Search PubMed⌕ Search

SEARCH · Search PubMed

Results for “FLASH”

Search indexed PubMed citations on genomics, clinical trials, systematic reviews and public health. Explore titles, authors and supplied subject terms, then open the PubMed record.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 1,261 records · Page 70Linked to original sources
PubMed1996

The use of flash visual evoked potentials in the early diagnosis of suspected optic nerve lesions due to craniofacial trauma.

Craniofacial trauma encroaching on the orbital apex and optic canal can result in direct or indirect optic nerve lesions, leading to visual impairment or blindness. Early diagnosis of a visual loss and immediate therapy are generally considered crucial for a successful restoration of vision in indirect trauma. However, in comatose or sedated patients the assessment of optic nerve function by testing pupillary reactivity may be severely compromised or impossible because of tensely swollen eyelids, conjunctival oedema, concussion of the ciliary muscle or pharmacological effects. In the event that clinical ophthalmic examination, computer tomography or nuclear magnetic resonance scanning fail to clarify the state of the optic nerve, visual evoked potentials (VEPs) to flash stimulation appear to provide reliable information on function within the visual pathway. On this basis, treatment with corticosteroids and/or surgical decompression can be rapidly initiated. Our results in a preliminary patient series confirm the value of acutely monitored VEPs as an objective test of optic nerve function in cases of suspected optic nerve injury immediately after admission to the emergency care unit. The imaging techniques usually applied may be complemented by VEPs to show the functional significance of structural abnormalities found in the vicinity of the optic nerve.

Adult↗
PubMed2000

Laser flash photolysis study of triphenylimidazole.

Laser flash photolysis of the photocyclization of triphenylimidazole (TPI) in ethyl alcohol at 308 nm. indicates that the dihydrophenanthroimidazole (DHPI) intermediate is produced rapidly, has a lifetime of 0.25 ms, and returns predominantly back to triphenylimidazole. Analysis of the decay channels for this intermediate indicates two rate constants: (1) k1 = 3.3 x 10(3) s(-1), associated with reversion back to triphenylimidazole and (2) k2 = 0.67 x 10(2) s(-1), which is associated with the conversion of the dihydrophenanthroimidazole to the photoproduct, 2-phenyl-9,10-phenanthroimidazole. The photoproduct is readily observed as an increasing component in the biexponential fluorescence decay data. Fluorescence lifetimes for triphenylimidazole and 2-phenyl-9,10-phenanthroimidazole (PPI) in ethyl alcohol were determined to be 1.76 and 8.21 ns, respectively, with no additional components in the fluorescence decay as the photochemistry proceeds. An additional transient absorption observed in the 450 nm. region, with a lifetime of 0.7 micros, decaying faster than the dihydrophenanthroimidazole intermediate, is assigned to the triplet state of triphenylimidazole.

Imidazoles↗
PubMed2003

Procedure for separation of GM2 ganglioside species with different ceramide structures by a flash reversed-phase silica gel liquid chromatography.

GM2 ganglioside, beta-GalNAc-(1-4)-[alpha-Neu5Ac-(2-3)-]beta-Gal-(1-4)-beta-Glc-(1-1)-Cer, is the main ganglioside in the brain of Tay-Sachs patients. In this work, GM2 ganglioside was extracted from a Variant B Tay-Sachs human brain, purified to homogeneity of the oligosaccharide moiety by silica gel chromatography. It was further fractionated for the first time into the molecular species differing in the ceramide structures by reverse-phase flash chromatography. The GM2 ganglioside species were characterized by gas-chromatography, nuclear magnetic resonance spectroscopy, and mass spectrometry. The major GM2 species contained the ceramides with d18:1-18:0 (40.5% of the total GM2 species), d20:1-18:0 (31%) and d18:1-20:0 (12%). We also found minor GM2 species with the ceramides with d18:1-24:1 (4%), d18:1-22:0 (2%) and d18:2-24:1 (1%), which have not been reported previously.

Chromatography, Gas↗
PubMed2000

Effects of preterm extrauterine visual experience on the development of the human visual system: a flash VEP study.

To compare the functional maturation of the human visual system between intra- and extrauterine course flash visual evoked potentials (VEPs) in preterm infants (gestational age 24 to 36 weeks). Previously established normal values, with special reference to the two components of the N1 wave, were employed (Tsuneishi 1995). A cross-sectional analysis of 124 infants at 36 weeks postmenstrual age (PMA), showed that there are no differences in the absolute values of VEP peak latencies depending on the postnatal age (PNA). Conversely, the N1 wave form changes with increasing PNA from a wave in which the early peak (N1a) has a higher amplitude than the late peak (N1b) into the reverse situation with a higher amplitude of the N1b as compared to N1a. This observation may correlate with the maturation of the neuronal networks in the visual cortex. In a longitudinal analysis of 50 infants followed for more than 5 sessions of weekly recordings, we found that the individual rapid decrease in the N1a latency, which may reflect the initiation of myelination in the optic radiation, most frequently occurs at around 37 weeks PMA, regardless of PNA. Preterm extrauterine visual experience has little effect on the myelination process in the visual pathway, but has a marked effect on the developmental changes in VEP wave form which reflect the developmental changes of the neuronal networks in the visual cortex.

Cross-Sectional Studies↗
PubMedDate not supplied

Responses of regular spiking and fast spiking cells in turtle visual cortex to light flashes.

Sharp electrodes were used to record light-evoked postsynaptic potentials (PSPs) from neurons in turtle visual cortex in an in vitro preparation of the geniculocortical pathway. Neurons were placed into four groups based on the firing patterns produced by intracellular current injections: regular spiking (RS), fast spiking (FS), intrinsic bursting (IB), and chattering (CH) cells. RS cells have been shown to be pyramidal cells while FS cells are typically interneurons. Light stimuli were diffuse, 1-s flashes of 640-nm light with intensities (I) varying from 0 to 10(4) photons microm(-2) s(-1). The response (R) in each case was the maximal amplitude of the light-evoked depolarizing PSP. Cells of all four types showed sigmoidal intensity-response (IR) functions with a linear rising phase for stimuli above the intensity threshold followed by saturation at high light intensities. Responses at high intensities were variable and some cells showed indications of supersaturation. Light-evoked PSPs had longer latencies and times-to-peak response in RS cells than they did in FS cells. RS cells fired action potentials as much as 200 ms later than did FS cells. Since responses recorded in RS cells at light intensities just above threshold are unlikely to involve contributions from other pyramidal cells, these data indicate that the geniculocortical or feedforward pathway to pyramidal cells has a high gain. The fact that FS cells fire well before RS cells suggests that feedforward inhibition plays a role in controlling the gain of the geniculocortical pathway.

Animals↗
PubMedDate not supplied

Spatiotemporal structure of depolarization spread in cortical pyramidal cell populations evoked by diffuse retinal light flashes.

The spatiotemporal structure of cortical activity evoked by diffuse light flashes was investigated in an isolated eyecup-brain preparation of the pond turtle, Pseudemys scripta. By combining a photomicroscopic image of the preparation with voltage-sensitive dye signals recorded by a 464-element photodiode array, the spread of depolarization within different cortical areas could be directly visualized with millisecond temporal resolution. Diffuse stimulation of the contralateral eyecup initially depolarized the visual cortex at the junction between its lateral and medial divisions in a small area rostral of the ventricular eminence. From this point, the depolarization spread at different velocities (10-100 microm/ms) depending upon the direction of travel. Since the initial depolarization was always in the rostral pole, the largest spread invariably occurred in a rostral --> caudal direction. Within the confines of the medial visual cortex, depolarization spread at a constant velocity but slowed after entering the adjoining medial cortex. Increasing the stimulus illuminance increased the velocity of spread. Rostrocaudal spread of depolarization was also observed in response to electrical stimulation of the geniculocortical pathway and by direct focal stimulation of the cortical sheet. These data suggest that excitatory connections between pyramidal cell clusters play a prominent role in the initial activation of the cortex by diffuse retinal stimulation.

Animals↗
PubMed1996

P2 flash visual evoked response delay may be a marker of cognitive dysfunction in healthy elderly volunteers.

In an earlier study, 31 healthy elderly volunteers had normal cognitive function as measured by the Mini-Mental State Examination. Twenty-seven returned for further memory testing using the Wechsler Memory Scale-Revised (WMS-R). The P2 latency of the flash visual evoked response was positively correlated with age (p = .0008), but was not significantly related to gender. Nine of these 27 putatively healthy subjects had a delayed P2, suggestive of dementia. Although unaware of any memory deficits, 5 of the 27 had WMS-R Visual Memory Span percentile scores 1 or more standard deviations less than age-matched controls. Four of the five also had a significantly delayed P2 component. This positive correlation, adjusted for age, between poor Visual Memory Span performance and a delayed P2 was statistically significant (p < .025). These findings suggest that a delayed P2 in putatively healthy subjects is indicative of a visuospatial deficit which might be a precursor of dementia later.

Aged↗
PubMedDate not supplied

A laser flash photolysis study of di-, tri- and tetrafluorinated phenylnitrenes; implications for photoaffinity labeling.

Several fluorinated aryl azides (2,6 and 2,4-difluorophenyl azide, 2,4,6-triflurorophenyl azide, and methyl 4-azido-2,3,5,6-tetrafluorobenzoate) were studied by laser flash photolysis techniques. In each case, the azides from singlet nitrenes which can be intercepted with pyridine to form ylides. This allowed determination of the rate constant for ring expansion of the singlet nitrene to the dehydroazepine and measurement of the absolute rate constants for reaction of the singlet nitrenes with typical quenchers.

Affinity Labels↗
PubMed2000

Conformational modulation of human cytochrome P450 2E1 by ethanol and other substrates: a CO flash photolysis study.

The alcohol-inducible cytochrome P450 2E1 is a major human hepatic P450 which metabolizes a broad array of endogenous and exogenous compounds, including ethanol, low-molecular weight toxins, and fatty acids. Several substrates are known to stabilize this P450 and inhibit its cellular degradation. Furthermore, ethanol is a known modulator of P450 2E1 substrate metabolism. We examined the CO binding kinetics of P450 2E1 after laser flash photolysis of the heme-CO bond, to probe the effects of ethanol and other substrates on protein conformation and dynamics. Ethanol had an effect on the two kinetic parameters that describe CO binding: it decreased the rate of CO binding, suggesting a decrease in the protein's conformational flexibility, and increased the photosensitivity, which indicates a local effect in the active site region such as strengthening of the heme-CO bond. Other substrates decreased the CO binding rate to varying degrees. Of particular interest is the effect of arachidonic acid, which abolished photodissociation in the absence of ethanol but had no effect in the presence of ethanol. These results are consistent with a model of P450 2E1 whereby arachidonic acid binds along a long hydrophobic binding pocket and blocks exit of CO from the heme region.

Arachidonic Acid↗
PubMed1995

Laser-flash kinetic analysis of the fast electron transfer from plastocyanin and cytochrome c6 to photosystem I. Experimental evidence on the evolution of the reaction mechanism.

The reaction mechanism of electron transfer from the interchangeable metalloproteins plastocyanin (Pc) and cytochrome c6 (Cyt) to photooxidized P700 in photosystem I (PSI) has been studied by laser-flash absorption spectroscopy using a number of evolutionarily differentiated organisms such as cyanobacteria (Anabaena sp. PCC 7119 and Synechocystis sp. PCC 6803), green algae (Monoraphidium braunii), and higher plants (spinach). PSI reduction by Pc or Cyt shows different kinetics depending on the organism from which the photosystem and metalloproteins are isolated. According to the experimental data herein reported, three different kinetic models are proposed by assuming either an oriented collisional reaction mechanism (type I), a minimal two-step mechanism involving complex formation followed by intracomplex electron transfer (type II), or rearrangement of the reaction partners within the complex before electron transfer takes place (type III). Our findings suggest that PSI was able to first optimize its interaction with positively charged Cyt and that the evolutionary replacement of the ancestral Cyt by Pc, as well as the appearance of the fast kinetic phase in the Pc/PSI system of higher plants, would involve structural modifications in both the donor protein and PSI.

Biological Evolution↗
PubMed1993

Flow-flash study of the reaction between cytochrome bo and oxygen.

The reaction between reduced cytochrome bo from Escherichia coli and oxygen has been studied using flash photolysis of the CO complex of the reduced protein after rapid mixing with oxygen. Absorbance changes were monitored in the alpha and Soret spectral regions. Two kinetic phases taking place at catalytically competent rates could be detected. The apparent rate constant obtained for both the first and second phase showed a hyperbolic dependence on the oxygen concentration. For the first phase, we obtained limiting first- and second-order rate constants at saturating and low oxygen concentrations of 4.5 x 10(4) s-1 and 1.6 x 10(8) M-1 s-1, respectively. The corresponding values for the second phase were 5 x 10(3) s-1 and 1.7 x 10(7) M-1 s-1. The first phase accounted for 30% of the total absorbance change in the Soret band (430 nm) and 15% of the total absorbance change in the alpha band (555 nm). These reactions are followed by a very slow phase with a lifetime of about 1 s. We have also studied the interaction between the fully oxidized enzyme and hydrogen peroxide, and we have found that peroxide binding induces an absorbance increase in the alpha band and a red shift of the Soret band. A consideration of the magnitude of the absorbance changes taking place during the first phase suggests that this reaction includes at least partial oxidation of the low-spin cytochrome b.

Cytochrome b Group↗
PubMed1993

Amino acid residues in Anabaena ferredoxin crucial to interaction with ferredoxin-NADP+ reductase: site-directed mutagenesis and laser flash photolysis.

Ferredoxin (Fd) functions in photosynthesis to transfer electrons from photosystem I to ferredoxin-NADP+ reductase (FNR). We have made several site-directed mutants of Anabaena 7120 Fd and have used laser flash photolysis to investigate the effects of these mutations on the kinetics of reduction of oxidized Fd by deazariboflavin semiquinone (dRfH.) and the reduction of oxidized Anabaena FNR by reduced Fd. None of the mutations influenced the second-order rate constant for dRfH. reduction by more than a factor of 2, suggesting that the ability of the [2Fe-2S] cluster to participate in electron transfer was not seriously affected. In contrast, a surface charge reversal mutation, E94K, resulted in a 20,000-fold decrease in the second-order rate constant for electron transfer from Fd to FNR, whereas a similar mutation at an adjacent site, E95K, produced little or no change in reaction rate constant compared to wild-type Fd. Such a dramatic difference between contiguous surface mutations suggests a very precise surface complementarity at the protein-protein interface. Mutations introduced at F65 (F65I and F65A) also decreased the rate constant for the Fd/FNR electron transfer reaction by more than 3 orders of magnitude. Spectroscopic and thermodynamic measurements with both the E94 and F65 mutants indicated that the kinetic differences cannot be ascribed to changes in gross conformation, redox potential, or FNR binding constant but rather reflect the protein-protein interactions that control electron transfer. Several mutations at other sites in the vicinity of E94 and F65 (R42, T48, D68, and D69) resulted in little or no perturbation of the Fd/FNR interaction.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acids↗
PubMed1992

Thylakoids from pea seedlings grown under intermittent light: biochemical and flash-spectrophotometric properties.

Thylakoid membranes were isolated from pea seedlings grown under intermittent light (2-min light/118-min dark cycles). These preparations differed from controls (thylakoids from plants grown under 16-h light/8-h dark cycles) in the following respects: 15 times smaller chlorophyll/protein ratio, 10 times greater chlorophyll a/b ratio, absence of light-harvesting chlorophyll a/b binding proteins, and 2-3-fold greater ratio of photosystem II over photosystem I. In addition we found the following: (1) Electrogenic electron transfer around cytochrome b6/f under flashing light was greatly enhanced, probably as a consequence of the greater photosystem II/photosystem I ratio. (2) The rate of proton uptake from the medium at the acceptor side of photosystem II was enhanced, probably by unshielding of the quinone binding domain. (3) The N,N'-dicyclohexylcarbodiimide sensitivity of the proton-pumping activity of photosystem II was absent, which was consistent with the attribution of a N,N'-dicyclohexylcarbodiimide-induced protonic short circuit to chlorophyll a/b binding proteins. (4) The sensitivity of oxygen evolution under continuous light to variations of pH or the concentration of Ca2+ was altered. Chlorophyll a/b binding proteins serve as light-harvesting antennas. We found in addition that they modulated the activity of water oxidation and, in particular, the proteolytic reactions around photosystem II.

Cell Fractionation↗
PubMed1994

Laser flash absorption spectroscopy study of ferredoxin reduction by photosystem I in Synechocystis sp. PCC 6803: evidence for submicrosecond and microsecond kinetics.

The kinetics of reduction of soluble ferredoxin by photosystem I (PSI), both purified from the cyanobacterium Synechocystis sp. PCC 6803, were investigated by flash-absorption spectroscopy between 460 and 600 nm. Most experiments were made with isolated monomeric PSI reaction centers prepared with the detergent beta-dodecyl maltoside. Analysis of absorption transients, in parallel at 480 and 580 nm and under several conditions, shows the existence of three different first-order components in the presence of ferredoxin (t1/2 approximately 500 ns, 20 microseconds, and 100 microseconds). A second-order phase of ferredoxin reduction is also present [k = (2-5) x 10(8) s-1 at pH 8 and at moderate ionic strength]. Similar first-order kinetic components were found with membranes from Synechocystis, with dissolved crystals of trimeric PSI reaction centers from Synechococcus, and also when ferredoxin from Synechocystis is replaced by ferredoxin from Chlamydomonas reinhardtii. The three first-order phases exhibit similar, though not identical, spectra which are consistent with electron transfer from the [4Fe-4S] centers of PSI to the [2Fe-2S] center of ferredoxin and are all attributed to reduction of ferredoxin bound to PSI. At pH 8 and at moderate ionic strength, the dissociation constants associated with each of these components are also similar, with a global value varying between 0.2 and 0.8 microM in different cyanobacterial preparations. The presence of three exponential components is discussed assuming homogeneity of the two partners and using the estimated values for the shortest possible distance of approach of soluble ferredoxin from the different iron-sulfur centers of PSI. It is concluded that the 500-ns phase corresponds to electron transfer from either FA- or FB-, the terminal iron-sulfur acceptors of PSI, to ferredoxin and that the immediate electron donor to ferredoxin is reduced within less than 500 ns. The presence of at least two different types of PSI-ferredoxin complex, all competent in electron transfer, is also deduced from the kinetic behavior.

Cyanobacteria↗
PubMed1994

High-pressure flash photolysis study of hemoprotein: effects of substrate analogues on the recombination of carbon monoxide to cytochrome P450CAM.

The effects of camphor and camphor analogues on the CO recombination kinetics of ferrous cytochrome P450CAM (P450CAM) at 293 K have been studied as a function of hydrostatic pressure (0.1-200 MPa) by means of flash photolysis. At 0.1 MPa, the association rate constant (kon) for substrate-free P450CAM is 8.5 x 10(6) M-1 s-1. Measurements as a function of pressure lead to a determination of the activation volume (delta V not equal to) of +4 cm3 mol-1 for substrate-free protein. This positive delta V not equal to is interesting because the CO association reaction of various hemoproteins, such as myoglobin and hemoglobin, exhibit negative delta V not equal to values [Adachi, S., & Morishima, I. (1989) J. Biol. Chem. 264, 18896-18901; Unno, M., Ishimori, K., & Morishima, I. (1990) Biochemistry 29, 10199-10205]. The binding of d-camphor and some camphor analogues (d-fenchone, 3-endo-bromocamphor, and 3,3,5,5-tetramethylcyclohexanone) into the heme pocket strongly influences the kinetics, i.e., kon is reduced ((1-10) x 10(5) M-1 s-1) and delta V not equal to is altered to a negative value (-14 to -32 cm3 mol-1). The negative delta V not equal to suggests that the effects of camphor and these camphor analogues are due to an increase in the iron-ligand bond formation barrier. On the other hand, the binding of adamantane and norcamphor does not affect the kinetics. This result is particularly surprising because both substrate analogues are located in the immediate vicinity of the CO binding site.(ABSTRACT TRUNCATED AT 250 WORDS)

Camphor↗
PubMed1994

Analyses of pH-induced modifications of the period four oscillation of flash-induced oxygen evolution reveal distinct structural changes of the photosystem II donor side at characteristic pH values.

This study presents a thorough analysis of the reaction pattern of flash-induced oxygen evolution in spinach thylakoids as a function of pH (4.5 < or = pH < or = 9) and the redox state of tyrosine YD in polypeptide D2. Evaluation of the experimental data within the conventional Kok model [Kok, B., Forbush, B., & McGloin, M. (1970) Photochem. Photobiol. 11, 457-475] led to the following results: (1) the probability of the miss factor is strongly pH dependent (with a pronounced minimum near neutral pH) while the double hit factor is less affected; (2) a marked increase of the apparent S0 population arises at alkaline pH in dark-adapted samples where most of the YD is reduced, but this effect is absent if the percentage of PS II containing the oxidized form YDox is high; and (3) the lifetimes of S2 and S3 exhibit a characteristic pH dependence that is indicative of conformational changes of functional relevance within the water-oxidizing complex and its environment; (4) the kinetic interaction of redox states S2 and S3 with YD is characterized by a change of its behavior at a threshold pH of 6.5-7.0; and (5) at acidic pH values the extent of S2 and S3 reduction by YD decreases concomitant with the occurrence of a very fast decay kinetics. On the basis of a detailed discussion of these results and data from the literature, the water oxidase is inferred to undergo structural changes at pH values of 5-5.5 and 6.5-7.0. These transitions are almost independent of the redox state Si and modify the reaction coordinates of the water oxidase toward endogenous reductants.

Hydrogen-Ion Concentration↗
PubMed1991

Uptake and release of protons during the reaction between cytochrome c oxidase and molecular oxygen: a flow-flash investigation.

Changes in pH during the reactions of the fully reduced and mixed-valence cytochrome oxidase with molecular oxygen have been followed in flow-flash experiments, using the pH indicator phenol red. Solubilized enzyme as well as enzyme reconstituted into phospholipid vesicles has been studied. With the solubilized enzyme, a biphasic uptake of one proton from the medium was observed, whereas the reconstituted enzyme gave release of 1.3 protons to the extravesicular medium. It is concluded from these results that a total of two to three protons are taken up during oxidation of the fully reduced enzyme. Kinetic analysis suggests that the proton uptake is initiated by the transfer of the third electron to the oxygen binding site. A reaction scheme that integrates proton transfers and oxygen chemistry is presented.

Animals↗
PubMed1991

Laser flash photolysis studies of the reduction kinetics of NADPH:cytochrome P-450 reductase.

The reduction kinetics of NADPH:cytochrome P-450 reductase have been investigated by the laser flash photolysis technique, using the semiquinone of 5-deazariboflavin (5-dRfH.) as the reductant. Transients observed at 470 nm at neutral pH indicated that the oxidized reductase was reduced via second-order kinetics with a rate constant of 6.8 X 10(7) M-1 s-1. The second-order rate constant corresponding to the formation of the protein-bound semiquinone (measured at 585 nm) was essentially the same as that obtained at 470 nm (7.1 X 10(7) M-1 s-1). Subsequent to this rapid formation of protein-bound semiquinone, a partial exponential decay was observed at 585 nm. The rate of this decay remained invariant with protein concentration between pH 5.0 and 7.0, and a first-order rate constant of 70 s-1 was obtained for this process. This is assigned to intramolecular electron transfer from FADH. to FMN. Prior reduction of the enzyme to the one-electron level led to a decrease in both the second-order rate constant for reduction (2 X 10(7) M-1 s-1) and the first-order intraflavin electron transfer rate constant (15 s-1). The protein-bound FAD moiety of FMN-depleted reductase was reduced by 5-dRfH. with a second-order rate constant that was identical with that observed with the native enzyme (6.9 X 10(7) M-1 s-1). However, with this species no significant decay of the FAD semiquinone was observed at 585 nm following its rapid formation, consistent with the above assignment of this kinetic process.(ABSTRACT TRUNCATED AT 250 WORDS)

Animals↗