Enzyme and membrane conformation in biochemical control

Characterization of liposomes and other lipid assemblies by multiprobe fluorescence polarization

This minireview describes the use of fluorescence polarization to characterize the dynamics and organization of biological membranes, lipoproteins, liposomes and other lipid-based assemblies. The advantages, disadvantages and limitations of this method are discussed. There is special emphasis on the multiprobe approach, which can monitor various parts of the lipid membrane or assembly. The fluorescence polarization, anisotropy and the total fluorescence intensity are defined and their graphic presentation described. A user guide for the actual measurements is given. It describes how to apply fluorescence polarization, especially how to select the proper extrinsic probe and to optimize the measurement regarding theoretical and practical issues, the determination of a fluorescent probe location in the assembly, how to introduce the probe to the lipid assembly, what is the ideal ratio of the probe to the lipid, dealing with the spectrofluorimeter problems and how to overcome disturbances introduced by light scattering. It is shown here, that even though there are disadvantages in using fluorescence polarization, this approach provides a highly sensitive and simple means for assessing the subtle and more pronounced changes in the structure of lipid assemblies.

The interaction of thioridazine with cardiac cytochrome oxidase; enzyme activity and drug binding studies

Thioridazine interacts with purified cardiac cytochrome oxidase altering both the activity of the enzyme and the optical spectrum of the drug. Cytochrome oxidase activity, as measured by oxidation of cytochrome c, exhibits a biphasic response to changing drug concentration. Lower concentrations of thioridazine increased cytochrome oxidase activity up to 20% at 65 microM and higher concentrations inhibit activity until almost complete inhibition is observed. Both the activation and the inhibition of cytochrome oxidase by thioridazine follow Michaelis-Menton kinetics with Vmax changing but Km remaining constant. The analysis of the 2 nm shift in the UV absorption spectrum of the thioridazine suggest that the binding of thioridazine to cytochrome oxidase involves multiple (535) binding sites on the enzyme with an average dissociation constant of 20 microM.

Determination of the critical micelle concentration of surfactants using the fluorescent probe N-phenyl-1-naphthylamine

The fluorescence quantum yield of N-phenyl-1-naphthylamine (NPN) increases about 10-fold and the wavelength of maximum fluorescence emission is blue-shifted when this molecule partitions into the apolar core of micellar structures from the aqueous phase. This property allowed the utilization of NPN as a fluorescent indicator of micelle formation by 14 different surfactants belonging to the families of alkyltrimethylammonium halides, alkylsulfates, alkylbetaines, alkylglucosides, and bile salts. The critical micelle concentrations (CMCs) determined with NPN agreed well with literature values. In this work NPN was used at a concentration of 10(-6) M which allowed determination of CMCs in the range between approximately 10(-5) and greater than 10(-2) M. With high-sensitivity instrumentation considerably lower NPN concentrations can be used and consequently considerably lower CMCs can be rapidly and accurately determined.

Spectra, membrane binding, and potentiometric responses of new charge shift probes

The properties of a series of new potentiometric membrane probes have been explored. The probes all contain an (aminostyryl)pyridinium chromophore or a more highly conjugated analogue. The spectral properties of the dyes are discussed in terms of the excitation-induced charge shift from the pyridine to the aniline; this charge shift also provides the basis for the voltage dependence of the spectra according to an electrochromic mechanism. The spectral responses to a membrane potential on a hemispherical bilayer have been obtained and, grossly, are quite similar for all probes tested. The more subtle variations from dye to dye can be partially rationalized by consideration of binding parameters, the depth within the membrane, and structural factors. The most potential sensitive dye in this collection has been designated di-4-ANEPPS and has a 6-amino-2-naphthyl group in place of the p-anilino on the parent chromophore. Both the relative fluorescence emission and excitation responses have maxima of 8% per 100 mV, and these two spectra display a striking symmetry.

Alterations in liver mitochondrial function as a result of fasting and exhaustive exercise

The effect of exercise upon liver mitochondria structure and function was examined in fasted and fed rats, following a single run to exhaustion on a motor-driven treadmill. Exercise alone and exercise coupled with fasting both produced a significant decrease in the amount of hexokinase bound to the mitochondria, as well as reduction in the ADP/O ratio and acceptor control index measured in the presence of succinate. The mitochondria of the exercised animals, when exposed to freeze-fracture analysis while in state 3, displayed fewer deflections in the fracture plane between the inner and outer membrane than those isolated from control animals. This suggests that fewer contacts existed between the two membranes. Measurements based upon the binding of 8-anilinonaphthalene 1-sulphonate indicated that there was an increase in the net negative charge on the surface of the mitochondrial membranes of the exercised animals. All of these effects could be mimicked by incubation of mitochondria from control animals with free fatty acids. This fact, coupled with the observation that washing of the mitochondria with a solution comprising 5% (w/v) albumin could reverse all of the consequences of exercise, suggests that these alterations in mitochondrial structure and function may be the result of the increase in plasma free fatty acids that accompanies long-term exercise. Furthermore, the observation that the exercise-induced changes are dynamic and readily reversible indicates that the mitochondria were not necessarily damaged, but rather that the coupling of oxidative phosphorylation may be subject to physiological regulation.

Effects of octyl beta-glucoside on insulin binding to solubilized membrane receptors

Octyl beta-glucoside (1%), a dialyzable detergent, was used to solubilize the insulin receptor of the turkey erythrocyte membrane. Insulin binding capacity was stable for at least 1 week when the receptor was kept in 1% octyl beta-glucoside at 4 degrees C. The binding properties of the solubilized receptor were examined at detergent concentrations above (1%) and below (0.6%) the critical micelle concentration. A reduction in insulin binding occurred when the detergent concentration was raised above the critical micelle concentration, due to an apparent decrease in the number of binding sites. The specificity of the receptor for insulin analogues was preserved, and the relative affinity of the solubilized receptor, desoctapeptide insulin greater than proinsulin greater than porcine insulin, was similar in 0.6% and 1% detergent. Addition of divalent cations increased insulin binding to a similar extent at both detergent concentrations, but there was a slightly greater stimulation of binding in 0.6% detergent as compared to 1% detergent. The pH optimum for binding was not affected by changes in the detergent concentration. These results indicate that the insulin receptor can be successfully solubilized by octyl beta-glucoside and that the binding activity is quite stable. Therefore, octyl beta-glucoside may be a useful detergent for purification of this receptor. In addition, the data indicate that the binding properties of the insulin receptor can be affected by changes in the physical state of the octyl beta-glucoside.

Dynamic properties of the lipid-water interface of model membranes as revealed by lifetime-resolved fluorescence emission spectra

We examined the dynamic properties of the lipid-water interface region of model membranes, on the nanosecond time scale, by using the fluorescent probe 2-p-toluidinylnaphthalene-6-sulfonic acid (TNS). In particular, we examined the steady-state emission spectra of TNS as its average lifetime was decreased by oxygen quenching. Under these quenching conditions the centers of gravity (Vcg) of the emission spectra shift ot shorter wavelengths. The lifetime dependence of these shifts reveals the time dependence of membrane relaxation around the excited-state dipole moment of TNS. The lipids investigated include dioleoyl-, dimyristoyl-, and dipalmitoyl-L-alpha-phosphatidylcholines, bilayers containing cholesterol, and an ether analogue of dipalmitoyl-L-alpha-phosphatidylcholine. For these lipids, the spectral relaxation times and the temperature dependence of the relaxations are similar in magnitude. Most relaxation times fall in the range of 0.6-6 ns, and except for the ether analogue, the inactivation energies for spectral relaxation are 10 plus or minus 2 kcal/mol. The average energy loss during spectral relaxation was 1000 cm(-1). However, for the saturated phosphatidylcholines at temperatures below their transition temperatures, smaller relaxation losses were observed (approximately 600 cm(-1)). We attribute these smaller losses to ordering of the polar head groups around the ground-state dipole moment of TNS. Overall, these results indicate that the dynamic properties of the lipid-water interface region are similar among the phosphatidylcholines and depend only slightly on the chemical composition and phase state of the acyl side chains.

Fluorescence spectroscopic studies of Huntington fibroblast membranes

Using fluorescence spectroscopic methods, we compared the membrane properties of intact fibroblasts from both normal subjects and patients with Huntington disease (HD). Cells were stained with various fluorophores, including 1-anilino-8-naphthalene sulfonic acid (ANS), 2-toluidinyl-6-naphthalene sulfonic acid (TNS), 1,6-diphenyl-1,3,5-hexatriene (DPH), and 6-lauroyl-2-(dimethylamino)-naphthalene (LAURDAN). Using these labeled cells, we measured fluorescence yields and emission maxima (ANS, TNS, and LAURDAN), polarizations (TNS, DPH, and LAURDAN), lifetimes (TNS), and differential polarized lifetimes (DPH). In each instance, comparisons were made between cells from normal and from HD individuals. These cultures were controlled for passage number in culture and for age of donor. We found no significant differences between the HD and the control fibroblasts in experiments using the above-mentioned probes and spectroscopic parameters.

Electron spin resonance and fluorescence observations on erythrocytes, erythrocyte membranes, 13762 MAT-A ascites adenocarcinoma cells, and their membranes, effects of membrane perturbations

Membranes from erythrocytes or MAT-A 13762 tumor cells were labeled with the fatty acid spin probe I(5,10) or ANS and examined by spin resonance (ESR) or fluorescence polarization in the presence or absence of the perturbants EDTA, trypsin, glutaraldehyde, and dodecylsulfate. Extraction of cell membranes with hypotonic EDTA produced fragments in which the order parameters and fluorescence polarization values increased. Fluorescence polarization values using membranes labeled with diphenylhexatriene showed an apparent increase in membrane fluidity. A large portion of both I(5,10) and both fluorescence probes coextract with the peripheral membrane proteins in both membrane systems. Paramagnetic quenching of tryptophan fluorescence with I(5,10) and the spectral characteristics of ANS in these membranes indicated further that significant amounts of both probes bind either at or near the protein-lipid interface or directly to protein moieties. Trypsinization of cell membranes, which preferentially cleaves the large cytoskeletal proteins, fragmented the membranes and reduced the ESR order parameter. Glutaraldehyde immobilized I(5,10) in both types of membranes. These studies suggest that the association of cytoskeletal proteins with the membrane does not have any pronounced, consistent effect on biophysical properties of the bilayer. Attempts to apply these same probes to studies of the plasma membranes of intact cells were not successful because of the diffusion of the probes into the cells. These studies also point out some difficulties in using probe-group techniques to determine the nature of changes in bilayer structural parameters and emphasize the need for a better understanding of probe-group localization and behavior in such studies.

Fluorescent probes to detect lymphocyte activation

Fluorescent probes can monitor events in lymphocytes stimulated by mitogens and antigens. Early activation is associated with conformational changes in membrane macromolecules, and has been studied by measurement of fluorescence intensity or polarization of the membrane-localizing probes ANS, NPN, DPH and TMRITC. Subsequent changes in cytoplasmic macromolecules have been detected by altered fluorescence polarization of intracellular fluorescein. Altered metabolic activity in the activated lymphocyte is also revealed by fluorescent probes: the increased red fluorescence of lysosomes seen by AO staining, is attributable to altered lysosome membrane permeability. AO fluorescence has also detected early changes in the nuclear nucleoprotein complex. The later synthesis of new DNA is readily demonstrated by increased staining with the nuclear probes AO, ethidium bromide, propidium iodide, mithramycin and the Hoechst dyes. Adaptation of fluorescent probe analyses to the now rapidly developing flow microfluorimeters is providing rapid and sensitive assays of lymphocyte stimulation. Such methods will permit routine detection of lymphocyte response to particular antigens or mitogens, as well as identification of antigenic substances by their stimulation of known reactive lymphocytes. Last but not least, fluorescent probes are providing new understanding of the cellular events and regulatory mechanisms associated with lymphocyte activation.

The binding of a neutral aromatic molecule to a negatively-charged lipid membrane

This paper presents a detailed study of the binding of the fluorescent dye N-phenyl naphthylamine (NPN) to bilayers composed of the negatively-charged phospholipid methylphosphatidic acid. Binding to the liquid-crystalline membrane is enthalpy-driven. It is shown by determination of the binding constant and confirmed by n.m.r. that most of the dye ("guest") molecules reside between the lipid hydrocarbon chains at a fixed distance from the head-group, and are not distributed uniformly throughout the hydrocarbon phase. Each guest molecule is surrounded by about four lipid molecules. Transition of the membrane from the liquid-crystalline to the crystalline state results in almost total expulsion of the bound NPN into the water phase. Electrostatic theory is developed to find the effect of electrostatics upon the binding of a neutral molecule to charged membranes. Although the charge product is zero, electrostatic interactions play a part in determining the strength of binding, if each guest molecule incorporated increases the area of the membrane. For NPN this increase was found to be ca. 41 A(2).

Cholesterol-lipid interactions in membranes. The saturation concentration of cholesterol in bilayers of various lipids

1. The integration of cholesterol in a lipid bilayer can be visualized by changes in the fluorescence properties of the probe N-phenyl-1-naphthylamine (NPN). An increasing cholesterol content in the lipid phase corresponds to decreasing fluorescence intensity of NPN and a short wave shift of the emission spectrum. 2. Equilibrium constants for the partition of NPN between water and the various lipid phases are reported. An increasing cholesterol content in a bilayer decreases the solubility of NPN in the bilayer. 3. The saturation concentration of cholesterol in bilayers of various lipids prepared by ultrasonication is determined using the flourescence probe NPN. The maximal molar ratio of cholesterol : lipid is 2 : 1 for sphingomyelin or egg phosphatidylcholine and 1 : 1 for cerebroside, dipalmitoyl phosphatidylcholine, or dipalmitoyl phosphatidylethanolamine. 4. The comparison of the maximal molar ratio of cholesterol : lipid with the number of proton donor and proton acceptor sites in the lipid moiety is used for a discussion of the polar interactions of cholesterol within a lipid bilayer.

Fluorescent localization of membrane sites in glycerinated chicken skeletal muscle fibers and the relationship of these sites to the protein composition of the Z disc

Didansyl derivatives of amino acids and N-phenyl-1-naphthylamine were used to localize membrane hydrophobic sites in glycerol-extracted chicken skeletal muscle fibers. Epifluorescence microscopy revealed that such sites coincide with the distribution of mitochondria, the transverse tubular (T) system and the sarcoplasmic reticulum (SR). They are specifically associated with myofibril Z lines and occasionally extend from one Z plane to the next longitudinally along the muscle fiber. The hydrophobic probes interact noncovalently with the Z lines, and their induced fluorescence can be eliminated by exposure of the myofibrils to ionic detergents, nonionic detergents, or phospholipase C, before or after addition of the hydrophobic label. Extraction of glycerinated fibers with 0.6 M KI removes the majority of sarcomeric actin and myosin and leaves a scaffold of longitudinally interconnected Z planes. Membrane fluorescence remains tightly associated with these Z planes and with the remnant mitochondria. Shearing of such scaffolds results in the cleavage of the longitudinal connections and the production of large sheets of interconnected, close-packed Z discs in a honeycomb-like array. Comparison of the localization of two Z disc proteins, desmin and alpha-actinin, with that of the membrane material reveals that alpha-actinin is localized in the interior of each myofibril Z disc whereas both desmin and the membrane material surround each disc. Thus, glycerination and KI extraction of muscle fibers leaves remnants of T system and SR membranes tightly associated with the Z disc honeycomb lattice. Because the Z discs are connected at their peripheries through the T system appear to the plasma membrane, desmin and this membrane structure appear to be connected throughout the whole Z plane up to and including the plasma membrane. The congruent localization of desmin and the T system strongly suggests that this molecule mediates the adhesion of this membrane system around each Z disc.

Small-angle X-ray scattering from mitochondria

X-ray (CuKalpha) scattering curves of rat liver mitochondria are characterized by continuously decreasing intensity from 0.5 to 5 mrad and a broad maximum centered near 20 mrad. The condensed-to-orthodox morphological transition of the inner membranes of intact mitochondria causes a dramatic decrease in scattering at very small angle and a marked shift of the 20 mrad maximum to smaller angle. A similar small-angle scattering maximum is observed with inner mitochondrial membrane fractions prepared by digitonin treatment and osmotic shock/step gradient centrifugation procedures. However, the small-angle X-ray scattering curves of mitochondria after acetone treatment and osmoticlysis/sonication are essentially continuous. These characteristics of mitochondrial X-ray scattering are discussed in terms of known structural features of the organelle.

4-Chloro-7-nitrobenzo-2-oxa-1,3-diazole as a reactivity probe for the investigation of the thiol proteinases. evidence that ficin and bromelain may lack carboxyl groups conformationally equivalent to that of aspartic acid-158 of papain

1. 4-Chloro-7-nitrobenzo-2-oxa-1,3-diazole (Nbd chloride) was used as a reactivity probe to characterize the active centres of papin (EC 3.4.22.2), ficin (EC 3.4.22.3) and bromelain (EC 3.4.22.4). 2. In the pH range 0-8 Nbd chloride probably exists mainly as a monocation, possibly with the proton located on N-1 of the oxadiazole ring. 3. Spectroscopic evidence is presented for the intermediacy of Meisenheimer-type adducts in the reaction of Nbd chloride with nucleophiles. 4. The pH-dependence of the second-order rate constants (k) of the reactions of the three enzymes with Nbd chloride was determined at 25 degrees C, I = 0.1 mol/litre in 6.7% (v/v) ethanol in the pH range 2.5-5, where, at least for papain and ficin, the reactions occur specifically with their active-centre thiol groups. The pH-k profile for the papain reaction is bell-shaped (pKaI = 3.24, pKaII = 3.44 and k = 86M(-1)-s(-1), whereas that for ficin is sigmoidal (pKa = 3.6, k = 0.36M(-1)-s(-1), the rate increasing with increasing pH. The profile for the bromelain reaction appears to resemble that for the ficin reaction, but is complicated by amino-group labelling. 5. The bell-shaped profile of the papain reaction is considered to arise from the reaction of the thiolate ion of cysteine-25, maintained in acidic media by interaction with the side chain of histidine-159, with the Nbd chloride monocation hydrogen-bonded at its nitro group to the un-ionized form of the carboxyl group of aspartic acid-158. The lack of acid catalysis in the corresponding reactions of ficin and probably of bromelain suggests that these enzymes may lack carboxyl groups conformationally equivalent to that of aspartic acid-158 of papain. The possible consequences of this for the catalytic sites of these enzymes is discussed.

Evidence for an increase in microviscosity of plasma membranes from soybean hypocotyls induced by the plant hormone, indole-3-acetic Acid

The plant hormone indole-3-acetic acid (IAA or auxin) added at a concentration for half-maximal promotion of cell elongation (1 mum) caused an increase of 25% in the fluorescence polarization of the membrane-bound probe N-phenyl-1-naphthylamine, when added to fractions enriched in plasma membranes from soybean hypocotyls (Glycine max L. var. Wayne), with no measurable change in fluorescence lifetime. The amplitude of the polarization increase was maximal in the temperature range 12 to 22 C. The findings provide evidence for a cell-free response of isolated plasma membranes to the hormone and imply that the response involves an increase in the microviscosity of hydrocarbon regions of the membrane.

Membrane changes in Escherichia coli induced by colicin Ia and agents known to disrupt energy transduction

The addition of colicin Ia to a suspension of intact Escherichia coli in the presence of the hydrophobic fluorescent probe N-phenyl-l-naphthylamine causes dramatic changes in the fluorescence of the probe. The fluorescence intensity increases several fold, the emission spectrum shifts to the blue, the fluorescence lifetime approximately doubles, and the polarization increases. These changes do not appear to result from an increase in membrane microviscosity, as has been previously postulated to be the case for the N-phenyl-l-naphthylamine fluorescence changes seen with colicin El-treated cells (Helgerson, S.L., Cramer, W.A., Harris, J.M., and Lytle, F.E. (1974), Biochemistry, 13, 3057); rather, they result from an increased binding of the dye to the cell envelope. A variety of agents have been used to demonstrate that a very similar fluorescence response results whenever the cells are "deenergized." These agents include electron transport inhibitors (malonate, amytal, cyanide) as well as the uncouplers CCCP and azide. In addition, depleting the cells of either endogenous substrates or oxygen results in the same fluorescence response. In these cases, the fluorescence response is reversed upon addition of an oxidizable substrate or oxygen. It is clear that there are significant changes in the Escherichia coli envelope as energy transduction processes are disrupted and restored. The changes reported by the fluorescent probe may prove useful indeciphering structure-function relationships in the Escherichia coli envelope.

Spin-labelled AMP - an activator of phosphorylase

1. A spin-labelled AMP derivative and its diamagnetic analogue activate phosphorylase b in the same way, but do not activate phosphorylase a. 2. The electron-spin-resonance spectra of the spin-labelled AMP derivative bound to phosphorylase b and a have "powderlike" characteristics indicating that the spin label is immobilised on the protein. From changes in the electron-spin-resonance spectrum of spin-labelled AMP as phosphorylase b or a is added, the dissociation constants were calculated. 3. The interactions of spin-labelled AMP and the diamagnetic analogue with phosphorylase b and a have been monitored by observing changes in the spectral properties of fluorescent and spin-label probes covalently attached to the enzyme. 4. The dissociation constants of spin-labelled AMP and phosphorylase b or a are 175 +/- 25 muM and 15 +/- 5 muM respectively. Similar dissociation constants are obtained for the diamagnetic analogue. The effect of these AMP derivatives on the covalently attached probe groups and on phosphorylase activity is compared to the effect of AMP and IMP.

Measurement and interpretation of fluorescence polarisations in phospholipid dispersions

An instrument that measures the temperature dependence of fluorescence polarisation and intensity directly and continuously is described. The behaviour of four fluorescent probes bound to a number of well characterised model systems was then examined. The motional properties of the probes were determined from the polarisation and intensity data and were found to be sensitive to the crystalline-liquid crystalline phase transitions in phospholipid vesicles of dimyristoly and dipalmitoly phosphatidylcholine. Binary mixture of dilauroyl and dipalmitoyl phosphatidylcholine show lateral phase separation and in this system the probes parition preferentially into the more 'fluid' phase. In systems that have been reported to contain 'short range order' or 'liquid clustering', such as dioleoyl phosphatidylcholine and liquid paraffin, the motion of the probes was found to have anomalous Arrhenius behaviour consistent with the idea that homogeneous phases were not being sampled. The significance of these findings for the interpretation of the behaviour of fluorescent probes bound to natural membranes is discussed.

Changes in E. coli cell envelope structure caused by uncouplers of active transport and colicin E1

It is of interest to inquire whether agents that uncouple or deenergize membranes cause concomitant structural changes. The agents considered here are the uncoupler carbonyl cyanide-p-trifluoromethoxyphenylhydrazone and the bacteriocidal protein colicin E1, agents for which there is some precedent for believing that they interact with membranes. In intact E. coli ML 308-225 cells the inhibition of [14C]-PROLINE ACtive transport by FCCP increases with uncoupler concentration from approximately 20% at 2 muM to approximately 100% at 5 muM. The increase in the rotational relaxation time (rho) of the cell-bound fluorescent probe N-phenyl-1-naphthylamine (PhNap)1 and 8-anilino-1-naphthalene-sulfonate (ANS) under these conditions shows the same dependence on FCCP concentration. For cells treated with EDTA to remove part of the outer lipopolysaccharide layer, inhibition of proline transport and the increase in rho value of ANS show the same dependence on FCCP concentration with saturation at 0.3 muM. EDTA treatment causes a large increase in the binding and rotational relaxation time of PhNap, the latter quantity approaching a value obtained with purified inner membrane. Similar effects are produced in untreated cells by 5muM FCCP...