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At least 1,027 records · Page 57Linked to original sources
PubMed1989

Location and dynamics of alamethicin in unilamellar vesicles and thylakoids as model systems. A spin label study.

Location and dynamics of the voltage-dependent pore-forming icosapeptide alamethicin have been studied using spin labels which were linked directly and via spacers to the C-terminus of the amphiphilic alpha-helix. Ion-transport activities of these derivatives were found to be very similar to those of natural alamethicin in green plant thylakoids chosen as a model system. The shape of the electron spin resonance spectra indicates segmental motion of the nitroxide rather than rotation of the whole peptide. A population of spins showing narrow lines in the presence of thylakoids or lipid vesicles is attributed to alamethicin in the aqueous solution. A second population shows rotational correlation times greater than 10(-9) s and is bound to the membranes, the C-termini residing in an environment with a polarity close to that of water. This population is inaccessible to the hydrophilic, charged line broadening agent chromium oxalate. Since spectral shapes and amplitudes of spectra are unchanged by additions of unlabelled peptide, it is concluded that the ESR detectable spins are bound to peptides essentially in the monomeric state. Alamethicin induced pore formation under flash illumination is demonstrated by measurement of kinetics of proton deposition in the thylakoid interior. When pores are opened by illuminating thylakoids and thus applying a membrane potential, mainly the bound population is affected by a process reversibly suppressing the signal, whereas only limited disappearance of label from the external medium is detected. Apparently, the potential causes a change in the conformation of the peptide which leads to a further immobilisation of the label, possibly due to a deeper insertion of the alpha-helices into the lipid membrane. However, evidence has been presented experimentally that there is no detectable change of potential prior to the opening of the pore.

Alamethicin↗
PubMed1997

Advanced EPR spectroscopy on electron transfer processes in photosynthesis and biomimetic model systems.

This review focuses on the recent advances in EPR spectroscopy as they are applied both to photoinduced electron transfer in the photosynthetic apparatus and to biomimetic systems. The review deals with time-resolved direct-detection cw and pulsed EPR and ENDOR methods, both at conventional bands [X-(9.5 GHz), K-(24 GHz), and Q-(35 GHz)(] and at high frequency bands (W-band, 95 GHz, and even higher frequency bands). EPR studies on photosynthetic and model systems in their doublet, triplet and radical pair states are surveyed, including their static and dynamic properties. APplications of time-resolved EPR in studying photoinduced electron and energy transfer in isotropic and anisotropic environments, and the concepts of electron spin polarization and magnetic field effects in photochemical reactions are also reviewed.

Electron Spin Resonance Spectroscopy↗
PubMed2004

Comparative study on pressure and temperature stability of 5-methyltetrahydrofolic acid in model systems and in food products.

A comparative study on the pressure and temperature stability of 5-methyltetrahydrofolic acid (5-CH(3)-H(4)folate) was performed in model/buffer systems and food products (i.e., orange juice, kiwi puree, carrot juice, and asparagus). Effects of pH and ascorbic acid (0.5 mg/g) on 5-CH(3)-H(4)folate stability in buffer systems were studied on a kinetic basis at different temperatures (from 65 to 160 degrees C) and different pressure/temperature combinations (from 100 to 700 MPa/from 20 to 65 degrees C). These studies showed that (i) the degradation of 5-CH(3)-H(4)folate in all model systems could be described by first-order reaction kinetics, (ii) the thermostability of 5-CH(3)-H(4)folate was enhanced by increasing pH up to 7, (iii) 5-CH(3)-H(4)folate was relatively pressure stable at temperatures lower than 40 degrees C, and (iv) ascorbic acid enhanced both the thermo- and barostabilities of 5-CH(3)-H(4)folate. In food products, temperature and pressure stabilities of 5-CH(3)-H(4)folate were studied at different temperatures (70-120 degrees C) and different pressure/temperature combinations (from 50 to 200 MPa/25 degrees C and 500 MPa/60 degrees C). 5-CH(3)-H(4)folate in orange juice and kiwi puree was relatively temperature (up to 120 degrees C) and pressure (up to 500 MPa/60 degrees C) stable in contrast to carrot juice and asparagus. Addition of ascorbic acid (0.5 mg/g) in carrot juice resulted in a remarkable protective effect on pressure (500 MPa/60 degrees C/40 min) and temperature degradation (120 degrees C/40 min) of 5-CH(3)-H(4)folate.

Ascorbic Acid↗
PubMed2003

Increased insulin-like growth factor-I gene expression precedes left ventricular cardiomyocyte hypertrophy in a rapidly-hypertrophying rat model system.

Chronic pressure overload leads to an increase in the size, i.e. hypertrophy, of cardiomyocytes in the heart. However, the molecular mechanisms underlying this hypertrophy are not understood. Insulin-like growth factor-I (IGF-I) synthesized locally in the heart is known to be associated with the hypertrophic process. So far, however, cardiac IGF-I gene expression in the widely used rat model system has only been shown to be increased when the hypertrophy induced by pressure-overload was already established. Therefore, the question of whether IGF-I serves as an initiating or early-enhancing factor for the cardiac hypertrophy remains unanswered. Here, cardiac hypertension and hypertrophy were rapidly induced in the rat by complete constriction of the abdominal aorta between the origins of the renal arteries. Carotid arterial systolic blood pressure remained unchanged in sham rats but increased rapidly in the pressure-overloaded constricted rats with a sustained hypertension established by 3 days. Hypertrophy of left ventricular (LV) cardiomyocytes in constricted rats also occurred by 3 days. However, this hypertrophy was preceded by increases in LV IGF-I mRNA and protein which occurred within 1 day. These results support the hypothesis that cardiac-synthesized IGF-I is an initiating or early-enhancing factor for hypertrophy of LV cardiomyocytes.

Animals↗
PubMed2000

A model system for activation-induced alternative splicing of CD45 pre-mRNA in T cells implicates protein kinase C and Ras.

Multiple isoforms of the protein tyrosine phosphatase CD45 are expressed on the surface of human T cells. Interestingly, the expression of these isoforms has been shown to vary significantly upon T-cell activation. In this report, we describe a novel cell line-based model system in which we can mimic the activation-induced alternative splicing of CD45 observed in primary T cells. Of the many proximal signaling events induced by T-cell stimulation, we show that activation of protein kinase C and activation of Ras are important for the switch toward the exclusion of CD45 variable exons, whereas events related to Ca(2+) flux are not. In addition, the ability of cycloheximide to block the activation-induced alternative splicing of CD45 suggests a requirement for de novo protein synthesis. We further demonstrate that sequences which have previously been implicated in the tissue-specific regulation of CD45 variable exons are likewise necessary and sufficient for activation-induced splicing. These results provide an initial understanding of the requirements for CD45 alternative splicing upon T-cell activation, and they confirm the importance of this novel cell line in facilitating a more detailed analysis of the activation-induced regulation of CD45 than has been previously possible.

Alternative Splicing↗
PubMed1983

In vitro perfusion of isolated mouse testes: a model system for investigating testicular steroidogenesis.

1. A procedure was developed to perfuse isolated mouse testes in vitro and investigate testosterone secretion. 2. The perfused testis preparation was validated by determining organ edema, glucose uptake, enzymatic and morphological indices of cellular integrity, and testosterone secretion in the absence and presence of LH and other pituitary peptides. 3. The results establish that this procedure can be exploited as a model system to investigate biochemical, physiological, and genetic aspects of steroidogenesis and Leydig cell function in the mouse, and presumably other small rodents as well.

Animals↗
PubMed1994

Loading of a DNA helicase on the DNA unwinding element in the yeast replication origin: mechanism of DNA replication in a model system.

We found that initiation of DNA replication occurs from the region containing the yeast autonomously replicating sequence 1 (ARS1), by incubating negatively supercoiled plasmid DNA with the proteins required for SV40 DNA replication in addition to DNA gyrase (Ishimi, Y., & Matsumoto, K. (1993) Proc. Natl. Acad. Sci. U.S.A. 90, 5399-5403). Here, the mechanism of DNA replication and the roles of the replication proteins in this model system were analyzed. Both SV40 T antigen as a DNA helicase and multisubunit human single-stranded DNA binding protein (HSSB) (also called RP-A) were required for the initial step of DNA synthesis. Furthermore, it has been shown that T antigen plays an essential role in the initiation of DNA replication from the ARS region in this system. The digestion of negatively supercoiled DNA with the single-strand-specific nuclease P1 revealed that regions containing A, B, and C domains of ARS1 can be unwound under the conditions used for DNA replication. Footprinting with KMnO4 indicated that T antigen interacted with the unwound B domain where initiation of DNA replication mainly occurred. When circular DNAs of different negative-superhelical densities were replicated in the absence of DNA gyrase, short fragments were synthesized from the ARS region in proportion to its density and they were elongated by addition of HeLa topoisomerase I, which inhibits the initiation of DNA replication in this system.(ABSTRACT TRUNCATED AT 250 WORDS)

Antigens, Polyomavirus Transforming↗
PubMed1991

Model system for optimising mRNA non-isotopic in situ hybridisation: riboprobe detection of lysozyme mRNA in archival gut biopsy specimens.

The aim of this study was to optimise conditions for mRNA detection by nonisotopic in situ hybridisation (NISH) using biotinylated and digoxigenin labelled riboprobes. Because lysozyme gene transcripts are present at high concentrations in Paneth and other alimentary cells, archival gut biopsy specimens were chosen as a model system for these experiments. Most of the variables in NISH, from unmasking of mRNA, to its ultimate detection by peroxidase or alkaline phosphatase based detection systems, were examined in detail. The most important findings were that simultaneous heating of tissue targets and riboprobes at 95 degrees C for 15 minutes before hybridisation at 50 degrees C for two hours gave the most intense signal for lysozyme mRNA in Paneth cells, Brunner's glands, and lamina propria macrophages; digoxigenin labelled riboprobes gave a higher signal to noise ratio than their biotinylated counterparts, and probes 600 base pairs long were superior to shorter probes. It is concluded that the mRNA NISH method may be generally useful for detecting gene transcription in archival clinical biopsy specimens.

Archives↗
PubMed2004

Synchronization of decentralized multiple-model systems by market-based optimization.

Market-based optimization is a new optimization method for large decentralized systems where the distributed resource allocation of an economic system is adopted. Market-based algorithms can be interpreted as multi-agent scenarios where producer and consumer agents both compete and cooperate on a market of specified commodities. The market-based approach is applied to the synchronization of a set of local multiple-model systems. The method is extended to the case where each of the subsystems is represented by a Takagi-Sugeno (TS) fuzzy system. Although all local systems are provided with the same control input, the behaviors of the local systems are, in general, different because of different parameters in the subsystems. The task of the market-based optimization is to find an appropriate composition of subsystems so that all local systems exhibit a similar dynamical behavior. Examples show that even systems with potentially unstable local systems can be synchronized if there exists a stable combination of weighted subsystems.

Journal Article↗
PubMed2002

Apoptosis in yeast: a new model system with applications in cell biology and medicine.

Apoptosis is a highly coordinated cellular suicide program crucial for metazoan health and diseases. Although its increasing importance in cancer, neurodegenerative disorders and AIDS led to intense research and a better understanding of apoptosis, many details of its regulation or the apoptotic phenotypes are poorly understood. The complex regulatory network and the often contradictory results obtained with human cell lines made application of an easier model system desirable. Apoptosis in yeast promises to provide a better understanding of the genetics of apoptosis. During the past 2 years, scientists were successful in identifying new cell-death regulators of humans, plants and fungi using Saccharomyces cerevisiae. The finding of apoptotic phenotypes, even in protists, suggests that apoptosis developed in unicellular organisms long before the evolutionary separation between fungi, plants and metazoan animals occurred.

Aging↗
PubMed2001

Congenital disorders of glycosylation: genetic model systems lead the way.

N-linked glycosylation is the most frequent modification of secretory proteins in eukaryotic cells. The highly conserved glycosylation process is initiated in the endoplasmic reticulum (ER), where the Glc(3)Man(9)GlcNAc(2) oligosaccharide is assembled on the lipid carrier dolichylpyrophosphate and then transferred to selected asparagine residues of polypeptide chains. In recent years, several inherited human diseases, congenital disorders of glycosylation (CDG), have been associated with deficiencies in this pathway. The ER-associated glycosylation pathway has been studied in the budding yeast Saccharomyces cerevisiae, and this model system has been invaluable in elucidating the molecular basis of novel types of CDG.

Animals↗
PubMed1999

Dictyostelium as model system for studies of the actin cytoskeleton by molecular genetics.

The actin cytoskeleton is an essential structure for most movements at the cellular and intracellular level. Whereas for contraction a muscle cell requires a rather static organisation of cytoskeletal proteins, cell motility of amoeboid cells relies on a tremendously dynamic turnover of filamentous networks in a matter of seconds and at distinct regions inside the cell. The best model system for studying cell motility is Dictyostelium discoideum. The cells live as single amoebae but can also start a developmental program that leads to multicellular stages and differentiation into simple types of tissues. Thus, cell motility can be studied on single cells and on cells in a tissue-like aggregate. The ability to combine protein purification and biochemistry with fairly easy molecular genetics is a unique feature for investigation of the cytoskeleton and cell motility. The actin cytoskeleton in Dictyostelium harbours essentially all classes of actin-binding proteins that have been found throughout eukaryotes. By conventional mutagenesis, gene disruption, antisense approaches, or gene replacements many genes that code for cytoskeletal proteins have been disrupted, and altered phenotypes in transformants that lacked one or more of those cytoskeletal proteins allowed solid conclusions about their in vivo function. In addition, tagging the proteins or selected domains with green fluorescent protein allows the monitoring of protein redistribution during cell movement. Gene tagging by restriction enzyme mediated integration of vectors and the ongoing international genome and cDNA sequencing projects offer the chance to understand the dynamics of the cytoskeleton by identification and functional characterisation of all proteins involved.

Actins↗
PubMed1996

Direct oxidation of polyunsaturated cis-parinaric fatty acid by phenoxyl radicals generated by peroxidase/H2O2 in model systems and in HL-60 cells.

Reactivity of phenoxyl radicals towards biomolecules (proteins, nucleic acids and lipids) is essential for antioxidant (protective) versus prooxidant (cytotoxic) effects of phenolic compounds (antioxidants, phytochemicals, environmental pollutants and toxic chemicals). The present study demonstrates for the first time that phenoxyl radicals formed by peroxidase/H2O2-catalyzed oxidation of phenol can directly oxidize a natural polyunsaturated fatty acid, cis-parinaric acid (PnA) both in model systems and in membrane phospholipids of HL-60 cells. Endogenous antioxidants-ascorbate and glutathione-can act as one-electron reductants of phenoxyl radicals and provide effective protection against phenoxyl radical-induced oxidation of PnA.

Analysis of Variance↗
PubMed1999

Effectiveness of ascorbate and ascorbate peroxidase in promoting nitrogen fixation in model systems.

Ascorbate and ascorbate peroxidase are important antioxidants that are abundant in N2-fixing legume root nodules. Antioxidants are especially critical in root nodules because leghemoglobin, which is present at high concentrations in nodules, is prone to autoxidation and production of activated oxygen species such as O2.- and H2O2. The merits of ascorbate and ascorbate peroxidase for maintaining conditions favorable for N2 fixation were examined in two model systems containing oxygen-binding proteins (purified myoglobin or leghemoglobin) and N2-fixing microorganisms (free-living Azorhizobium or bacteroids of Bradyrhizobium japonicum) in sealed vials. The inclusion of ascorbate alone to these systems led to enhanced oxygenation of hemeproteins, as well as to increases in nitrogenase (acetylene reduction) activity. The inclusion of both ascorbate and ascorbate peroxidase resulted in even greater positive responses, including increases of up to 4.5-fold in nitrogenase activity. In contrast, superoxide dismutase did not provide beneficial antioxidant action and catalase alone provided only very marginal benefit. Optimal concentrations were 2 mM for ascorbate and 200 micrograms/ml for ascorbate peroxidase. These concentrations are similar to those found in intact soybean nodules. These results support the conclusion that ascorbate and ascorbate peroxidase are beneficial for maintaining conditions favorable for N2 fixation in nodules.

Ascorbate Peroxidases↗
PubMed2005

Derived release limits for the greek research reactor site based on a diagnostic atmospheric modeling system for irregular terrain.

The upper limits for the rate of release of radionuclides into the atmosphere, i.e., the "derived release limits," are calculated for the Greek Research Reactor (GRR-1) in order to determine possible operational schemes compatible with the effective dose limits for the general population. GRR-1 is located at the northwestern foot of Hymettos Mountain and at the eastern border of the urbanized area of Athens basin. Due to the topographic complexity of the region, the meteorological and atmospheric dispersion calculations were based on a numerical modeling system that is especially designed to work over irregular terrains by using a prismatic unstructured grid. The calculation of derived release limits was made using guidelines and methods that conform to the system of dose limits prescribed by the European radiation protection regulations.

Air Pollutants, Radioactive↗
PubMed1987

The role of group structure in the action of some morpholinium chloride derivatives on model systems.

Effects of morpholinium chlorides, which exhibit fungicidal activity, on model lipid systems is discussed. It was shown by means of DSC and BLM techniques that for different salts possessing alkyl chains of identical length, the interaction with lipids depends strongly on the polar head charge and the structure of the salt. The results obtained and conclusions drawn can be useful in explaining the possible mechanism of the biological effectiveness of the compounds tested; this is suggested by the observation that salts tested showed the same sequence of activity in both biological tests and model experiments.

Antifungal Agents↗
PubMed2000

Stress transmission through a model system of cohesionless elastic grains

Understanding the mechanical properties of granular materials is important for applications in civil and chemical engineering, geophysical sciences and the food industry, as well as for the control or prevention of avalanches and landslides. Unlike continuous media, granular materials lack cohesion, and cannot resist tensile stresses. Current descriptions of the mechanical properties of collections of cohesionless grains have relied either on elasto-plastic models classically used in civil engineering, or on a recent model involving hyperbolic equations. The former models suggest that collections of elastic grains submitted to a compressive load will behave elastically. Here we present the results of an experiment on a two-dimensional model system--made of discrete square cells submitted to a point load--in which the region in which the stress is confined is photoelastically visualized as a parabola. These results, which can be interpreted within a statistical framework, demonstrate that the collective response of the pile contradicts the standard elastic predictions and supports a diffusive description of stress transmission. We expect that these findings will be applicable to problems in soil mechanics, such as the behaviour of cohesionless soils or sand piles.

Journal Article↗
PubMed1999

Antifungal activities of antineoplastic agents: Saccharomyces cerevisiae as a model system to study drug action.

Recent evolutionary studies reveal that microorganisms including yeasts and fungi are more closely related to mammals than was previously appreciated. Possibly as a consequence, many natural-product toxins that have antimicrobial activity are also toxic to mammalian cells. While this makes it difficult to discover antifungal agents without toxic side effects, it also has enabled detailed studies of drug action in simple genetic model systems. We review here studies on the antifungal actions of antineoplasmic agents. Topics covered include the mechanisms of action of inhibitors of topoisomerases I and II; the immunosuppressants rapamycin, cyclosporin A, and FK506; the phosphatidylinositol 3-kinase inhibitor wortmannin; the angiogenesis inhibitors fumagillin and ovalicin; the HSP90 inhibitor geldanamycin; and agents that inhibit sphingolipid metabolism. In general, these natural products inhibit target proteins conserved from microorganisms to humans. These studies highlight the potential of microorganisms as screening tools to elucidate the mechanisms of action of novel pharmacological agents with unique effects against specific mammalian cell types, including neoplastic cells. In addition, this analysis suggests that antineoplastic agents and derivatives might find novel indications in the treatment of fungal infections, for which few agents are presently available, toxicity remains a serious concern, and drug resistance is emerging.

Acyltransferases↗