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Biomedical subjects

R Schwyzer

Publications and source records attributed to R Schwyzer.

At least 19 recordsLinked to original sources
PubMed1992

Interaction of glucagon with artificial lipid bilayer membranes.

The enhancement of fluorescence emission from the tryptophan residue of glucagon, the quenching of that emission with acrylamide and with 5-doxyl and 16-doxyl stearic acid, circular dichroism spectra, the release of 6-carboxyfluorescein, and polarized infrared attenuated total reflection (IR-ATR) spectra were used to study the interaction of glucagon with intact lipid vesicles and flat bilayers. Dimyristoylphosphatidylcholine bound the peptide only below the main transition temperature, thus confirming earlier results of Epand et al. (1977). However, the peptide is also bound by vesicles of unsaturated lipids above their transition temperature, suggesting an influence of lipid area on the binding process. Circular dichroism showed that binding to such vesicles also increases the helix content of glucagon. The IR-ATR study and a comparison with dynorphin-A-(1-13)-tridecapeptide revealed profound differences in orientation of the two peptides. The dichroic ratios and the derived order parameters indicated an isotropic orientation of the helical segments of glucagon, but did not exclude a principal orientation of the molecules lying flat on the membrane surface. In contrast, the axis of the dynorphin helix is clearly oriented normal to the interface. The two peptides also differ in their rates of 6-carboxyfluorescein release, suggesting a deeper penetration of the primary amphiphilic helix of dynorphin A-(1-13) than of the secondary amphiphilic helix of glucagon.

Amino Acid Sequence↗
PubMed1992

How do peptides interact with lipid membranes and how does this affect their biological activity?

1. A short review is given of the chemical, physical, and pharmacological development of the idea that target cell lipid membranes may catalyze the interaction between regulatory peptides (or other pharmacologic agents) and their cell surface receptors. 2. The message-address and the membrane compartments concepts explain the observed correlations between the three-dimensional structures of peptides induced by a membrane surface and their preference for a certain receptor subtype. 3. Examples are given for opioid peptides (enkephalin, dynorphin, etc.), tachykinin peptides (substance P, neurokinin A, etc.), and melanocortin peptides (ACTH, alpha-MSH, etc.). 4. Relationships between the conformation of substance P induced by membrane association and that of a non-peptide substance P mimetic are discussed. Possible reasons for the difference between agonistic and antagonistic properties in the peptide field are revealed by this case.

Animals↗
PubMed1991

New principle in QSAR: membrane requirements.

In this review I discuss our recent work on the possible role of the lipid phase of the target cell membrane in mediating receptor subtype selectivity of peptides.

Amino Acid Sequence↗
PubMed1990

Transfusion-related human immunodeficiency virus in patients with haemophilia in Johannesburg.

Tests were carried out on 198 patients with inherited coagulation disorders attending haemophilia clinics of Johannesburg and Baragwanath Hospitals for the prevalence of antibodies to the human immunodeficiency virus (HIV). This cohort of patients has been treated with locally produced (South African) blood products from volunteer donors, except for a 15-month period in 1982-1984 when, owing to a shortage of locally produced material, an imported large donor-pool US factor VIII concentrate was used. Not all patients received this material. Of the haemophilia A patients who received the imported factor VIII concentrate, 85% were seropositive, while only 3% of the patients who received locally produced small donor-pool products were seropositive. No factor VIII-deficient patients have seroconverted while using small donor-pool products, since the introduction of routine screening of blood donations and strict exclusion criteria of donors. However, despite testing of blood products, 3 patients receiving locally produced factor IX concentrate (4,000 donors) seroconverted in 1988, having previously been HIV-negative. Factors influencing the choice of blood products to be used, especially in South Africa, are discussed.

Adolescent↗
PubMed1989

Reversible binding of substance P to artificial lipid membranes studied by capacitance minimization techniques.

Interaction of substance P with electrically neutral, planar lipid bilayers prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine and with anionic bilayers prepared from mixtures of 1,2-dioleoyl-sn-glycero-3-phosphocholine and brain phosphatidylserine was measured using the capacitance minimization method for monitoring the membrane surface potential caused by the positive charges and electric dipole moment of adsorbed peptide. Substance P bound to the electrically neutral bilayers from 9 mM KCl (buffered to pH 5.5 with 2.0 mM 2-(N-morpholino)ethanesulfonate) with a maximal binding density of about 1 x 10(-2) molecules per nm2 and a dissociation constant of about 2 x 10(-4) M. Measurement of the surface potential at different ionic strengths (shielding of surface charges) allowed distinction between the fixed-charge surface potential and a dipole potential. Ascribing this dipole potential to membrane-bound substance P would imply an effective dipole moment normal to the bilayer surface of about 20 Debye per molecule. Magnitude and polarity are consistent with an alpha-helical domain at the C-terminal end of substance P which is oriented normal to the surface of the membrane, and inserted so as to be inaccessible to the aqueous phase. Consistent measurements were obtained with anionic membranes at low substance P concentrations (10(-7)-10(-6) M; pH 7.2). They indicated electrostatic accumulation of the triply charged peptide on the surface of the membrane followed by hydrophobic interaction with the same parameters as for neutral membranes. The results agree with the membrane structure of substance P determined with infrared attenuated total reflection spectroscopy, circular dichroism measurements, and thermodynamic estimations.

Kinetics↗
PubMed1988

Cation dependence of opioid receptor binding supports theory on membrane-mediated receptor selectivity.

A quantitative analysis of the binding of dynorphin A-(1-9)-nonapeptide to the opioid Kappa-receptors of the guinea pig cerebellum [Paterson et al. (1986) Proc. Natl. Acad. Sci. U.S.A. 83, 6216-6220] shows that changes in electrostatic surface accumulation of the ligand fully account for the observed suppression of binding by a series of univalent and divalent salts. Binding to mu- and delta-receptors, on the other hand, is subject to additional ion-specific effects. These observations support the membrane locations for the receptor sites proposed by the "membrane compartments" theory for opioid receptor selection.

Algorithms↗
PubMed1988

Conformation and orientation of regulatory peptides on lipid membranes. Key to the molecular mechanism of receptor selection.

The reaction of regulatory peptides with their membrane-bound receptors often occurs via a membrane-associated state of the peptide. From infrared studies on thin lipid films, we have shown that several ligands of the opioid kappa receptor and the neurokinin NK-1 receptor insert their message segments as an alpha-helix, more or less perpendicularly, into the membrane. The binding parameters for these membrane-associated states were determined from the capacitance minimization potential of lipid bilayers. A theory has been developed to account for the observed binding constants and the preferred conformation and orientation of these peptides. In contrast to the kappa and NK-1 receptors, ligands of the opioid mu and delta, and the neurokinin NK-2 and NK-3 receptors, are predicted not to form the inserted alpha-helical structure. A selection between the mu and delta (or NK-2 and NK-3) receptors appears to be made on the basis of an electrostatic gradient near the membrane surface. The molecular mechanism of receptor selection thus appears to be based to a large extent on the membrane-induced compartmentalization of ligands for the different receptors.

Hormones↗
PubMed1988

Estimated membrane structure and receptor subtype selection of an opioid alkaloid-peptide hybrid.

Preferred conformation, orientation, and accumulation of dynorphin A-(1-8)-octapeptide, naltrexone, and N beta-(D-Leu-D-Arg-D-Arg-D-Leu-D-Phe)-naltrexamine (Lipkowsky et al., 1988) were estimated according to established procedures. Opioid binding site affinities and selectivities available from the literature were correlated with the estimated parameters of lipid membrane interaction. The results agreed with the molecular mechanism of opioid receptor subtype selection proposed earlier.

Animals↗
PubMed1988

Ligand/receptor interactions--the influence of the microenvironment on macroscopic properties. Electrostatic interactions with the membrane phase.

The heterogeneous environment in which ligand/receptor interactions occur often leads to complex binding behaviour. We consider here the ligand/membrane interaction, emphasizing the possibilities of electrostatic modulation of the overall binding characteristics. The binding of Substance P to neutral or negatively charged planar lipid bilayers was monitored using the capacitance minimization technique. The electrostatic attraction to the charged bilayer potentiates the interaction by more than two orders of magnitude and leads to a nonlinearity in the Scatchard plot of bound vs. bulk concentrations. The Boltzmann accumulation factor, along with the direct measurement of the surface potential, provides an easy explanation of the effect. The general importance of electrostatic accumulation (or repulsion) at surfaces is discussed and the concept applied to examples from the literature.

Chemical Phenomena↗
PubMed1987

Membrane structure of bombesin studied by infrared spectroscopy. Prediction of membrane interactions of gastrin-releasing peptide, neuromedin B, and neuromedin C.

Bombesin, in contact with flat phospholipid bilayer membranes, was shown to adopt a membrane structure similar to that of substance P, dynorphin-(1-13)-tridecapeptide, and adrenocorticotropin-(1-24)-tetracosapeptide. The C-terminal message segment, comprising 8-10 amino acid residues, is inserted into a relatively hydrophobic membrane compartment as an alpha-helical domain oriented perpendicularly on the membrane surface. The N-terminal, hydrophilic tetrapeptide segment remains in the aqueous compartment as a random coil. This was shown with IR and IR attenuated total reflection spectroscopy. Equilibrium thermodynamic estimations confirmed the observed membrane structure with respect to helix length, strength of hydrophobic membrane association, and orientation (caused by favorably oriented molecular amphiphilic and helix electric dipole moments). The membrane structure may explain why Trp-8 and His-12 are essential for biologic activity. Neuromedin B is predicted to be able to adopt a membrane structure similar to that of bombesin. However, gastrin-releasing peptide and neuromedin C are predicted not to behave in the same manner. The molecular mechanism of receptor subtype selection by bombesin-like peptides may prove to be similar to that observed earlier for opioid peptides and the neurokinins.

Bombesin↗
PubMed1987

Membrane-assisted molecular mechanism of neurokinin receptor subtype selection.

Based on the observed membrane structures of substance P, physalaemin, and eledoisin, preferred conformations, orientations and accumulations of 13 mammalian neurokinins and non-mammalian tachykinins were estimated and compared with pharmacologic and selective binding data taken from the literature. Principal site affinities and relative affinities supported the view that neurokinins bind to three principal mammalian sites: the NK-1 (preferring substance P), the NK-2 (preferring neurokinin A), and the NK-3 site (preferring neurokinin B). Strong hydrophobic membrane interaction of the C-terminal message segment as a perpendicularly oriented alpha-helical domain correlated with NK-1 selection. Electrostatic accumulation of the peptide at the anionic fixed charge layer of the membrane without hydrophobic interaction through a helix correlated with NK-2 preference. Electrostatic repulsion by the anionic fixed charge layer correlated with NK-3 selection. Thus, neurokinin receptor selection is guided by the same principles as opioid receptor selection. Membrane catalysis of specific agonist--receptor interactions may prove to be a quite general phenomenon, and the membrane structure of a peptide more important for its structure--activity relationship than its crystal structure or its mixture of conformers in solution or in vacuo.

Amino Acid Sequence↗
PubMed1986

Molecular mechanism of opioid receptor selection.

Preferred conformations, orientations, and accumulations of 26 opioid peptides on lipid membranes were estimated and compared with pharmacologic and selective binding data taken from the literature. Interaction with mu-receptors was governed by the net positive charge effective at the message domain of the agonist peptides z(eff) as the Boltzmann term ez(eff) that determines relative accumulation on anionic biologic membranes. Selection for delta-receptors was reduced by z(eff) and correlated with e-z(eff). Selection for kappa-receptors was governed by the peptide amphiphilic moment A. A pronounced scalar magnitude A and almost perpendicular orientation of the N-terminal message domain as an alpha-helix were favorable for kappa-site selection. Potencies as kappa-agonists and binding affinities correlated with A X ez(eff). The classical site selectivity caused by the receptor requirements for a complementary fit of the agonist to the discriminator site is thus crucially supplemented by a selection mechanism based on peptide membrane interactions (membrane requirements). In the model presented here, the delta-site is exposed to the aqueous compartment surrounding the target cell at a distance comparable to or greater than the Debye-Hückel length and is in a cationic vicinity. The mu-site is exposed to the anionic fixed-charge compartment of the membrane in aqueous surroundings. The kappa-site is buried in a more hydrophobic membrane compartment close to the fixed-charge compartment. The relative accumulation of the opioid message domains in these compartments is determined by the address domains and constitutes a major part of the site selection mechanism.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗
PubMed1986

Estimated conformation, orientation, and accumulation of dynorphin A-(1-13)-tridecapeptide on the surface of neutral lipid membranes.

Equilibrium thermodynamic and kinetic estimations were used to confirm the rather unusual conformation, orientation, and accumulation of dynorphin A-(1-13)-tridecapeptide (dynorphin1-13) on the surface of neutral lipid membranes, as observed by Erne et al. [Erne, D., Sargent, D. F., & Schwyzer, R. (1985) Biochemistry 24, 4261-4263]. I started from the premise that the most stable conformation of molecularly disperse peptides in contact with the hydrophobic phase of a membrane is helical [Henderson, R. (1979) Soc. Gen. Physiol. Ser. 33, 3-15]. Calculation of the Gibbs free energy difference for the transfer of increasing numbers m of N-terminal residues of dynorphin1-13 from their random-coil conformation in water to their alpha-helical conformation in a hydrophobic phase, with the values provided by Von Heijne and Blomberg [Von Heijne, G., & Blomberg, C. (1979) Eur. J. Biochem. 97, 175-181], showed an energy minimum at m = 9 that corresponded to the observed apparent association constant of 9 X 10(4) L/mol. This confirmed our experimental observations. The orientation of dynorphin1-13 in the interphase was estimated by calculation of the molecular amphiphilic moment A. This force vector was defined in analogy to the "helical" and "structural" hydrophobic moments of Eisenberg et al. [Eisenberg, D., Weiss, R. M., & Terwilliger, T. C. (1982) Nature (London) 299, 371-374]. It takes into account the segregation of hydrophobic and hydrophilic residues with respect to the center of the alpha-helix. A peptide located in a hydrophobic-hydrophilic gradient experiences a torque that tends to orient A in a direction perpendicular to the surfaces of equal hydrophobicity.(ABSTRACT TRUNCATED AT 250 WORDS)

Dynorphins↗
PubMed1986

Membrane lipid phase as catalyst for peptide-receptor interactions.

Catalysis of ligand-receptor interactions is proposed as an important function of the lipid phase of the cell membrane. The catalytic mechanism is deduced from observed specific interactions of amphiphilic peptides with artificial lipid bilayers. In our model a direct ligand-receptor reaction is replaced by multiple sequential steps including surface accumulation of charged ligands, ligand-membrane interactions, and ultimately binding to the receptor itself. By dividing the total free energy of binding among several steps, the energy per step, including the intrinsic receptor interaction energy, is kept to moderate values. The model thereby yields simple explanations for the large apparent association constants, the high association and dissociation rates, and the heterogeneity of binding sites so frequently found with pharmacological and biochemical ligand-receptor interactions. Furthermore, the measured apparent association constant is a function of the whole system rather than just the receptor. The same, fully functional receptor may show different binding characteristics in different surroundings, such as in another tissue or in a reconstituted system.

Adrenocorticotropic Hormone↗
PubMed1985

Preferred conformation, orientation, and accumulation of dynorphin A-(1-13)-tridecapeptide on the surface of neutral lipid membranes.

Infrared attenuated total reflection (IR-ATR) spectroscopy and capacitance minimization (CM) were used to study the secondary structure, orientation, and accumulation of dynorphin A-(1-13)-tridecapeptide (dynorphin1-13) molecules on the surface of planar membranes prepared from 1-palmitoyl-2-oleoyl-sn-glycero-3-phosphocholine. The peptide assumed a helical structure oriented perpendicularly on the membrane surface. Binding from aqueous solutions containing 10 mM KCl saturated reversibly at about a bilayer area of 110 nm2 per peptide molecule, an apparent dissociation constant of 11 microM, and rate constants of 2 X 10(2) s-1 (adsorption) and 2 X 10(-3) s-1 (desorption). The results complement those obtained by vesicle-mediated hydrophobic labeling [Gysin, B., & Schwyzer, R. (1983) Arch. Biochem. Biophys. 225, 467-474]. They indicate that the behavior of this amphiphilic peptide in contact with neutral lipid membranes may be quite different from that in molecularly disperse or micellar solutions of detergents or lysolecithins and that, in the case of dynorphin1-13, primary amphiphilicity overrules secondary amphiphilicity.

Dynorphins↗
PubMed1979

Fibrinogen: agreement of experimental and calculated hydrodynamic data with electron-microscopic models.

There is no general agreement on the size and shape of the fibrinogen molecule. We have studied the diffusion of the fibrinogen in solution by means of dynamic light scattering, nanosecond fluorescence depolarization and analytical ultracentrifugation. The results obtained under physiological concentration, pH an ionic strength are DT = 2.0 X 10(7) cm2sec-1, DR perpendicular = 40'000 sec 1. Nanosecond fluorescence depolarization yielded DR parallel = 1.6 x 10(6) sec-1. Tentatively this value is interpreted as DR parallel, namely rotational diffusion about the major axis of the molecule. The sedimentation coefficient is 8.1 S. The hydrodynamic parameters derived from our measurements were compared with those calculated on the basis of the models proposed by Hall and Slayter, Hudry-Clergeon and Marguerie et al., Bachmann and Lederer, and Köppel. The agreement is poor even if the degree of hydration is varied within wide limits. However, satisfactory agreement can be achieved by assuming a flexible molecule of about 900 A length corresponding to two end-to-end bound trinodular structures of the Hall and Slayter type, with a nodule diameter of about 45 A. Experimental evidence indicates that the discrepancies between the different models might be due to different techniques of sample preparation leading to different conformations of the molecule.

Fibrinogen↗
PubMed1979

Capacitance and conductance as tools for the measurement of asymmetric surface potentials and energy barriers of lipid bilayer membranes.

A simple method for determination of asymmetric surface potentials in lipid bilayers is described. The method is based on the dependence of bilayer capacitance on transmembrane voltage. The capacitance is measured by rectifying in 90 degrees component of an applied alternating current signal. A superimposed slow triangular wave results in a hysteresis-like time course of capacitance. The center of the hysteresis figure is shifted along the voltage axis by an amount equal to the difference of the dipole plus surface-charge potentials on the two sides of the bilayer (capacitance minimization potential). Alternatively, such bilayer asymmetry was studied by using the current-voltage characteristics in the presence of nonactin as a carrier. This analysis was based on the integrated Nernst-Planck equation, assuming a trapezoidal energy barrier and equilibrium of the surface reactions. The two methods gave consistent results for the surface potentials of phosphatidyl serine membranes asymmetrically shielded with calcium. In addition, the current analysis yields the positions of the corners of the barrier, found to be set in 13% for this lipid.

Calcium↗