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Biomedical subjects

J Eichler

Publications and source records attributed to J Eichler.

At least 19 recordsLinked to original sources
PubMed2001

Near-threshold photoionization of hydrogenlike uranium studied in ion-atom collisions via the time-reversed process.

Radiative electron capture, the time-reversed photoionization process occurring in ion-atom collisions, provides presently the only access to photoionization studies for very highly charged ions. By applying the deceleration mode of the ESR storage ring, we studied this process in low-energy collisions of bare uranium ions with low- Z target atoms. This technique allows us to extend the current information about photoionization to much lower energies than those accessible for neutral heavy elements in the direct reaction channel. The results prove that for high- Z systems, higher-order multipole contributions and magnetic corrections persist even at energies close to the threshold.

Journal Article↗
PubMed2001

[Introduction to tissue optics and optical dosimetry].

The spatial distribution of radiation during medical laser application is determined by the characteristics of the beam (power, time, beam geometry) and the optical properties of the tissue. The irradiance E (in W/m2) describes the primary laser beam. Scattered radiation, in turn, is taken into account by the fluence rate phi (also in W/m2). The basic parameters of tissue optics are the absorption coefficient mu a, the scattering coefficient mu s and the anisotropy factor g. In addition, derived parameters are also used, i.e., total attenuation coefficient mu t, reduced scattering coefficient mu s', effective attenuation coefficient mu eff, mean free path of a photon d and penetration depth delta. Further tissue properties are the diffuse reflectance Rd and the back-scattering factor k. In an one-dimensional model the fluence rate phi in tissue is a nearly exponential function characterized by the penetration depth delta. At the tissue surface, the relationship exists phi = kE. This model is compared with the results of a computer program based on the finite element method.

Dose-Response Relationship, Radiation↗
PubMed2001

The signal recognition particle of Archaea.

It is becoming increasingly clear that similarities exist in the manner in which extracytoplasmic proteins are targeted to complexes responsible for translocating these proteins across membranes in each of the three domains of life. In Eukarya and Bacteria, the signal recognition particle (SRP) directs nascent polypeptides to membrane-embedded translocation sites. In Archaea, the SRP protein targeting pathway apparently represents an intermediate between the bacterial and eukaryal systems. Understanding the archaeal SRP pathway could therefore reveal universal aspects of targeting not detected in current comparisons of the eukaryal and bacterial systems while possibly identifying aspects of the process either not previously reported or unique to Archaea.

Amino Acid Sequence↗
PubMed2001

Post-translational modification of the S-layer glycoprotein occurs following translocation across the plasma membrane of the haloarchaeon Haloferax volcanii.

The halophilic archaeon Haloferax volcanii is surrounded by a protein shell solely comprised of the S-layer glycoprotein. While the gene sequence and glycosylation pattern of the protein and indeed the three-dimensional structure of the surface layer formed by the protein have been described, little is known of the biosynthesis of the S-layer glycoprotein. In the following, pulse-chase radiolabeling and cell-fractionation studies were employed to reveal that newly synthesized S-layer glycoprotein undergoes a maturation step following translocation of the protein across the plasma membrane. The processing step, detected as an increase in the apparent molecular mass of the S-layer glycoprotein, is unaffected by inhibition of protein synthesis and is apparently unrelated to glycosylation of the protein. Maturation requires the presence of magnesium ions, involved in membrane association of the S-layer glycoprotein, and results in increased hydrophobicity of the protein as revealed by enhanced detergent binding. Thus, along with protein glycosylation, additional post-translational modifications apparently occur on the external face of the haloarchaeal plasma membrane, the proposed topological homologue of the lumenal face of the eukaryal endoplasmic reticulum membrane.

Amino Acid Sequence↗
PubMed2001

Influence of pulse duration on ultrashort laser pulse ablation of biological tissues.

Ablation characteristics of ultrashort laser pulses were investigated for pulse durations in the range of 130 fs-10 ps. Tissue samples used in the study were dental hard tissue (dentin) and water. We observed differences in ablation crater morphology for craters generated with pulse durations in the 130 fs-1 ps and the 5 ps-10 ps range. For the water experiment, the surface ablation and subsequent propagation of stress waves were monitored using Mach-Zehnder interferometry. For 130 fs-1 ps, energy is deposited on the surface while for longer pulses the beam penetrates into the sample. Both studies indicate that a transition occurs between 1 and 5 ps.

Differential Threshold↗
PubMed2000

Collagen structure and nonlinear susceptibility: effects of heat, glycation, and enzymatic cleavage on second harmonic signal intensity.

BACKGROUND AND OBJECTIVE: Helical macromolecules such as collagen and DNA are characterized by nonlinear optical properties, including nonlinear susceptibility. Because collagen is the predominant component of most biological tissues, as well as the major source of second harmonic generation (SHG), it is reasonable to assume that changes in harmonic signal can be attributed to structural changes in collagen. The purpose of this study is to determine whether various modifications of collagen structure affect second harmonic intensity. STUDY DESIGN/MATERIALS AND METHODS: SHG was measured in tissues from cows, humans, and chickens. The effects of beam polarization, thermal denaturation, glyco-oxidative damage, and enzymatic cleavage of tissues on second harmonic intensity was studied. RESULTS: The second harmonic intensity differed considerably among different tissues, as did the effect of the incident beam polarization. In structurally modified collagen, SHG was significantly degraded from SHG in intact collagen. CONCLUSION: These structural modifications are representative of changes that occur in pathophysiologic conditions such as thermal injury, diabetes, tumor invasion, and abnormal wound healing. The ability to assess these changes rapidly and noninvasively has considerable clinical applicability. SHG analysis might provide a unique tool for monitoring these structural changes of collagen.

Animals↗
PubMed2000

Characterization of acetohydroxy acid synthase activity in the archaeon Haloferax volcanii.

Whereas the biochemistry of acetohydroxy acid synthase has been extensively studied in bacteria and eukaryotes, relatively little is known about the enzyme in archaea, the third kingdom of life. The present study biochemically characterizes acetohydroxy acid synthase activity in the halophilic archaea Haloferax volcanii. In addressing ion requirements, enzyme inhibition and antibody labeling, the results reveal that, except for its elevated salt requirements, the haloarchaeal enzyme is remarkably similar to its bacterial counterpart.

Acetolactate Synthase↗
PubMed2000

Temperature distribution for combined laser hyperthermia-photodynamic therapy in the esophagus.

In recent years photodynamic laser therapy (PDT) has been tested in animal and clinical studies for treatment of esophageal cancer. In several animal experiments a synergistic effect was found by simultaneously applying PDT and hyperthermia (HT). In this paper an optical fibre system is described which can be used in the esophagus for combined PDT with a 1 W dye laser and HT with a 15 W Nd:YAG laser. A phantom was built simulating the geometry of the esophagus using cow muscle. The spatial temperature field during HT was measured versus irradiation time. The results were compared with calculations using a coupled Monte Carlo laser transport/finite difference heat transport model using the LATIS computer program. Measurements and calculations yield a realistic description of the temperature distribution during HT under various experimental conditions. The LATIS program allows the prediction of the effects of blood perfusion for in vivo situations. The results show that perfusion has considerable influence on the temperature field, reducing the effective depth in tissue for HT.

Animals↗
PubMed2000

Archaeal protein translocation crossing membranes in the third domain of life.

Proper cell function relies on correct protein localization. As a first step in the delivery of extracytoplasmic proteins to their ultimate destinations, the hydrophobic barrier presented by lipid-based membranes must be overcome. In contrast to the well-defined bacterial and eukaryotic protein translocation systems, little is known about how proteins cross the membranes of archaea, the third and most recently described domain of life. In bacteria and eukaryotes, protein translocation occurs at proteinaceous sites comprised of evolutionarily conserved core components acting in concert with other, domain-specific elements. Examination of available archaeal genomes as well as cloning of individual genes from other archaeal strains reveals the presence of homologues to selected elements of the bacterial or eukaryotic translocation machines. Archaeal genomic searches, however, also reveal an apparent absence of other, important components of these two systems. Archaeal translocation may therefore represent a hybrid of the bacterial and eukaryotic models yet may also rely on components or themes particular to this domain of life. Indeed, considering the unique chemical composition of the archaeal membrane as well as the extreme conditions in which archaea thrive, the involvement of archaeal-specific translocation elements could be expected. Thus, understanding archaeal protein translocation could reveal the universal nature of certain features of protein translocation which, in some cases, may not be readily obvious from current comparisons of bacterial and eukaryotic systems. Alternatively, elucidation of archaeal translocation could uncover facets of the translocation process either not yet identified in bacteria or eukaryotes, or which are unique to archaea. In the following, the current status of our understanding of protein translocation in archaea is reviewed.

Amino Acid Sequence↗
PubMed2000

[In vitro experiments with Nd:YAG laser surgery on the turbinates].

BACKGROUND: Turbinate surgery is a therapeutic method for the treatment of the obstruction of nasal respiration. In this paper the dimensions of the laser lesions are described. In addition macro- and microscopical findings after laser surgery are given. METHODS: 10 human lower and 4 middle turbinates in vitro were treated with the Nd:YAG-laser in the non-contact mode (1064 nm, 2.5-25 W, cw). Stripe-like lesion with 3 cm length were produced. In addition the posterior end of the lower turbinates and the head of the middle turbinates were vaporized. RESULTS: Width, depth and volume of the lesions are given in dependence of laser power and irradiation time. The histological changes immediately after laser treatment are described. CONCLUSIONS: The energy doses for a clinical relevant stripe-like laser lesion of 3-4 cm in length of the lower turbinate is about 1500 Ws using Nd:YAG-laser. For evaporation of a posterior end of the lower turbinate 360 Ws are required using Nd:YAG-laser. For evaporation of the head of the middle turbinate a doses of about 1500 Ws are required using Nd:YAG-laser.

Feasibility Studies↗
PubMed1998

Endogenous SecA catalyzes preprotein translocation at SecYEG.

SecA is found in the cytosol and bound to the plasma membrane of Escherichia coli. Binding occurs either with high affinity at SecYEG or with low affinity to lipid. Domains of 65 and 30 kDa of SecYEG-bound SecA insert into the membrane upon interaction with preprotein and ATP. Azide blocks preprotein translocation, in vivo and in vitro, through interacting with SecA and preventing SecA deinsertion. This provides a measure of the translocation relevance of each form of SecA membrane association. We now report that azide acts exclusively on SecA that is cycling at SecYEG and has no effect on SecA lipid associations. SecA molecules recovered with sucrose gradient-purified inner membrane vesicles ("endogenous" SecA) support translocation at the same rate as "added" SecA molecules bound at SecYEG. Both endogenous and added SecA yield the same proteolytic fragments, which are distinct from those obtained from SecA once it has inserted into membranes at SecYEG or from SecA at lipidic sites. Endogenous and added SecA differ, however, in their resistance to urea extraction. The translocation supported by either endogenous or added SecA is blocked by azide or by antibody to SecY. We conclude that SecA functions in preprotein translocation only through cycling at SecYEG.

Adenosine Triphosphatases↗
PubMed1998

125I-labeled fasciculin 2: a new tool for quantitation of acetylcholinesterase densities at synaptic sites by EM-autoradiography.

Radio-iodinated fasciculin 2 (Fas2), a polypeptide anticholinesterase toxin from Mamba venom, was used as a new probe for localizing and quantifying acetylcholinesterase (AChE) at mouse neuromuscular junctions (NMJs) by quantitative electron microscope autoradiography. We demonstrate that 125I-Fas2 binds very specifically to the NMJs of mouse sternomastoid muscles, with very little binding to other regions in the muscles. Junctional AChE-site densities obtained from the autoradiograms were similar to those previously obtained for the same muscles using 3H-DFP. The use of 125I-Fas2 with EM-autoradiography is simpler and provides higher resolution and sensitivity, as well as considerably lower non-specific binding than previously attainable with 3H-DFP. The advantages and limitations of this procedure are discussed.

Acetylcholinesterase↗
PubMed1998

The SecA subunit of Escherichia coli preprotein translocase is exposed to the periplasm.

SecA undergoes conformational changes during translocation, inserting domains into and across the membrane or enhancing the protease resistance of these domains. We now show that some SecA bound at SecYEG is accessible from the periplasm to a membrane-impermeant probe in cells with a permeabilized outer membrane but an intact plasma membrane.

Adenosine Triphosphatases↗
PubMed1997

Both an N-terminal 65-kDa domain and a C-terminal 30-kDa domain of SecA cycle into the membrane at SecYEG during translocation.

SecA, a 102-kDa hydrophilic protein, couples the energy of ATP binding to the translocation of preprotein across the bacterial inner membrane. SecA function and topology were studied with metabolically labeled [35S]SecA and with inner membrane vesicles from cells that overexpressed SecYEGDFyajC, the integral domain of preprotein translocase. During translocation in the presence of ATP and preprotein, a 65-kDa N-terminal domain of SecA is protected from proteolytic digestion through insertion into the membrane, as previously reported for a 30-kDa C-terminal domain [Economou, A. & Wickner, W. (1994) Cell 78, 835-843]. Insertion of both domains occurs at saturable SecYEGDFyajC sites and is rapidly followed by deinsertion. SecA also associates nonsaturably and unproductively with lipid. In the presence of ATP, yet without involvement of preprotein or SecYEG, lipid-bound SecA forms domains that are protease-resistant and that remain so even upon subsequent membrane disruption. Unlike the [35S]SecA that inserts into the membrane at SecYEGDFyajC as it promotes preprotein translocation, lipid-associated [35S]SecA does not chase from its protease-resistant state upon the addition of excess SecA. The finding that two domains of SecA (which together represent most regions of the polypeptide chain) cycle into the membrane during preprotein translocation, as well as the distinction between the membrane association of SecA at translocation sites of SecYEGDFyajC and at nonproductive lipid sites, are fundamental to the study of the role of SecA in preprotein movement.

Adenosine Triphosphatases↗
PubMed1997

The protease-protected 30 kDa domain of SecA is largely inaccessible to the membrane lipid phase.

SecA binds to the inner membrane of Escherichia coli through low affinity lipid interactions or with high affinity at SecYEG, the integral domain of preprotein translocase. Upon addition of preprotein and nucleotide, a 30 kDa domain of SecYEG-bound SecA is protected from proteolysis via membrane insertion. Such protection could result from some combination of insertion into the lipid phase, into a proteinaceous environment or across the membrane. To assess the exposure of SecYEG-bound SecA to membrane lipids, a radiolabeled, photoactivatable and lipid-partitioning crosslinker, 3-trifluoromethyl-3-(m[125I]iodophenyl) diazirine benzoic acid ester, was incorporated into inner membrane vesicles. The 30 kDa domain of SecYEG-bound SecA, inserted into the membrane in response to translocation ligands, is 18-fold less labeled than SecY, which is labeled effectively. In contrast, incorporation of the purified 30 kDa SecA fragment into crosslinker-containing detergent micelles or addition of detergent to crosslinker-containing membranes bearing the protease-protected SecA domain readily allows for labeling of this domain. We propose that the protease-inaccessible 30 kDa SecA domain is shielded from the fatty acyl membrane phase by membrane-spanning SecYEG helices and/or is largely exposed to the periplasm.

Adenosine Triphosphatases↗
PubMed1996

Novel alpha-glucosidase inhibitors identified using multiple cyclic peptide combinatorial libraries.

Twenty-six cyclic synthetic peptide combinatorial libraries (disulfides and lactams) of varying size and composition, representing 6.8 x 10(3) to 4.7 x 10(7) individual peptides, were synthesized along with their respective linear analogs. One of the hexapeptide lactam libraries (cyclo[xXxXxN]) was found to have significant alpha-glucosidase inhibitory activity. This library was carried through an iterative process of synthesis and screening, during which all of the five mixture positions (x and X) were successively defined. As the result of this process, potent and selective alpha-glucosidase inhibitors were identified.

Amino Acid Sequence↗