMS-I below 10 mM indicates that the close proximity of the two mitochondrial membranes excludes ions only at low, nonphysiological I. Since the similarity of IMS-I and bulk I is unaffected by either IMS-pH or IMS-volume above a bulk I of 10 mM, at cytosolic physiological I (i.e., 100-150 mM) cytochrome c can be expected to be a free, three-dimensional diffusant in the IMS irrespective of the pH or volume of the IMS.

Investigations of the possible glycosylation of monoamine oxidase B from pig leucocytes

Monoamine oxidase B (MAO B) from pig liver has been reported to be a sialoglycoprotein. However, when that enzyme from pig lymphocytes and granulocytes was separated by polyacrylamide gel electrophoresis after labelling with the specific irreversible inhibitor [3H]pargyline, staining with 1-ethyl-2-[3-(1-ethyl-naphtho [1,2d] thiazolin-2-ylidene)-2-methylpropenyl] naphtho [1,2d] thiazolium bromide ("Stains-all") failed to detect the presence of sialic acid residues. Treatment of the enzyme in disrupted lymphocytes and granulocytes, or in mitochondrial fractions prepared from them, with neuraminidase resulted in a decrease in MAO activity. However, after the enzyme was rendered soluble by treatment with octylglucoside, treatment with neuraminidase had no effect on the activity. These results indicate that sialic acid residues are not an intrinsic component of MAO B, although associated material containing such groups appears to affect the activity of the membrane-bound enzyme. The activities of membrane-bound preparations of MAO B from pig lymphocytes and granulocytes were unaffected by treatment with trypsin or beta-chymotrypsin. After the preparations had been rendered soluble by treatment with octylglucoside there was a decrease in the activity on treatment with beta-chymotrypsin, but trypsin treatment had no effect. Thus solubilization resulted in residues sensitive to cleavage by the former enzyme becoming accessible to it. Tryptic and chymotryptic peptides separated from the sodium dodecyl sulphate denatured enzymes by polyacrylamide gel electrophoresis revealed no differences between MAO B prepared from lymphocytes and granulocytes.

Membrane lateral pressure as a modulator of glycerol-3-phosphate dehydrogenase activity

Michaelis-Menten kinetics of glycerol-3-phosphate dehydrogenase activity in proteoliposomes from brown adipose tissue mitochondria with exogenously added phospholipids or cholesterol was measured. It was shown that changes in membrane lipid composition affected the membrane lateral pressure and therefore modulated the enzyme activity, namely Vmax value. Contrarily, changes in surface charge caused by minute amounts of phosphatidylserine or charged organic substances influenced only the apparent Km value. The role of bulk phospholipids in regulation of glycerol-3-phosphate dehydrogenase is discussed.

Incorporation of mitochondrial L-glycerol-3-phosphate dehydrogenase into liposomes; effect of sodium oleate and calcium ions

FAD-linked L-glycerol-3-phosphate dehydrogenase purified from liver mitochondria of hyperthyroid rats was incorporated into unilamellar phospholipid liposomes. The incorporation influenced both Vmax,app and Km,app values of the enzyme for its substrate, L-glycerol 3-phosphate. The Km,app for the electron acceptor remained unchanged with a simultaneous slight enhancement of the corresponding Vmax,app value. The steady-state fluorescence anisotropies of the fluorescein isothiocyanate and trimethylammoniumdiphenylhexatriene labels were affected by sodium oleate and calcium ions in the case of both solubilized and liposome-incorporated L-glycerol-3-phosphate dehydrogenase. These results indicate that calcium ions cause a significant alteration of the enzyme conformation. Sodium oleate (used as a model of free fatty acids), besides its direct action on the enzyme itself, affects the enzyme indirectly as well, via alteration of the physical properties of the membrane.

Electrostatic analysis of effects of ions on the inhibition of corn root plasma membrane Mg2+-ATPase by the bivalent orthovanadate

The microelectrophoretic mobility of corn root plasma membranes and the inhibition of the Mg+2-ATPase by vanadate were investigated under different ionic conditions. The Mg2+-ATPase was uncompetitively inhibited and a 10-fold variation of the apparent inhibition constant was observed, depending on the addition of K+ and Mg2+. The determination of the zeta potential indicated that a 5-fold decrease of the apparent inhibition constant was due to aspecific electrostatic interactions of the vanadate anion and the negative charge of the membrane. The screening and masking effects of 6 mM free Mg2+ totally abolished electrostatic interactions and allowed the direct determination of the intrinsic vanadate inhibition constant (KIi). On the other hand, a specific, non-electrostatic, effect of K+ caused a 2-fold decrease of the inhibition constant in addition to the electrostatic effect. Finally, the electrostatic analysis indicates that the Mg2+-ATPase is inhibited by the monomeric bivalent anion HVO4(2-).

Modification of membrane permeability by animal viruses

Animal viruses modify membrane permeability during lytic infection. There is a co-entry of macromolecules and virion particules during virus penetration and a drastic change in transport and membrane permeability at the late stages of the lytic cycle. Both events are of importance to understand different molecular aspects of viral infection, as virus entry into the cell and the interference of virus infection with cellular metabolism. Other methods of cell permeabilization of potential relevance to understand the mechanism of viral damage of the membrane are also discussed.

Inhibition of nuclear T3 binding by fatty acids

Studies were performed to evaluate a possible modulatory role of lipids on the binding of T3 to rat liver nuclear receptors in vitro. Unsaturated fatty acids were potent inhibitors of the binding of [125I] T3 to isolated rat liver nuclei. Doses (in mumol/L) causing a 50% inhibition of nuclear T3 binding were 10 for palmitoleic acid, 11 for linoleic acid, 22 for oleic acid, 24 for arachidonic acid, and 37 for linolenic acid. Other lipids had less or no inhibitory activity. Unsaturated fatty acids reduced the affinity constant (Ka) of the binding of T3 to nuclear receptors to 57.4% +/- 11.0% that of controls (mean +/- SE 1.04 +/- 0.14 v 1.97 +/- 0.23 10(9) L/M, n = 5; P less than .02) but did not affect the maximal binding capacity (MBC) (1.47 +/- 0.20 v 1.55 +/- 0.10 10(-10) M/L; NS). Evaporated ether extracts of rat liver homogenate pretreated with phospholipase A2 for five to 20 minutes (that liberates unsaturated fatty acids from phospholipids) demonstrated a progressive inhibition of nuclear T3 binding with time when compared with ether extracts of untreated rat liver homogenate (F = 16.1; P less than .01). Evaporated, fatty-acid-rich ether extracts of human sera caused a dose-dependent inhibition in the binding of [125I] T3 to nuclear T3 receptors.(ABSTRACT TRUNCATED AT 250 WORDS)

Ca2+ and Mg2+ as modulators of mitochondrial L-glycerol-3-phosphate dehydrogenase

1. Physiological concentrations of either Ca2+ or Mg2+ stimulated L-glycerol 3-phosphate oxidation by intact mitochondria isolated from various mammalian tissues (hamster brown adipose tissue, rat brain, liver of normal and hyperthyroid rats). A higher cation concentration was required for stimulation by Mg2+ than by Ca2+. L-glycerol-3-phosphate dehydrogenase was the target of the stimulation by both cations as revealed by measurements with intact mitochondria as well as with the solubilized enzyme. With different electron acceptors Ca2+ and Mg2+ stimulation occurred at significantly different cation concentrations. 2. Substrate activation of mitochondrial L-glycerol-3-phosphate dehydrogenase was observed in intact mitochondria and with the solubilized enzyme isolated from hyperthyroid rats in the absence of Ca2+ and Mg2+. According to kinetic analysis two independent binding sites, functioning with different turnovers and with different affinities for the substrate, could account for the phenomenon. In the presence of Ca2+ or Mg2+ substrate activation could not be detected; the kinetic parameters apparently correspond to the tight substrate-binding site functioning with high turnover. 3. Thiol group(s), which in the absence of Ca2+ and Mg2+ did not participate in the functioning of the enzyme, played an essential role in the binding of these cations to the enzyme, as shown by chemical modification studies. 4. From the solubilized mitochondrial proteins L-glycerol-3-phosphate dehydrogenase was bound selectively to the hydrophobic phenyl-Sepharose 4B matrix in the presence Ca2+, and the bound enzyme could be eluted with EDTA. This suggests that Ca2+ caused an alteration in the conformation of the enzyme.

Effect of oleate on the apparent Km of monoamine oxidase and the amount of membrane-bound hexokinase in isolated rat hepatocytes: further evidence for the controlling role of the surface charge in hexokinase binding

Brief incubation of isolated rat hepatocytes in the presence of the oleate-bovine serum albumin complex resulted in a release to the cytosol of a portion of hexokinase (EC 2.7.1.1) normally bound to intracellular membranes. This was correlated with an increase of the negative surface potential of the outer mitochondrial membrane, as measured in situ by determining changes of Km of monoamine oxidase (EC 1.4.3.4). It is suggested that non-esterified fatty acids produce a partial release of bound hexokinase in the liver cell by changing the surface charge of intracellular membranes.

Action of long-chain fatty acids in vitro on Ca2+-stimulatable, Mg2+-dependent ATPase activity in human red cell membranes

Human red cell membrane Ca2+-stimulatable, Mg2+-dependent adenosine triphosphatase (Ca2+-ATPase) activity and its response to thyroid hormone have been studied following exposure of membranes in vitro to specific long-chain fatty acids. Basal enzyme activity (no added thyroid hormone) was significantly decreased by additions of 10(-9)-10(-4) M-stearic (18:0) and oleic (18:1 cis-9) acids. Methyl oleate and elaidic (18:1 trans-9), palmitic (16:0) and lauric (12:0) acids at 10(-6) and 10(-4) M were not inhibitory, nor were arachidonic (20:4) and linolenic (18:3) acids. Myristic acid (14:0) was inhibitory only at 10(-4) M. Thus, chain length of 18 carbon atoms and anionic charge were the principal determinants of inhibitory activity. Introduction of a cis-9 double bond (oleic acid) did not alter the inhibitory activity of the 18-carbon moiety (stearic acid), but the trans-9 elaidic acid did not cause enzyme inhibition. While the predominant effect of fatty acids on erythrocyte Ca2+-ATPase in situ is inhibition of basal activity, elaidic, linoleic (18:2) and palmitoleic (16:1) acids at 10(-6) and 10(-4) M stimulated the enzyme. Methyl elaidate was not stimulatory. These structure-activity relationships differ from those described for fatty acids and purified red cell Ca2+-ATPase reconstituted in liposomes. Thyroid hormone stimulation of Ca2+-ATPase was significantly decreased by stearic and oleic acids (10(-9)-10(-4) M), but also by elaidic, linoleic, palmitoleic and myristic acids. Arachidonic, palmitic and lauric acids were ineffective, as were the methyl esters of oleic and elaidic acids. Thus, inhibition of the iodothyronine effect on Ca2+-ATPase by fatty acids has similar, but not identical, structure-activity relationships to those for basal enzyme activity. To examine mechanisms for these fatty acid effects, we studied the action of oleic and stearic acids on responsiveness of the enzyme to purified calmodulin, the Ca2+-binding activator protein for Ca2+-ATPase. Oleic and stearic acids (10(-9)-10(-4) M) progressively inhibited, but did not abolish, enzyme stimulation by calmodulin (10(-9) M). Double-reciprocal analysis of the effect of oleic acid on calmodulin stimulation indicated noncompetitive inhibition. Addition of calmodulin to membranes in the presence of equimolar oleic acid restored basal enzyme activity. Oleic acid also reduced 125I-calmodulin binding to membranes, but had no effect on the binding of [125I]T4 by ghosts. The mechanism of the decrease by long chain fatty acids of Ca2+-ATPase activity in situ in human red cell ghosts thus is calmodulin-dependent and involves reduction in membrane binding of calmodulin.

Effects of nondenaturating zwitterionic detergent CHAPS, 3-[(3-cholamido propyl) dimethyl ammonio]-1-propanesulfonate, on rat liver mitochondrial and microsomal monoamine oxidase

It has been reported that monoamine oxidase (MAO) activity (EC1.4.3.4) and, in general, enzymes possessing cationic substrates, were activated and inhibited by anionic and cationic detergents, respectively. In order to examine this hypothesis, the effect of the zwitterionic detergent CHAPS 3-[(3-cholamido propyl) dimethyl ammonio]-1-propanesulphonate was studied in comparison with the effects of cationic, anionic, and non-ionic detergents. The non-denaturating zwitterionic detergent CHAPS was used to solubilise rat liver monoamine oxidase MAO (EC1.4.3.4) of mitochondrial and microsomal origin; the solubilisation conditions, purification, inhibition and kinetic studies were then determined. These results are compared with those previously obtained with the non-ionic detergent Triton X-100, which would also be expected to have no net charge, and are interpreted in terms of specific ionic effects.

Ionic strength-dependent alterations of membrane structure of red blood cells

Electron paramagnetic resonance (EPR) measurements using various fatty acid spin labels were performed on membranes of intact human erythrocytes at physiological, and at low ionic strength. In the case of spin probes bearing the nitroxide near the polar head group, a less restricted motion at low ionic strength was seen than with those labels with a nitroxide deeper within the hydrophobic tail of the membrane. Although these data clearly show an influence of ionic strength on membrane structure, and possibly a modified protein-lipid interaction, they cannot be simply discussed in terms of an altered membrane fluidity.

Further evidence for a membrane potential-dependent shape transformation of the human erythrocyte membrane

The influence of various factors (pH, temperature, sodium gluconate) on the ionic strength-dependent stomatocyte-discocyte-echinocyte transformation of the human erythrocyte membrane was investigated. The results give further evidence for a correlation between shape of erythrocyte membrane and the transmembrane potential of the cells.

Action of surfactants on porcine heart malate dehydrogenase isoenzymes and a simple method for the differential assay of these isoenzymes

The cationic surfactant, cetyl (hexadecyl) trimethylammonium bromide (CTAB), completely inactivates porcine heart cytoplasmic malate dehydrogenase (L-malate:NAD+ oxidoreductase, EC 1.1.1.37) at concentrations (of surfactant) which do not affect the activity of the mitochondrial isoenzyme. These concentrations are close to, or higher than, the critical micelle concentration of CTAB. An increase in the ionic strength of the medium significantly retards the CTAB-induced inactivation of the cytoplasmic enzyme. The enzyme is also markedly protected against CTAB inactivation by NADH; L-malate on its own has no effect but a combination of NADH and L-malate affords greater protection than NADH alone. The CTAB inactivation is not reversed by dilution of the surfactant. The highly selective action of CTAB on the two malate dehydrogenases, which correlates well with their electrostatic charges, has been exploited for a simple and reliable differential assay of these isoenzymes. The anionic surfactant, sodium dodecyl sulphate (SDS), at concentrations well below the critical micelle concentration, inactivates both isoenzymes, but the mitochondrial enzyme is significantly more sensitive than its cytoplasmic counterpart. There is thus some correlation, though not as strong as with CTAB, between SDS inactivation and the charges of the two malate dehydrogenases. An increase in ionic strength has opposite effects on the two isoenzymes: the mitochondrial enzyme becomes more resistant and the cytoplasmic enzyme less so. Both isoenzymes are rendered more resistant to SDS by the inclusion of NADH. Inactivation of the enzymes caused by short exposure to SDS is largely reversed by dilution of the detergent, but longer exposure leads to progressive irreversible loss of activity. NADH very effectively protects the isoenzymes against irreversible inactivation. It is likely that a reversible phase of inactivation precedes an irreversible phase and that in the former phase SDS acts competitively with NADH. Both malate dehydrogenases possess considerable resistance to the nonionic detergent, Triton X-100.

The regulation of exogenous NAD(P)H oxidation in spinach (Spinacia oleracea) leaf mitochondria by pH and cations

Essentially chlorophyll-free mitochondria were isolated from green leaves of spinach (Spinacia oleracea L. cv. Viking II). Uncoupled oxidation of exogenous NADPH (1 mM) to oxygen had an optimum at pH 6.0, and activity was relatively low at pH 7.0, even in the presence of 1 mM-CaCl2. There was a proportional increase in the apparent Km for NADPH with decreasing H+ concentrations, suggesting that NADPH protonated on the 2'-phosphate group was the true substrate. Exogenous NADH was oxidized by oxygen with an optimum at pH 6.9. Under low-cation conditions, EGTA or EDTA (both 1 mM) had no effect on the Vmax. of NADH oxidation, although the removal of bivalent cations from the membrane surface by the chelators could be observed by use of 9-aminoacridine fluorescence. In contrast, under high-cation conditions, chelators lowered the Vmax. by about 50%, probably due to a better approach of the negatively charged chelators to the negative membrane surface than under low-cation conditions. In a low-cation medium, the Vmax. of NADH oxidation was increased by about 50% by the addition of cations. This was caused by a lowering of the size of the negative surface potential through charge screening. In contrast with other cations, La3+ inhibited NADH oxidation, possibly through binding to lipids essential for NADH oxidation. The apparent Km for NADH varied 6-fold in response to changes in the size of the surface potential, suggesting that the approach of the negatively charged NADH to the active site is hampered by the negative surface potential. The results demonstrate that the spinach leaf cell can regulate the mitochondrial NAD(P)H oxidation through several mechanisms: the pH; the cation concentration in general; and the concentration of Ca2+ in particular. The results also emphasize the importance of electrostatic considerations when investigating the kinetic behaviour of membrane-bound enzymes.