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11.
Analysis of the effects of cesium ions on potassium channel currents in biological membranes 总被引:3,自引:0,他引:3
Cesium ions block potassium channels in biological membranes in a voltage dependent manner. For example, external cesium blocks inward current with little or no effect on outward current. Consequently, it produces a characteristic N-shaped current-voltage relationship. We have modeled this result by single file diffusion of ions in a narrow channel spanning the membrane with a special blocking site in the channel for cesium ions. The model enables us to make detailed comparisons of the effects of cesium on potassium channels in different types of biological membranes. 相似文献
12.
ABSTRACT. Recordings of avian vocal signals in natural habitats include ambient noise. Often this background noise corrupts across all frequencies and is of substantial amplitude. Reducing this ambient noise to prepare vocal signals for playback stimuli or to remove habitat-specific noise signatures prior to analyzing a signal's acoustic characteristics can be useful. We conducted experimental evaluations of three noise reduction procedures to determine their effectiveness. We embedded two bird vocalizations ("clean" signals) in four kinds of natural noise, resulting in eight noise-signal combinations. We then applied three noise reduction procedures (Noise Profile, Band Pass, and Noise Estimate) to each of the embedded signals and compared the recovered signals to the original (clean) signals. Noise Profile filtering was effective in reducing noise and returning fairly high-quality signals from even severe levels of masking noise. The other two noise reduction procedures did not perform as well. For the two most corrupting maskers, however, Noise Profile filtering also altered the signal properties by reducing signal amplitude at those frequencies containing high levels of noise. Apart from this loss of amplitude, the quantitative features of the filtered signals were similar to those of the original model sounds. We conclude that Noise Profile filtering produces good results for cases where noise is approximately constant over the signal duration and the signal intensity exceeds noise intensity over the frequencies of interest. 相似文献
13.
When grown with nitrate as terminal electron acceptor both the soluble (periplasm, cytoplasm) and the membrane fraction of Spirillum strain 5175 exhibited high nitrite reductase activity. The nitrite reductase obtained from the soluble fraction was purified 76-fold to electrophoretical homogeneity. The enzyme reduced nitrite to ammonia with a specific activity of 723 mol NO
inf2
sup-
× (mg protein × min)-1. The molecular mass was 58±1 kDa by SDS-PAGE compared to 59±2 kDa determined by size exclusion chromatography under nondenaturing conditions. The enzyme (as isolated) contained 5.97±0.15 heme c molecules/Mr 58 kDa. The absorption spectrum was typical for c-type cytochrome with maxima at 280, 408, 532 and 610 nm (oxidized) and at 420, 523 and 553 nm (dithionite-reduced). The enzyme (as isolated) exhibited a complex set of high-spin and lowspin ferric heme resonances with g-values at 9.82, 3,85, 3.31, 2.95, 2.30 and 1.49 in agreement with data reported for electron paramagnetic resonance spectra of nitrite reductases from Desulfovibrio desulfuricans, Wolinella succinogenes and Escherichia coli.Abbreviations DNRA
dissimilatory nitrate reduction to ammonia
- EPR
electron paramagnetic resonance
- PAGE
polyacrylamide gel electrophoresis
- NaPi
sodium phosphate
- SDS
sodium dodecylsulfate 相似文献
14.
J. T. Keltjens J. M. H. Hermans G. J. F. A. Rijsdijk C. Van der Drift G. D. Vogels 《Antonie van Leeuwenhoek》1988,54(3):207-220
F430 is the prosthetic group of the methylcoenzyme M reductase of methanogenic bacteria. The compound isolated from Methanosarcina barkeri appears to be identical to the one obtained from the only distinctly related Methanobacterium thermoautotrophicum. F430 is thermolabile and in the presence of acetonitrile or C10
in4
sup-
two epimerization products are obtained upon heating; in the absence of these compounds F430 is oxidized to 12, 13-didehydro-F430. The latter is stereoselectively reduced under H2 atmosphere to F430 by cell-free extracts of M. barkeri or M. thermoautotrophicum. H2 may be replaced by the reduced methanogenic electron carrier coenzyme F420.Abbreviations CH3S-CoM
methylcoenzyme M, 2-methylthioethanesulfonic acid
- HS-CoM
coenzyme M, 2-mercaptoethanesulfonic acid
- F430
Ni(II) tetrahydro-(12, 13)-corphin with a uroporphinoid (III) ligand skeleton
- 13-epi-F430 and 12,13-di-epi-F430
the 12, 13- and 12, 13-derivatives of F430
- 12, 13-didehydro-F430
F430 oxidized at C-12 and C-13
- coenzyme F420
7,8-didemethyl-8-hydroxy-5-deazaflavin derivative
- coenzyme F420H2
reduced coenzyme F420
- MV+
methylviologen semiquinone
- HPLC
high-performance liquid chromatography 相似文献
15.
M. V. Gusev T. G. Korzhenevskaya L. V. Pyvovarova O. I. Baulina R. G. Butenko 《Planta》1986,167(1):1-8
Tobacco (Nicotiana tabacum L.) shoots associated with the nitrogen-fixing cyanobacterium Anabaena variabilis Kütz. (ATCC 29413) were regenerated in mixed cultures of tobacco callus and the cyanobacterium. The cyanobacteria were localized inside the tissues as well as on the surface of regenerated shoots, formed heterocysts, and were capable of acetylene reduction. 相似文献
16.
S. Abramov Y. Aharonowitz M. Harnik R. Lamed A. Freeman 《Enzyme and microbial technology》1990,12(12):982-988
The development of a continuous anaerobic process for stereospecific Δ4-3-keto-steroid reduction by immobilized Clostridium paraputrificum cells cells is described. Following a study on conditions for cell growth and sporulation, spores of C. paraputrificum were aseptically immobilized in PAAH beads. Conditions for cell growth and induction in the immobilized state were determined, as well as the medium composition required to maintain a stabilized immobilized cell population. The effect of the concentration of ethylene glycol added as selected cosolvent on reaction kinetics, substrate solubility, specific activity, and cell growth, was investigated. A 10% (v/v) cosolvent input provided maximal activity along with enhanced solubility of the steroidal substrate. It was shown that cell growth was enhanced in the presence of the added cosolvent in addition to its effect on substrate solubility and enzymic activity. The immobilized cells readily performed Δ4, as well as 3-keto steroid reduction of several steroids, including ADD, AD, 16-dehydroprogesterone, progesterone, and hydrocortisone. It was shown that repeated batch-wise reduction cycle—in the presence of the cosolvent—resulted in rapid loss of activity, while the continuous uninterrupted process permitted the attaining of full bioconversion level, maintained stable for at least the period of 5 days of continuous operation tested. 相似文献
17.
18.
Effects of fire on water and salinity relations of riparian woody taxa 总被引:12,自引:0,他引:12
Water and salinity relations were evaluated in recovering burned individuals of the dominant woody taxa from low-elevation riparian plant communities of the southwestern U.S. Soil elemental analyses indicated that concentrations of most nutrients increased following fire, contributing to a potential nutrient abundance but also elevated alluvium salinity. Boron, to which naturalized Tamarix ramosissima is tolerant, was also elevated in soils following fire. Lower moisture in the upper 30 cm of burned site soil profiles was attributed to shifts in evapotranspiration following fire. Higher leaf stomatal conductance occurred in all taxa on burned sites. This is apparently due to higher photosynthetic photon flux density at the midcanopy level and may be partially mitigated by reduced unit growth in resprouting burned individuals. Predawn water potentials varied little among sites, as was expected for plants exhibiting largely phreatophytic water uptake. Midday water potentials in recovering Salix gooddingii growing in the Colorado River floodplain reached levels which are considered stressful. Decreased hydraulic efficiency was also indicated for this species by examining transpiration-water potential regressions. Recovering, burned Tamarix and Tessaria sericea had enriched leaf tissue 13C relative to unburned controls. Higher water use efficiency following fire in these taxa may be attributed to halophytic adaptations, and to elevated foliar nitrogen in Tessaria. Consequently, mechanisms are proposed which would facilitate increased community dominance of Tamarix and Tessaria in association with fire. The theory that whole ecosystem processes are altered by invading species may thus be extended to include those processes related to disturbance. 相似文献
19.
David L. Musso Nariman B. Mehta Francis E. Soroko Robert M. Ferris Elizabeth B. Hollingsworth Bernard T. Kenney 《Chirality》1993,5(7):495-500
The synthesis of the enantiomers of bupropion, (rac)-2-tert-butylamino-3′-chloropropiophenone 1 (Wellbutrin®) is described. The enantiomers were compared with the racemate in both the tetrabenazine-induced sedation model and the inhibition of uptake of biogenic amine assay. No significant differences were found in their potencies to reverse tetrabenazine-induced sedation in mice or in their IC50 values as inhibitors of biogenic amine uptake into nerve endings obtained from mouse brain. © 1993 Wiley-Liss, Inc. 相似文献
20.
David H. Jewson 《Journal of phycology》1992,28(6):856-866
Studies of the life cycle of a centric diatom, tentatively identified as Stephanodiscus neoastraea Håkansson & Hickel, showed that sexual reproduction occurred every year in a freshwater lake (Lough Neagh, Northern Ireland). Male and female gametes were produced in cells below 55% of the maximum diameter during a 3–4-week period in late summer, following the return of nitrate concentrations above 10 μM NO3-N. The frequency of sexual reproduction was linked to the cycle of diameter size reduction and regeneration. The times of largest decreases in cell diameter were during nutrient stress in summer and low light conditions in late autumn, rather than during the main spring growth period. So, environmental conditions (combined with the limited life-spans of individual cells) affected the rate of diameter reduction and, therefore, the length of the life cycle (3–4 years). 相似文献