Solution Structures of Two Different FRP-OCP Complexes as Revealed via SEC-SANS

Photosynthetic organisms have established photoprotective mechanisms in order to dissipate excess light energy into heat, which is commonly known as non-photochemical quenching. Cyanobacteria utilize the orange carotenoid protein (OCP) as a high-light sensor and quencher to regulate the energy flow in the photosynthetic apparatus. Triggered by strong light, OCP undergoes conformational changes to form the active red state (OCPR). In many cyanobacteria, the back conversion of OCP to the dark-adapted state is assisted by the fluorescence recovery protein (FRP). However, the exact molecular events involving OCP and its interaction with FRP remain largely unraveled so far due to their metastability. Here, we use small-angle neutron scattering combined with size exclusion chromatography (SEC-SANS) to unravel the solution structures of FRP-OCP complexes using a compact mutant of OCP lacking the N-terminal extension (∆NTEOCPO) and wild-type FRP. The results are consistent with the simultaneous presence of stable 2:2 and 2:1 FRP-∆NTEOCPO complexes in solution, where the former complex type is observed for the first time. For both complex types, we provide ab initio low-resolution shape reconstructions and compare them to homology models based on available crystal structures. It is likely that both complexes represent intermediate states of the back conversion of OCP to its dark-adapted state in the presence of FRP, which are of transient nature in the photocycle of wild-type OCP. This study demonstrates the large potential of SEC-SANS in revealing the solution structures of protein complexes in polydisperse solutions that would otherwise be averaged, leading to unspecific results.

Recent Progress in Solution Structure Studies of Photosynthetic Proteins Using Small-Angle Scattering Methods

Utilized for gaining structural insights, small-angle neutron and X-ray scattering techniques (SANS and SAXS, respectively) enable an examination of biomolecules, including photosynthetic pigment-protein complexes, in solution at physiological temperatures. These methods can be seen as instrumental bridges between the high-resolution structural information achieved by crystallography or cryo-electron microscopy and functional explorations conducted in a solution state. The review starts with a comprehensive overview about the fundamental principles and applications of SANS and SAXS, with a particular focus on the recent advancements permitting to enhance the efficiency of these techniques in photosynthesis research. Among the recent developments discussed are: (i) the advent of novel modeling tools whereby a direct connection between SANS and SAXS data and high-resolution structures is created; (ii) the employment of selective deuteration, which is utilized to enhance spatial selectivity and contrast matching; (iii) the potential symbioses with molecular dynamics simulations; and (iv) the amalgamations with functional studies that are conducted to unearth structure-function relationships. Finally, reference is made to time-resolved SANS/SAXS experiments, which enable the monitoring of large-scale structural transformations of proteins in a real-time framework.

Stages of OCP-FRP Interactions in the Regulation of Photoprotection in Cyanobacteria, Part 1: Time-Resolved Spectroscopy

Most cyanobacteria utilize a water-soluble Orange Carotenoid Protein (OCP) to protect their light-harvesting complexes from photodamage. The Fluorescence Recovery Protein (FRP) is used to restore photosynthetic activity by inactivating OCP via dynamic OCP-FRP interactions, a multistage process that remains underexplored. In this work, applying time-resolved spectroscopy, we demonstrate that the interaction of FRP with the photoactivated OCP begins early in the photocycle. Interacting with the compact OCP state, FRP completely prevents the possibility of OCP domain separation and formation of the signaling state capable of interacting with the antenna. The structural element that prevents FRP binding and formation of the complex is the short α-helix at the beginning of the N-terminal domain of OCP, which masks the primary site in the C-terminal domain of OCP. We determined the rate of opening of this site and show that it remains exposed long after the relaxation of the red OCP states. Observations of the OCP transitions on the ms time scale revealed that the relaxation of the orange photocycle intermediates is accompanied by an increase in the interaction of the carotenoid keto group with the hydrogen bond donor tyrosine-201. Our data refine the current model of photoinduced OCP transitions and the interaction of its intermediates with FRP.

Stages of OCP-FRP Interactions in the Regulation of Photoprotection in Cyanobacteria, Part 2: Small-Angle Neutron Scattering with Partial Deuteration

We used small-angle neutron scattering partially coupled with size-exclusion chromatography to unravel the solution structures of two variants of the Orange Carotenoid Protein (OCP) lacking the N-terminal extension (OCP-ΔNTE) and its complex formation with the Fluorescence Recovery Protein (FRP). The dark-adapted, orange form OCP-ΔNTE^O is fully photoswitchable and preferentially binds the pigment echinenone. Its complex with FRP consists of a monomeric OCP component, which closely resembles the compact structure expected for the OCP ground state, OCP^O. In contrast, the pink form OCP-ΔNTE^P, preferentially binding the pigment canthaxanthin, is mostly nonswitchable. The pink OCP form appears to occur as a dimer and is characterized by a separation of the N- and C-terminal domains, with the canthaxanthin embedded only into the N-terminal domain. Therefore, OCP-ΔNTE^P can be viewed as a prototypical model system for the active, spectrally red-shifted state of OCP, OCP^R. The dimeric structure of OCP-ΔNTE^P is retained in its complex with FRP. Small-angle neutron scattering using partially deuterated OCP-FRP complexes reveals that FRP undergoes significant structural changes upon complex formation with OCP. The observed structures are assigned to individual intermediates of the OCP photocycle in the presence of FRP.

Light-Induced Conformational Flexibility of the Orange Carotenoid Protein Studied by Quasielastic Neutron Scattering with In Situ Illumination

The orange carotenoid protein plays a vital role in the photoprotection of cyanobacteria and exhibits a significant structural change upon photoactivation. A rarely considered aspect is the importance of internal protein dynamics in facilitating the structural transition to the active state. In this study, we use quasielastic neutron scattering under (in situ) blue light illumination for the first time to directly probe the protein dynamics of the orange carotenoid protein in the dark-adapted and active states. This shows that the localized internal dynamics of amino acid residues is significantly enhanced upon photoactivation. This is attributed to the photoinduced structural changes exposing larger areas of the protein surface to the solvent, thus resulting in a higher degree of motional freedom. However, the flexibility of the W288A mutant assumed to mimic the active state structure is found to be different, thus highlighting the importance of in situ experiments.

"Invisible" Detergents Enable a Reliable Determination of Solution Structures of Native Photosystems by Small-Angle Neutron Scattering

Photosystems I (PSI) and II (PSII) are pigment-protein complexes capable of performing the light-induced charge separation necessary to convert solar energy into a biochemically storable form, an essential step in photosynthesis. Small-angle neutron scattering (SANS) is unique in providing structural information on PSI and PSII in solution under nearly physiological conditions without the need for crystallization or temperature decrease. We show that the reliability of the solution structure critically depends on proper contrast matching of the detergent belt surrounding the protein. Especially, specifically deuterated ("invisible") detergents are shown to be properly matched out in SANS experiments by a direct, quantitative comparison with conventional matching strategies. In contrast, protonated detergents necessarily exhibit incomplete matching so that related SANS results systematically overestimate the size of the membrane protein under study. While the solution structures obtained are close to corresponding high-resolution structures, we show that temperature and solution state lead to individual structural differences compared with high-resolution structures. We attribute these differences to the presence of a manifold of conformational substates accessible by protein dynamics under physiological conditions.

Light-Harvesting Complex II Adopts Different Quaternary Structures in Solution as Observed Using Small-Angle Scattering

The high-resolution crystal structure of the trimeric major light-harvesting complex of photosystem II (LHCII) is often perceived as the basis for understanding its light-harvesting and photoprotective functions. However, the LHCII solution structure and its oligomerization or aggregation state may generally differ from the crystal structure and, moreover, also depend on its functional state. In this regard, small-angle scattering experiments provide the missing link by offering structural information in aqueous solution at physiological temperatures. Herein, we use small-angle scattering to investigate the solution structures of two different preparations of solubilized LHCII employing the nonionic detergents n-octyl-β-d-glucoside (OG) and n-dodecyl-β-D-maltoside (β-DM). The data reveal that the LHCII-OG complex is equivalent to the trimeric crystal structure. Remarkably, however, we observe─for the first time─a stable oligomer composed of three LHCII trimers in the case of the LHCII-β-DM preparation, implying additional pigment-pigment interactions. The latter complex is assumed to mimic trimer-trimer interactions which play an important role in the context of photoprotective nonphotochemical quenching.

Characterisation of a synthetic Archeal membrane reveals a possible new adaptation route to extreme conditions

It has been proposed that adaptation to high temperature involved the synthesis of monolayer-forming ether phospholipids. Recently, a novel membrane architecture was proposed to explain the membrane stability in polyextremophiles unable to synthesize such lipids, in which apolar polyisoprenoids populate the bilayer midplane and modify its physico-chemistry, extending its stability domain. Here, we have studied the effect of the apolar polyisoprenoid squalane on a model membrane analogue using neutron diffraction, SAXS and fluorescence spectroscopy. We show that squalane resides inside the bilayer midplane, extends its stability domain, reduces its permeability to protons but increases that of water, and induces a negative curvature in the membrane, allowing the transition to novel non-lamellar phases. This membrane architecture can be transposed to early membranes and could help explain their emergence and temperature tolerance if life originated near hydrothermal vents. Transposed to the archaeal bilayer, this membrane architecture could explain the tolerance to high temperature in hyperthermophiles which grow at temperatures over 100 °C while having a membrane bilayer. The induction of a negative curvature to the membrane could also facilitate crucial cell functions that require high bending membranes.

Solution Structure of the Detergent-Photosystem II Core Complex Investigated by Small-Angle Scattering Techniques

Albeit achieving the X-ray diffraction structure of dimeric photosystem II core complexes (dPSIIcc) at the atomic resolution, the nature of the detergent belt surrounding dPSIIcc remains ambiguous. Therefore, the solution structure of the whole detergent-protein complex of dPSIIcc of Thermosynechococcus elongatus (T. elongatus) solubilized in n-dodecyl-ß-d-maltoside (ßDM) was investigated by a combination of small-angle X-ray scattering (SAXS) and small-angle neutron scattering (SANS) with contrast variation. First, the structure of dPSIIcc was studied separately in SANS experiments using a contrast of 5% D₂O. Guinier analysis reveals that the dPSIIcc solution is virtually free of aggregation in the studied concentration range of 2-10 mg/mL dPSIIcc, and characterized by a radius of gyration of 62 Å. A structure reconstitution shows that dPSIIcc in buffer solution widely retains the crystal structure reported by X-ray free electron laser studies at room temperature with a slight expansion of the entire protein. Additional SANS experiments on dPSIIcc samples in a buffer solution containing 75% D₂O provide information about the size and shape of the whole detergent-dPSIIcc. The maximum position of P(r) function increases to 68 Å, i.e., it is about 6 Å larger than that of dPSIIcc only, thus indicating the presence of an additional structure. Thus, it can be concluded that dPSIIcc is surrounded by a monomolecular belt of detergent molecules under appropriate solubilization conditions. The homogeneity of the ßDM-dPSIIcc solutions was also verified using dynamic light scattering. Complementary SAXS experiments indicate the presence of unbound detergent micelles by a separate peak consistent with a spherical shape possessing a radius of about 40 Å. The latter structure also contributes to the SANS data but rather broadens the SANS curve artificially. Without the simultaneous inspection of SANS and SAXS data, this effect may lead to an apparent underestimation of the size of the PS II-detergent complex. The formation of larger unbound detergent aggregates in solution prior to crystallization may have a significant effect on the crystal formation or quality of the ßDM-dPSIIcc.

Current limits of structural biology: The transient interaction between cytochrome c 6 and photosystem I

Trimeric photosystem I from the cyanobacterium Thermosynechococcus elongatus (TePSI) is an intrinsic membrane protein, which converts solar energy into electrical energy by oxidizing the soluble redox mediator cytochrome c 6 (Cyt c 6 ) and reducing ferredoxin. Here, we use cryo-electron microscopy and small angle neutron scattering (SANS) to characterize the transient binding of Cyt c 6 to TePSI. The structure of TePSI cross-linked to Cyt c 6 was solved at a resolution of 2.9 Å and shows additional cofactors as well as side chain density for 84% of the peptide chain of subunit PsaK, revealing a hydrophobic, membrane intrinsic loop that enables binding of associated proteins. Due to the poor binding specificity, Cyt c 6 could not be localized with certainty in our cryo-EM analysis. SANS measurements confirm that Cyt c 6 does not bind to TePSI at protein concentrations comparable to those for cross-linking. However, SANS data indicate a complex formation between TePSI and the non-native mitochondrial cytochrome from horse heart (Cyt c HH ). Our study pinpoints the difficulty of identifying very small binding partners (less than 5% of the overall size) in EM structures when binding affinities are poor. We relate our results to well resolved co-structures with known binding affinities and recommend confirmatory methods for complexes with K M values higher than 20 μM.

Solution Structure and Conformational Flexibility in the Active State of the Orange Carotenoid Protein. Part II: Quasielastic Neutron Scattering

Orange carotenoid proteins (OCPs), which are protecting cyanobacterial light-harvesting antennae from photodamage, undergo a pronounced structural change upon light absorption. In addition, the active state is anticipated to boost a significantly higher molecular flexibility similar to a "molten globule" state. Here, we used quasielastic neutron scattering to directly characterize the vibrational and conformational molecular dynamics of OCP in its ground and active states, respectively, on the picosecond time scale. At a temperature of 100 K, we observe mainly (vibronic) inelastic features with peak energies at 5 and 6 meV (40 and 48 cm^-1, respectively). At physiological temperatures, however, two (Lorentzian) quasielastic components represent localized protein motions, that is, stochastic structural fluctuations of protein side chains between various conformational substates of the protein. Global diffusion of OCP is not observed on the given time scale. The slower Lorentzian component is affected by illumination and can be well-characterized by a jump-diffusion model. While the jump diffusion constant D is (2.82 ± 0.01) × 10^-5 cm²/s at 300 K in the ground state, it is increased by ∼20% to (3.48 ± 0.01) × 10^-5 cm²/s in the active state, revealing a strong enhancement of molecular mobility. The increased mobility is also reflected in the average atomic mean square displacement ⟨u²⟩; we determine a ⟨u²⟩ of 1.47 ± 0.05 Å in the ground state, but 1.86 ± 0.05 Å in the active state (at 300 K). This effect is assigned to two factors: (i) the elongated structure of the active state with two widely separated protein domains is characterized by a larger number of surface residues with a concomitantly higher degree of motional freedom and (ii) a larger number of hydration water molecules bound at the surface of the protein. We thus conclude that the active state of the orange carotenoid protein displays an enhanced conformational dynamics. The higher degree of flexibility may provide additional channels for nonradiative decay so that harmful excess energy can be more efficiently converted to heat.

The first study on the impact of osmolytes in whole cells of high temperature-adapted microorganisms

The hyperthermophilic piezophile, Thermococcus barophilus displays a strong stress response characterized by the accumulation of the organic osmolyte, mannosylglycerate during growth under sub-optimal pressure conditions (0.1 MPa). Taking advantage of this known effect, the impact of osmolytes in piezophiles in an otherwise identical cellular context was investigated, by comparing T. barophilus cells grown under low or optimal pressures (40 MPa). Using neutron scattering techniques, we studied the molecular dynamics of live cells of T. barophilus at different pressures and temperatures. We show that in the presence of osmolytes, cells present a higher diffusion coefficient of hydration water and an increase of bulk water motions at a high temperature. In the absence of osmolytes, the T. barophilus cellular dynamics is more responsive to high temperature and high hydrostatic pressure. These results therefore give clear evidence for a protecting effect of osmolytes on proteins.

Dynamics of a family of cyan fluorescent proteins probed by incoherent neutron scattering

Cyan fluorescent proteins (CFPs) are variants of green fluorescent proteins in which the central tyrosine of the chromophore has been replaced by a tryptophan. The increased bulk of the chromophore within a compact protein and the change in the positioning of atoms capable of hydrogen bonding have made it difficult to optimize their fluorescence properties, which took approximately 15 years between the availability of the first useable CFP, enhanced cyan fluorescent protein (ECFP), and that of a variant with almost perfect fluorescence efficiency, mTurquoise2. To understand the molecular bases of the progressive improvement in between these two CFPs, we have studied by incoherent neutron scattering the dynamics of five different variants exhibiting progressively increased fluorescence efficiency along the evolution pathway. Our results correlate well with the analysis of the previously determined X-ray crystallographic structures, which show an increase in flexibility between ECFP and the second variant, Cerulean, which is then hindered in the three later variants, SCFP3A (Super Cyan Fluorescent Protein 3A), mTurquoise and mTurquoise2. This confirms that increasing the rigidity of the direct environment of the fluorescent chromophore is not the sole parameter leading to brighter fluorescent proteins and that increased flexibility in some cases may be helpful.

Picosecond Dynamical Response to a Pressure-Induced Break of the Tertiary Structure Hydrogen Bonds in a Membrane Chromoprotein

We used elastic incoherent neutron scattering (EINS) to find out if structural changes accompanying local hydrogen bond rupture are also reflected in global dynamical response of the protein complex. Chromatophore membranes from LH2-only strains of the photosynthetic bacterium Rhodobacter sphaeroides, with spheroidenone or neurosporene as the major carotenoids, were subjected to high hydrostatic pressure at ambient temperature. Optical spectroscopy conducted at high pressure confirmed rupture of tertiary structure hydrogen bonds. In parallel, we used EINS to follow average motions of the hydrogen atoms in LH2, which reflect the flexibility of this complex. A decrease of the average atomic mean square displacements of hydrogen atoms was observed up to a pressure of 5 kbar in both carotenoid samples due to general stiffening of protein structures, while at higher pressures a slight increase of the displacements was detected in the neurosporene mutant LH2 sample only. These data show a correlation between the local pressure-induced breakage of H-bonds, observed in optical spectra, with the altered protein dynamics monitored by EINS. The slightly higher compressibility of the neurosporene mutant sample shows that even subtle alterations of carotenoids are manifested on a larger scale and emphasize a close connection between the local structure and global dynamics of this membrane protein complex.

The Effect of Crowding on Protein Stability, Rigidity, and High Pressure Sensitivity in Whole Cells

In live cells, high concentrations up to 300-400 mg/mL, as in Eschericia coli (Ellis, R. J. Curr. Opin. Struct. Biol. 2001, 11, 114) are achieved which have effects on their proper functioning. However, in many experiments only individual parts of the cells as proteins or membranes are studied in order to get insight into these specific components and to avoid the high complexity of whole cells, neglecting by the way the influence of crowding. In the present study, we investigated cells of the order of Thermococcales, which are known to live under extreme conditions, in their intact form and after cell lysis to extract the effect of crowding on the molecular dynamics of the proteome and of water molecules. We found that some parameters characterizing the dynamics within the cells seem to be intrinsic to the cell type, as flexibility typical for the proteome, others are more specific to the cellular environment, as bulk water's residence time and some fractions of particles participating to the different motions, which make the lysed cells' dynamics similar to the one of another Thermococcale adapted to live under high hydrostatic pressure. In contrast to studies on the impact of crowding on pure proteins we show here that the release of crowding constraints on proteins leads to an increase in the rigidity and a decrease in the high pressure sensitivity. In a way similar to high pressure adaptation in piezophiles, the hydration water layer is decreased for the lysed cells, demonstrating a first link between protein adaptation and the impact of crowding or osmolytes on proteins.

High Hydrostatic Pressure Induces a Lipid Phase Transition and Molecular Rearrangements in Low-Density Lipoprotein Nanoparticles

Low-density lipoproteins (LDL) are natural lipid transporter in human plasma whose chemically modified forms contribute to the progression of atherosclerosis and cardiovascular diseases accounting for a vast majority of deaths in westernized civilizations. For the development of new treatment strategies, it is important to have a detailed picture of LDL nanoparticles on a molecular basis. Through the combination of X-ray and neutron small-angle scattering (SAS) techniques with high hydrostatic pressure (HHP) this study describes structural features of normolipidemic, triglyceride-rich and oxidized forms of LDL. Due to the different scattering contrasts for X-rays and neutrons, information on the effects of HHP on the internal structure determined by lipid rearrangements and changes in particle shape becomes accessible. Independent pressure and temperature variations provoke a phase transition in the lipid core domain. With increasing pressure an inter-related anisotropic deformation and flattening of the particle are induced. All LDL nanoparticles maintain their structural integrity even at 3000 bar and show a reversible response toward pressure variations. The present work depicts the complementarity of pressure and temperature as independent thermodynamic parameters and introduces HHP as a tool to study molecular assembling and interaction processes in distinct lipoprotein particles in a nondestructive manner.

Influence of cosolvents, self-crowding, temperature and pressure on the sub-nanosecond dynamics and folding stability of lysozyme

We studied the effects of temperature and hydrostatic pressure on the dynamical properties and folding stability of highly concentrated lysozyme solutions in the absence and presence of the osmolytes trimethylamine-N-oxide (TMAO) and urea. Elastic incoherent neutron scattering (EINS) was applied to determine the mean-squared displacement (MSD) of the protein's hydrogen atoms to yield insights into the effects of these cosolvents on the averaged sub-nanosecond dynamics in the pressure range from ambient up to 4000 bar. To evaluate the additional effect of self-crowding, two protein concentrations (80 and 160 mg mL^-1) were used. We observed a distinct effect of TMAO on the internal hydrogen dynamics, namely a reduced mobility. Urea, on the other hand, revealed no marked effect and consequently, no counteracting effect in an urea-TMAO mixture was observed. Different from the less concentrated protein solution, no significant effect of pressure on the MSD was observed for 160 mg mL^-1 lysozyme. The EINS experiments were complemented by Fourier-transform infrared (FTIR) spectroscopy measurements, which led to additional insights into the folding stability of lysozyme under the various environmental conditions. We observed a stabilization of the protein in the presence of the compatible osmolyte TMAO and a destabilization in the presence of urea against temperature and pressure for both protein concentrations. Additionally, we noticed a slight destabilizing effect upon self-crowding at very high protein concentration (160 mg mL^-1), which is attributable to transient destabilizing intermolecular interactions. Furthermore, a pressure-temperature diagram could be obtained for lysozyme at these high protein concentrations that mimics densely packed intracellular conditions.

Excitation energy transfer in phycobiliproteins of the cyanobacterium Acaryochloris marina investigated by spectral hole burning

The cyanobacterium Acaryochloris marina developed two types of antenna complexes, which contain chlorophyll-d (Chl d) and phycocyanobilin (PCB) as light-harvesting pigment molecules, respectively. The latter membrane-extrinsic complexes are denoted as phycobiliproteins (PBPs). Spectral hole burning was employed to study excitation energy transfer and electron-phonon coupling in PBPs. The data reveal a rich spectral substructure with a total of four low-energy electronic states whose absorption bands peak at 633, 644, 654, and at about 673 nm. The electronic states at ~633 and 644 nm can be tentatively attributed to phycocyanin (PC) and allophycocyanin (APC), respectively. The remaining low-energy electronic states including the terminal emitter at 673 nm may be associated with different isoforms of PC, APC, or the linker protein. Furthermore, the hole burning data reveal a large number of excited state vibrational frequencies, which are characteristic for the chromophore PCB. In summary, the results are in good agreement with the low-energy level structure of PBPs and electron-phonon coupling parameters reported by Gryliuk et al. (BBA 1837:1490-1499, 2014) based on difference fluorescence line-narrowing experiments.

Solution structure of monomeric and trimeric photosystem I of Thermosynechococcus elongatus investigated by small-angle X-ray scattering

The structure of monomeric and trimeric photosystem I (PS I) of Thermosynechococcus elongatus BP1 (T. elongatus) was investigated by small-angle X-ray scattering (SAXS). The scattering data reveal that the protein-detergent complexes possess radii of gyration of 58 and 78 Å in the cases of monomeric and trimeric PS I, respectively. The results also show that the samples are monodisperse, virtually free of aggregation, and contain empty detergent micelles. The shape of the protein-detergent complexes can be well approximated by elliptical cylinders with a height of 78 Å. Monomeric PS I in buffer solution exhibits minor and major radii of the elliptical cylinder of about 50 and 85 Å, respectively. In the case of trimeric PS I, both radii are equal to about 110 Å. The latter model can be shown to accommodate three elliptical cylinders equal to those describing monomeric PS I. A structure reconstitution also reveals that the protein-detergent complexes are larger than their respective crystal structures. The reconstituted structures are larger by about 20 Å mainly in the region of the hydrophobic surfaces of the monomeric and trimeric PS I complexes. This seeming contradiction can be resolved by the addition of a detergent belt constituted by a monolayer of dodecyl-β-D-maltoside molecules. Assuming a closest possible packing, a number of roughly 1024 and 1472 detergent molecules can be determined for monomeric and trimeric PS I, respectively. Taking the monolayer of detergent molecules into account, the solution structure can be almost perfectly modeled by the crystal structures of monomeric and trimeric PS I.

High hydrostatic pressure specifically affects molecular dynamics and shape of low-density lipoprotein particles

Lipid composition of human low-density lipoprotein (LDL) and its physicochemical characteristics are relevant for proper functioning of lipid transport in the blood circulation. To explore dynamical and structural features of LDL particles with either a normal or a triglyceride-rich lipid composition we combined coherent and incoherent neutron scattering methods. The investigations were carried out under high hydrostatic pressure (HHP), which is a versatile tool to study the physicochemical behavior of biomolecules in solution at a molecular level. Within both neutron techniques we applied HHP to probe the shape and degree of freedom of the possible motions (within the time windows of 15 and 100 ps) and consequently the flexibility of LDL particles. We found that HHP does not change the types of motion in LDL, but influences the portion of motions participating. Contrary to our assumption that lipoprotein particles, like membranes, are highly sensitive to pressure we determined that LDL copes surprisingly well with high pressure conditions, although the lipid composition, particularly the triglyceride content of the particles, impacts the molecular dynamics and shape arrangement of LDL under pressure.

Protein dynamics tunes excited state positions in light-harvesting complex II

Light harvesting and excitation energy transfer in photosynthesis are relatively well understood at cryogenic temperatures up to ∼100 K, where crystal structures of several photosynthetic complexes including the major antenna complex of green plants (LHC II) are available at nearly atomic resolution. The situation is much more complex at higher or even physiological temperatures, because the spectroscopic properties of antenna complexes typically undergo drastic changes above ∼100 K. We have addressed this problem using a combination of quasielastic neutron scattering (QENS) and optical spectroscopy on native LHC II and mutant samples lacking the Chl 2/Chl a 612 pigment molecule. Absorption difference spectra of the Chl 2/Chl a 612 mutant of LHC II reveal pronounced changes of spectral position and their widths above temperatures as low as ∼80 K. The complementary QENS data indicate an onset of conformational protein motions at about the same temperature. This finding suggests that excited state positions in LHC II are affected by protein dynamics on the picosecond time scale. In more detail, this means that at cryogenic temperatures the antenna complex is trapped in certain protein conformations. At higher temperature, however, a variety of conformational substates with different spectral position may be thermally accessible. At the same time, an analysis of the widths of the absorption difference spectra of Chl 2/Chl a 612 reveals three different reorganization energies or Huang-Rhys factors in different temperature ranges, respectively. These findings imply that (dynamic) pigment-protein interactions fine-tune electronic energy levels and electron-phonon coupling of LHC II for efficient excitation energy transfer at physiological temperatures.

Color correction in planar optics configurations

Color correction in planar optics configurations can be achieved by resorting to gradient-index rather than uniform-refractive-index substrates. The basic configuration, principle of correction, and calculated and experimental results are presented. The results reveal that, with an appropriate refractive index distribution along the thickness of the substrates, the color can be corrected over a wavelength range up to 155 nm depending on incidence angles.

Stimulation of intestinal mucosal afferent nerves increases superior mesenteric artery and decreases mesenteric adipose tissue blood flow

We tested the hypothesis that stimulation of intestinal mucosal afferent nerves produces an increase in superior mesenteric artery (SMA) but a decrease in mesenteric adipose tissue (MAT) blood flow. In anesthetized rats, blood flow in the SMA (pulsed Doppler flowmetry) and MAT (hydrogen gas clearance) was measured simultaneously before and after administration of 0.9% saline, 640 microM capsaicin, or 5% dextrose into the intestinal lumen. The changes in the SMA were 3.8 +/- 3.0, 15.9 +/- 4.0, and 18.8 +/- 7.6%; and those in the MAT, 4.7 +/- 4.0, -11.5 +/- 3.4, and -0.07 +/- 3.4% of baseline, respectively. The data indicate that exposure of the intestinal lumen to an afferent nerve stimulant or nutrient induced a dichotomous pattern of blood flow changes, an increase in the SMA and a reduction in MAT. The capsaicin-sensitive afferent nerves may be instrumental in mediating these energy responses.

Tonic inhibition of renin secretion by the 12 lipoxygenase pathway: augmentation by high salt intake

Recent evidence suggests that lipoxygenase (LO) metabolites inhibit renin production in vitro. However, the physiological significance of this effect has not been determined. This study examined the role of the LO pathway in the regulation of plasma renin concentration (PRC) in vivo. The acute administration of two structurally unrelated LO inhibitors, phenidone (30 and 60 mg/kg) and esculetin (60 mg/kg), resulted in suppression of platelet 12 hydroxyeicosatetraenoic acid (12HETE) production, reduction in systemic arterial pressure and a 2- to 3-fold increase in PRC. To determine whether the esculetin-induced increase in PRC was secondary to hypotension, esculetin was also administered to rats preinfused with a pressor dose of norepinephrine. In these acutely hypertensive rats, esculetin still induced a 2.5-fold increase in PRC, whereas blood pressure remained over 40 mm Hg above basal levels. Further, esculetin (10(-6)M) increased renin release in renal slices from 150 +/- 10 to 310 +/- 20 ng/ml.h (P < 0.05) and this rise was entirely blocked in the presence of 12HETE (10(-7)M; 130 +/- 40 ng/ml.h). In rats placed on high salt intake, 12HETE concentration in renal slices from the outer cortex was considerably higher than in renal slices from salt-restricted rats (116.5 +/- 15.7 vs. 65 +/- 12 pg/mg protein; P < 0.05). Chronic administration of the LO inhibitor phenidone also resulted in an increase of PRC, which was independent of changes in blood pressure. On either high salt (3.15%0 or low salt (0.05%) diet phenidone-treated rats had higher PRC levels than the respective control groups [high salt 9.7 +/- 3.5 vs. 1.9 +/- 1.4 ng/ml.h; P < 0.05; low salt 33.2 +/- 5.3 vs. 19.4 +/- 3.10 ng/ml.h; P < 0.05]. The finding that LO blockers are potent stimulators of PRC in vivo suggests the existence of a physiological tonic inhibition of renin secretion by LO products that is operative under a wide range of salt intake. High salt intake enhances this inhibitory tone by increasing renal cortical 12 LO activity and, in fact, normal suppression of PRC during high salt diet does not occur in LO-blocked animals. Thus, the LO pathway exerts a tonic inhibitory effect on renin release, which appears particularly important for renin suppression during high salt intake.

The lipoxygenase inhibitor phenidone is a potent hypotensive agent in the spontaneously hypertensive rat

Previous studies from our laboratory indicated that the lipoxygenase inhibitor phenidone markedly attenuates angiotensin II (AII) induced vascular contractility. Phenidone was also shown to inhibit the formation of vascular lipoxygenase products and to reduce blood pressure in the AII-dependent renovascular hypertensive rat. We have now examined the effects of phenidone in the spontaneously hypertensive rat (SHR). A single dose of phenidone lowered intraarterial systolic pressure in a dose dependent manner in both SHR and Wistar-Kyoto (WKY) [(max 74 +/- 15 and 22 +/- 3 mm Hg, respectively; P < .001)], but the effect was substantially greater in SHR. Long-term oral phenidone administration arrested the evolution of hypertension in 6 week old SHR treated over a period of 4 weeks (control 190 +/- 2 mm Hg; phenidone treated rats 164 +/- 4 mm Hg; P < .01). To assess the role of AII related mechanisms in the hypotensive effect of phenidone, the acute effect was studied in SHR on high and low sodium intake. In addition, the effect of captopril was compared to that of phenidone alone or captopril and phenidone in salt restricted SHR. While a single dose of phenidone (30 mg/kg, intraperitoneally) elicited similar maximal effects in SHR on high and low sodium intake (54 +/- 6 and 52 +/- 5 mm Hg compared to basal blood pressure, respectively), the hypotensive effect in sodium restricted rats was more sustained. Phenidone had no further hypotensive effect in captopril treated, salt restricted SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

12-Lipoxygenase products modulate calcium signals in vascular smooth muscle cells

Previous studies have shown that inhibition of the lipoxygenase pathway of arachidonic acid metabolism can prevent the development of elevated blood pressure in renin-dependent models of hypertension. Agents that inhibit the lipoxygenase pathway such as phenidone and the flavonoid baicalein can selectively attenuate contractile responses to angiotensin II in vivo as well as in isolated vascular tissue. In the present study, the effects of lipoxygenase inhibitors on pressor-induced changes in cytosolic calcium were examined in cultured rat vascular smooth muscle cells using the fluorescent dye fura-2. Two structurally unrelated lipoxygenase inhibitors, baicalein and 5,8,11-eicosatriynoic acid, attenuated angiotensin II-stimulated increases in cytosolic calcium in both normal and calcium-poor buffer. The addition of 5-, 12-, or 15(S)-hydroxyeicosatetraenoic acid alone to the cells had no acute effect on intracellular calcium concentration. However, the addition of 12(S)-hydroxyeicosatetraenoic acid but not 5- or 15(S)-hydroxyeicosatetraenoic acid restored the initial calcium response to angiotensin II in vascular smooth muscle cells pretreated with both inhibitors; 5,8,11-eicosatriynoic acid also reduced [Arg8]-vasopressin and endothelin-stimulated increases in intracellular calcium. The attenuation of vasopressor-induced calcium transients by agents that inhibit lipoxygenase may explain their observed hypotensive effects in vivo. Moreover, lipoxygenase products, in particular 12(S)-hydroxyeicosatetraenoic acid, may act as mediators for the intracellular actions of angiotensin II and possibly other pressor hormones in vascular tissue by regulation of intracellular calcium metabolism.

Inhibition of lipoxygenase pathway reduces blood pressure in renovascular hypertensive rats

To assess the potential role of the lipoxygenase (LO) pathway in the vasculature in an angiotensin II (ANG II)-dependent model of hypertension, we investigated the effect of LO pathway inhibition on blood pressure in the two-kidney, one-clip (2K,1C) Goldblatt hypertensive rat. The development of renovascular hypertension in 2K,1C rats was attenuated by oral administration of phenidone (Phe, 60 mg.kg-1.day-1), a nonselective LO inhibitor, throughout the 3 wk of observation after renal artery constriction. In contrast, the same treatment protocol had no effect on the evolution of hypertension in the deoxycorticosterone acetate-salt rat, which is considered to be an ANG II-independent form of hypertension. The hypotensive effect of Phe was not associated with changes in plasma renin or aldosterone concentration (PRC and PAC, respectively). In vitro synthesis of 12-hydroxyeicosatetraenoic acid (12-HETE) by aortic segments was increased in 2K,1C hypertensive rats compared with sham-operated rats. In addition, the synthesis of 12-HETE was suppressed by the in vitro addition of Phe (10(-4) M) to aortic-segment incubates obtained from 2K,1C rats and sham-operated rats. Acute administration of Phe (30 or 60 mg/kg) in 2K,1C hypertensive rats produced a rapid and sustained decrease in mean blood pressure (MBP). This decrease in MBP was accompanied by a brisk rise in PRC and PAC. In contrast, bolus administration of indomethacin, a selective cyclooxygenase inhibitor, did not affect MBP, PRC, or PAC.(ABSTRACT TRUNCATED AT 250 WORDS)

Hypotensive effects of the lipoxygenase inhibitor phenidone in two-kidney, one clip Goldblatt hypertension

This study examined the role of the lipoxygenase (LO) pathway in the maintenance of hypertension in rats with two-kidney, one clip (2K,1C) Goldblatt hypertension. A single dose of the lipoxygenase blocker phenidone was injected intraperitoneally to 2K,1C rats during the early phase (14 days) of the development of hypertension (mean intraarterial blood pressure 137 +/- 3.9 mm Hg). Phenidone (60 mg/kg) markedly decreased arterial pressure to nadir levels of 58.9% of resting blood pressure. The maximal changes were observed 15 min after injection and the hypotensive response was sustained for at least 2 h. Plasma renin concentration (PRC) increased from 74.3 +/- 18.9 to 281.0 +/- 6.5 ng/mL/h after injection (P less than .05). Thus, the hypotensive effect of phenidone was not due to suppression of renin secretion but presumably due to inhibition of its effects. It is suggested that arachidonate metabolites of the LO pathway at the vascular bed may be involved in maintenance of high arterial pressure in 2K,1C renovascular hypertension in rat.

Selective inhibition of angiotensin II-mediated vasoconstriction by lipoxygenase blockade

We have previously demonstrated that the lipoxygenase (LO) pathway has a specific role in the effect of angiotensin II (ANG II) on aldosterone secretion. To elucidate whether the LO pathway also participates in the vascular effects of ANG II, the nonselective LO inhibitor phenidone (PHE; 30 mg/kg) was administered to rats 1 h before graded dose ANG II infusion. PHE reduced the LO product 12-hydroxyeicosatetraenoic acid (12-HETE) in deendothelialized aortas by an average of 36% as determined by radiometric detection with high-performance liquid chromatography and radioimmunoassay methods. In parallel, the peak systolic pressor response to ANG II was lowered from 36.2 +/- 3.7 to 16.8 +/- 2.0 mmHg. The peak pressor responses to ANG II were also reduced by two other LO inhibitors, baicalein (30 mg/kg) and esculetin (60 mg/kg) (13.9 +/- 2.4 and 22.1 +/- 4.7 mmHg, respectively; P less than 0.01 compared with control rats for both), but not by the cyclooxygenase inhibitor indomethacin. The LO inhibitors baicalein (7.5 X 10(-5) M) and PHE (10(-4) M) markedly attenuated the in vitro contractile response to ANG II of femoral artery rings. In contrast, neither the in vivo nor in vitro constrictor responses to norepinephrine were affected by baicalein. Thus lipoxygenase blockade induces a direct and selective inhibition of ANG II-induced vasoconstriction. The LO pathway may have an important role in mediating the pressor effect of ANG II.

Health education, public policy and disease prevention: a case history of the New York City Coalition to End Lead Poisoning

How can health educators influence public policy so as to increase the resources for disease prevention? The following case history of the New York City Coalition to End Lead Poisoning (NYCCELP) describes how an advocacy group sought to educate parents, health workers and policy makers about a major urban health problem. Using health education techniques such as small group meetings, mass media coverage and community organizing, NYCCELP hoped to persuade city agencies to enforce more systematically existing laws for the prevention of lead poisoning and to allocate more resources for screening and education. By defining the problem of lead poisoning broadly, the Coalition was able to attract diverse constituencies including housing organizations, public health advocacy groups, public interest lawyers, elected officials and children's rights groups. In part as a result of NYCCELP's efforts, the city government has screened more children for lead poisoning, hired additional health educators, enforced relevant sections of the housing code more often, and allocated new resources for lead poisoning control.

Effect of high calcium intake on pressor responsivity in hypertensive rats

Some proposed mechanisms for the hypotensive effect of high calcium intake involve reduction in vascular responsivity. To assess the effect of dietary calcium on vascular responsivity, spontaneously hypertensive rats (SHR) were placed on normal (N-Ca; 0.4%) or high (H-Ca; 2.8%) casein-based synthetic diet for 4 wk. Intraarterial pressure, pressor response to graded intravenous infusion of norepinephrine (NE) and angiotensin II (ANG II), and in vitro vascular reactivity of tail artery segments to NE and transmural nerve stimulation (TNS) were studied. Urinary electrolyte excretion, plasma renin activity (PRA), aldosterone, NE, and epinephrine (EPI) were also determined. H-Ca SHR had a lower intraarterial systolic and diastolic pressure. However, H-Ca SHR had greater in vivo pressor response to both ANG II and NE. Maximal contractile force developed by tail artery segments in vitro in response to NE and TNS was slightly, but not significantly, higher in H-Ca SHR. In vitro dose-response curves to NE and TNS were not significantly different. Although H-Ca SHR had increased urinary excretion of sodium throughout the study period, PRA and aldosterone levels were similar in both groups. Plasma NE and EPI levels in the two groups were also not different. Despite lowered intra-arterial blood pressure, H-Ca SHR exhibited enhanced pressor response to ANG II and NE in vivo and a similar in vitro vascular reactivity to NE and TNS when compared with N-Ca SHR. Our results do not support a role for alterations in vascular reactivity to NE or ANG II in the hypotensive effect of high calcium intake in SHR.(ABSTRACT TRUNCATED AT 250 WORDS)

Cyclooxygenase products and cyclic nucleotide levels with infusion of arachidonic acid, PGI2, PGE2, PGF2, and 6-keto-PGF1 in an isolated dog lung

Arachidonic acid (AA) was infused into the pulmonary artery of an isolated dog lung perfused with a physiologic salt solution. This led to elevations in pulmonary cyclic AMP and prostaglandins (PGs) including PGE2, PGF2 alpha, TXB2 (a metabolite of TXA2), and 6-keto-PGF1 alpha (a metabolite of PGI2). The elevations were prevented by PG synthesis inhibitors. A dose of PGI2 comparable to that produced from AA led to elevations in cyclic AMP. These elevations were not reduced by PG synthesis inhibitors; this indicated that the inhibitors did not reduce cyclic AMP except by inhibiting metabolism of AA. The PGE2 led to lesser elevations in cyclic AMP than did PGI2; PGF2 alpha and 6-keto-PGF1 alpha did not increase cyclic AMP. Levels of cyclic AMP were not elevated. We conclude that some of the elevation in cyclic AMP from AA was most likely from production of PGs since elevations in both were prevented by the inhibitors. However, the possibility remains that AA metabolites other than PGs also contributed to elevations in cyclic AMP. We also conclude that PGI2 most likely accounted for some of the cyclic AMP elevation from AA since PGI2 could be readily produced in amounts that elevate cyclic AMP. However, the possibility remains that PGE2, the less consistent cyclic AMP stimulators (l.e., PGF2 alpha and 6-keto-PGF1 alpha), TXA2 or TXB2, or PGs not measured in this study also contributed to the elevations in cyclic AMP from AA.

Dopaminergic modulation of meal-stimulated and circadian secretion of pancreatic polypeptide in man

This study investigated dopaminergic control of human pancreatic polypeptide (hPP) secretion in normal male volunteers. Dopamine infusion blunted the hPP response to a protein-rich meal. Dopamine antagonism with metoclopramide resulted in a hPP response at 5 min and a peak elevation of hPP 10 min after drug administration. Bromocriptine (2.5 mg, three times daily for 5 days) suppressed meal-induced secretory responses of hPP. Although bromocriptine did not alter the basic circadian pattern of hPP secretion, it did slightly increase nocturnal levels of this hormone. These results suggest that dopaminergic mechanisms exert a tonic inhibitory effect on hPP secretion in normal subjects.

Role of prolactin and the renin-angiotensin system in mediating dopaminergic control of aldosterone secretion in the rat

Blockade of dopaminergic pathways increases aldosterone levels by mechanisms that are not well delineated. Since both prolactin (PRL) and plasma renin activity (PRA) also increase after administration of dopaminergic antagonists, the aldosterone increments may be secondary to these changes. To address these questions, the relationship between plasma aldosterone (PA) and PRL responses to 2 different dopamine receptor antagonists, haloperidol and metoclopramide (MCP) was examined in rats. The PA response to MCP was compared before and after blockade of the renin-angiotensin system with saralasin and after pre-administration of L-dopa. MCP administration produced significant and parallel increments in PA and PRL whereas haloperidol increased PRL without any change in PA or PRA. L-dopa pre-treatment suppressed the early PA response to MCP. Hypophysectomy prior to MCP administration eliminated the PRL response but did not significantly alter the PA response to MCP. Our findings suggest that dopamine has an inhibitory action on the adrenal gland production of aldosterone acting independently of changes in PRL and the renin-angiotensin system.

Induction of renin release by exogenous prostaglandins in hyporeninemic hypoaldosteronism

A deficiency in renal prostaglandin synthesis has been proposed as the cause of the syndrome of hyporeninemic hypoaldosteronism. To determine if renin release could be stimulated by pharmacologic infusions of PGA1, we infused PGA1 0.075 to 0.60 microgram/kg/min to nine patients with the syndrome. Total renal PGE production as measured by urinary PGE excretion was normal (650 +/- 169 vs 400 +/- 55 ng/24hr in normal subjects). Renin (PRA) was markedly depressed in all patients despite stimulation with upright posture and furosemide (1.0 +/- 0.4 vs 9.3 +/- 0.7 ng/ml/hr, p < 0.001). But in two patients PGA1, induced an increase in renin similar to that of normal subjects. PRA increased to a lesser degree in two other patients and plasma aldosterone slightly increased. Five showed no response. Infusions of nitroprusside in doses and duration that mimicked the hypotensive effects of PGA1 failed to increase PRA or aldosterone. The data suggest that total renal PGE production is normal in patients with the syndrome of hyporeninemic hypoaldosteronism. Although orthostasis, furosemide and nitroprusside do not increase renin, prostaglandin A1 infusion appears to be a potent stimulus to renin release in some of the patients.

[Clinical, radiological and bacteriological study of pulmonary tuberculosis with bacteria showing drug resistance]

The authors have investigated two groups of patients of young age suffering from tuberculosis with germs showing primary resistance to chemotherapeutic agents (61 cases), and with germs displaying sensitivity to these agents (64 cases). The following conclusions have been reached: tuberculosis with germs showing primary drug resistance did not display onset modalities, clinically and radiologically, that differed significantly from other types of tuberculosis; sputum conversion is more slow in patients with resistant germs than in those with sensitive germs in the first two months of treatment, but following application of the treatment according to the data resulting from the antibiogram, this differences quickly disappeared. The presence of a smaller number of complete recoveries and of a surplus of doubtful recoveries can be explained in the same way in this group of patients: the final results are similar in both groups of patients, evidencing that the treatment of pulmonary tuberculosis due to germs showing parimary drug resistance does not give rise to particular problems.

Role of prostaglandins in the pathogenesis of Bartter's syndrome

Increased renal prostaglandins activated by beta-catecholamines could produce renal tubular sodium wasting and angiotensin pressor resistance observed in Bartter's syndrome. We therefore measured plasma renin activity (PRA), aldosterone and prostaglandin A (PGA) by radioimmunoassay, and body composition by isotope dilution prior to and following beta-adrenergic blockade with propranolol (200 mg/day for 4 days) and prostaglandin synthesis inhibition by indomethacin (200 mg/day for 4 days) in a patient with Bartter's syndrome on a 250 meq sodium diet. After the administration of propranolol, body weight increased 3 kg, daily urine sodium decreased within 24 hours from 230 to 64 meq, and urine potassium from 102 to 45 meq, but PRA and the aldosterone level remained elevated. With the administration of indomethacin, body weight increased 5 kg, daily urinary sodium decreased within 24 hours to 11meq and urine potassium to 16 meq, PRA (normal less than 3 ng/100 ml/hour) decreased from 55 to 4.3 ng/ml/hour, plasma aldosterone (normal less than 8 ng/100 ml) from 74.1 to 3.6 ng/100 ml, and whole blood PGA (normal 546 +/- 307 pg/ml) decreased from 1,390 and 945 to 86 pg/ml. After the administration of propranolol or indomethacin, exchangeable sodium, total body water, extracellular volume and plasma volume all increased from less than to greater than predicted, and pressor resistance to angiotensin was normalized. These results suggest that Bartter's syndrome results from beta adrenergic and prostaglandin-mediated proximal tubular rejection of sodium leading to increased distal sodium-potassium exchange.