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PubMed1977

The carotenoid shift in Rhodopseudomonas sphaeroides. The flash induced change.

A mutant, Rhodopseudomonas sphaeroides GIC, having only one major carotenoid, neurosporene, is described. The spectrum of the carotenoid shift in this mutant is analysed and it is concluded that only 7-11% of the pigment is involved under conditions of steady-state illumination and that this pigment undergoes a shift of 7 nm. The spectrum of the carotenoid shift under conditions of multi-flash illumination is examined for changes in shape concordant with a progressive red shift of the pigment with increasing membrane potential; the spectra of the fast change after each of three flashes does not agree well with predictions from a model involving a progressive shift of the pigment, the slow change shows qualitative agreement with such a model but the small size of the signal and the presence of more than one phase makes analysis of this phase more difficult. No separate pool of carotenoid, that might correspond to that postulated to participate in the carotenoid shift, could be identified by fourth derivative analysis of, or curve fitting to, the spectrum of the neurosporene.

Antimycin A↗

PubMed1977

The rise in chlorophyll a fluorescence yield and decay in delayed light emission in tris-washed chloroplasts in the 6-100 microseconds time range after an excitation flash.

Parallel measurements of the rise in chlorophyll a fluorescence yield and delayed light emission decay, after a 10 ns saturating excitation flash, have been made in tris (hydroxymethyl)aminomethane-washed chloroplasts. Various electron donor systems (Mn2+; ascorbate; reduced phenylenediamine and benzidine) were used in conjuction with different preillumination regimes to alter [P+-680], the oxidized form of the Photosystem II reaction center chlorophyll a. Conditions giving rise to high [p+ -680] resulted in only a small rise in fluorescence yield, an inhibition of a 6 microseconds component of delayed light emission. These results confirm the hypothesis that P+-680 acts as a quencher of fluorescence and that delayed light emission in the microsecond time range is due to the back reaction of P+-680 and Q-. (Q is the first "stable" electron acceptor of Photosystem II.) Two preillumination flashes are required before the full effect of Tris washing is observed in the delayed light emission decay and fluorescence yield rise; this suggests that a capacity to hold two charges exists between the Tris block and P+-680. Tris washing has no direct effect on the movement of electrons from Z (the first electron donor to P+-680. Finally, Mn2+ donates electrons to P+-680 via Z.

Ascorbic Acid↗

PubMed1978

Kinetics of reduction of the oxidized primary electron donor of photosystem II in spinach chloroplasts and in Chlorella cells in the microsecond and nanosecond time ranges following flash excitation.

Absorption changes (deltaA) at 820 nm, following laser flash excitation of spinach chloroplasts and Chlorella cells, were studied in order to obtain information on the reduction time of the photooxidized primary donor of Photosystem II at physiological temperatures. In the microsecond time range the difference spectrum of deltaA between 750 and 900 nm represents a peak at 820 nm, attributable to a radical-cation of chlorophyll a. In untreated dark-adapted material the signal can be attributed solely to P+-700; it decays in a polyphasic manner with half-times of 17 microseconds, 210 microseconds and over 1 ms. The oxidized primary donor of Photosystem II (P+II) is not detected with a time resolution of 3 microseconds. After treatment with 3--10 mM hydroxylamine, which inhibits the donor side of Photosystem II, P+II is observed and decays biphasically (a major phase with t1/2=20--40 microseconds, and a minor phase with t1/2 congruent to 200 microseconds), probably by reduction by an accessory electron donor. In the nanosecond range, which was made accessible by a new fast-response flash photometer operating at 820 nm, it was found the P+II is reduced with a half-time of 25--45 ns in untreated dark-adapted chloroplasts. It is assumed that the normal secondary electron donor is responsible for this fast reduction.

Chlorella↗

PubMed1979

Correlation between ATP synthesis and the decay of the arotenoid band shift after single flash activation of chromatophores from Rhodopseudomonas capsulata.

ATP synthesis and the acceleration of the decay of the carotenoid absorption band shift after single flash excitation of Rhodopseudomonas capsulata chromatophores were compared. The two processes behave similarly with respect to: (1) ADP and Pi concentration; (2) inhibition by efrapeptin and venturicidin, and (3) inhibition by valinomycin/K+ and by ionophores. Taken together with earlier evidence for the electrochromic nature of the carotenoid band shift the data support the contention that positive charge moves outwards across the chromatophore membrane during ATP synthesis and justify the method for determination of the H+/ATP ratio (Petty, K.M. and Jackson, J.B. (1979) FEBS Lett. 97, 367-372). The ability of nucleotide diphosphates in the presence of Pi and Mg2+ to give rise to the acceleration of the carotenoid shift decay closely correlates with the rate of phosphorylation of the nucleotides in steady-state light. Nucleotide triphosphates enhance the decay in parallel with their rate of hydrolysis. Adenylyl imidodiphosphate is itself without effect on the decay of the carotenoid shift and it does not prevent the ADP-induced acceleration. The analogue does prevent the ATP effect but only after repeated flashes.

Adenosine Triphosphatases↗

PubMed1980

The photosynthetic electron transfer chain of Chromatium vinosum chromatophores: flash-induced cytochrome b reduction.

Reduction of a cytochrome b following excitation by a single, short, near-saturating light flash has been demonstrated in Chromatium vinosum chromatophores. The extent of reduction is increased by addition of antimycin. The cytochrome has an alpha-band maximum at 562 nm in the presence of antimycin. The cytochrome b reduction is most readily observed in the presence of antimycin at high redox potential when cytochrome c-555 is oxidised before excitation. Under these conditions the half-time for reduction is about 20 ms, and the extent is about 0.5 mol of cytochrome b reduced per mol of reaction center oxidised. This extent of reduction is observed on the first flash-excitation from the dark-adapted state, and there was no indication that the reaction center quinone acceptor complex acted as a two-electron accumulating system. With cytochrome c-555 reduced before excitation, the extent of cytochrome b reduction is approximately halved. The factors which result in substoichiometric cytochrome b reduction are not yet understood. Agents which appear to inhibit primary acceptor oxidation by the secondary acceptor (UHDBT, PHDBT, DDAQQ, HOQNO, o-phenanthroline), inhibit reduction of the cytochrome b. DBMIB inhibits cytochrome b reduction but does not appear to inhibit primary acceptor oxidation. These observations confirm that a cytochrome b receives electrons delivered from the primary acceptor complex, and indicate that the photoreduced cytochrome b is reoxidised via an antimycin-sensitive pathway.

Antimycin A↗

PubMed1982

Electron transfer after flash photolysis of mixed-valence carboxycytochrome c oxidase.

The light-induced difference spectra of the fully reduced (a2+ a23+-CO) complex and the mixed-valence carboxycytochrome c oxidase (a3+ a23+-CO) during steady-state illumination and after flash photolysis showed marked differences. The differences appear to be due to electron transfer between the redox centres in the enzyme. The product of the absorbance coefficient and the quantum yield was found to be equal in both enzyme species, both when determined from the rates of photolysis and from the values of the dissociation constants of the cytochrome a23+-CO complex. This would confirm that the spectral properties of cytochrome a3 are not affected by the redox state of cytochrome a and CuA. When the absorbance changes after photolysis of cytochrome a23+-CO with a laser flash were followed on a time scale from 1 mus to 1 s in the fully reduced carboxycytochrome c oxidase, only the CO recombination reaction was observed. However, in the mixed-valence enzyme an additional fast absorbance change (k = 7 X 10(3) s-1) was detected. The kinetic difference spectrum of this fast change showed a peak at 415 nm and a trough at 445 nm, corresponding to oxidation of cytochrome a3. Concomitantly, a decrease of the 830 nm band was observed due to reduction of CuA. This demonstrates that in the partially reduced enzyme a pathway is present between CuA and the cytochrome a3-CuB pair, via which electrons are transferred rapidly.

Animals↗

PubMed1996

Photodynamic inactivation of gramicidin channels:a flash-photolysis study.

Photosensitized inactivation of ionic channels formed by gramicidin in the planar bilayer lipid membrane (BLM) has been studied upon exposure of the BLM to single flashes of visible light in the presence of tetrasulphonated aluminium phthalocyanine. The gramicidin photoinactivation is inhibited by the addition of unsaturated phospholipids to the membrane-forming solution as well as by the addition of azide to the bathing solution, consistent with involvement of singlet oxygen. The characteristic time of the photoinactivation (tau) does not change markedly under these conditions. Moreover, tau remains nearly constant upon alteration of the flash energy and the photosensitizer concentration. The value of tau appears to be sensitive to the gramicidin concentration and to the factors affecting the open time of the gramicidin channels, namely the temperature and the solvent used in the membrane-forming solution. The photoinactivation is not observed with covalent gramicidin dimers. The equations derived from the model of Bamberg and Laeuger (J. Membrane Biol. (1973) 11, 177-194), describing the relaxation of the gramicidin-induced conductance after a sudden distortion of the dimer-monomer equilibrium, are shown to explain consistently the time course of the photoinactivation provided that the damage of the gramicidin molecules leads to deviation from the equilibrium.

Gramicidin↗

PubMed1976

Reticular suppression of flash-evoked IPSPs in visual cortex neurons.

In a majority of visual cortex (VC) neurons recorded intracellularly in chronically implanted rabbits, light flashes evoked responses consisting of protracted IPSPs (duration 160-200 msec) followed in some cases by a rebound. Inhibition of spontaneous activity through high frequency reticular stimulation (MRF) or through presentations of non-visual arousing stimuli, was not associated with strong hyperpolarization of the cell membrane. Reticular stimulation evoking both activation and inhibition of spontaneous neuronal activity elicited attenuation or complete elimination elimination (disinhibition) of flash-evoked protracted IPSPs and of the postinhibitory rebounds. Rhythmic neuronal discharges elicited by stimulation of the visual pathway was also reduced during reticular activation, due to attenuation of the IPSP-rebound sequence.

Animals↗

PubMed1991

ON and OFF components of the receptive fields have different latencies to diffuse flashes of light in the superior colliculus of golden hamsters.

Single unit activities were recorded from the superior colliculus in anesthetized hamsters. ON center and OFF center cells without surround showed short (within 150 ms) and long latency (longer than 150 ms) responses to diffuse flashes of light, respectively; ON and OFF center cells with an antagonistic surround and ON-OFF center cells showed both short and long latency responses. We suggest that the short latency discharges represent the activity of the ON component of the receptive field, while the long latency responses correspond to the OFF component. Thus, latencies of unit response to diffuse flashes of light may provide a useful indication of the existence of ON or OFF components in the receptive field.

Animals↗

PubMed1984

Presenile dementia--the use of the flash and pattern VEP in diagnosis.

Flash, pattern reversal and pattern onset-offset VEPs were recorded in 17 patients with a clinical diagnosis of presenile dementia. The results were compared with those from two control groups--17 normal volunteers of equivalent ages and 17 patients from the same psychiatric hospital with affective disorders but no clinical evidence of dementia. The flash P2 component was found to be significantly delayed in the group of patients with dementia, but the latency of the P1 component and pattern VEP components were all within normal limits. This unusual finding presents a useful index for the diagnosis of presenile dementia.

Aged↗

PubMed1994

Calibration and time resolution of lumenal pH-transients in chromatophores of Rhodobacter capsulatus following a single turnover flash of light: proton release by the cytochrome bc1-complex is strongly electrogenic.

The flash-induced proton release into the lumen of chromatophores from Rhodobacter capsulatus was studied with Neutral red as pH-indicator. Calibration of the acidification jump after a single flash yielded a much larger figure, at least 0.8 units, than previously thought. A slow kinetic phase of proton release (85-90% of total) was sensitive to inhibitors of the cytochrome bc1-complex. Its half-rise time, about 10 ms, was the same as the rise time of the electrogenic reaction in the cytochrome bc1-complex that was recorded by electrochromism of carotenoids. The oxidoreduction of the two b-hemes was significantly faster (t1/2 approximately equal to 3 ms). Thus the major electrogenic event in the cytochome bc1-complex is proton and not electron transfer.

Bacterial Chromatophores↗

PubMed1994

Flash-induced electrogenic reactions in the SA(L223) reaction center mutant in Rhodobacter sphaeroides chromatophores.

The charge transfer events in the SA(L223) reaction center mutant Rhodobacter sphaeroides chromatophores were investigated by direct electrometry. Besides the primary charge separation, the small stigmatellin-sensitive electrogenic reaction due to the electron transfer from the primary to the secondary quinone acceptor in the reaction center complex was observed after the first flash. The second flash-induced electrogenic phase of the secondary quinone protonation and subsequent electrogenic reactions of the cytochrome bc1 complex were much slower than those in chromatophores of the wild type. It is suggested that replacement of Ser-L223 by Ala impairs both specific proton-conducting pathways leading to the secondary quinone QB.

Alanine↗

PubMed1981

Light-flash analysis of the photoenzymic repair process in yeast cells. II. Determination of the rate constant for formation of photoreactivating enzymes-pyrimidine dimer complexes and its activation energy term.

As reported in the previous paper, the number of deoxyribodipyrimidine photolyase or photoreactivating enzyme (PRE) molecules per yeast cell was determined by the use of intense light flashes. In the present work, the reaction rate constant for the formation of PRE-substrate complexes, k1, and the activation energy term of k1 were determined by yeast cells in vivo by the use of light flashes. At 30 degrees C, k1 equalled (6.5 +/- 1.1) x 10(-5) (nuclear volume) x (molecule)(-1) sec(-1), which corresponded to 1.1 x 10(5) 1 mole(-1) sec(-1), on the assumption that a nuclear volume is 3 x 10(-15) 1. k1 showed positive temperature dependence as described by the arrhenius expression with an activation energy of 11.8 +/- 1.6 kcal mole(-1).

DNA Repair↗

PubMed1984

Short test flashes produce large tilt aftereffects.

Using a staircase method, the tilt aftereffect (TAE) was measured with test flashes of durations ranging from 10 to 600 msec. The TAE is largest with brief duration test stimuli and reaches an asymptotic value at a test flash duration of 200-300 msec. A series of control experiments rule out a variety of explanations for this finding. The decline in TAE magnitude with increasing test duration is not due to decay, nor can the large TAEs produced by brief stimuli be explained in terms of their lower effective luminance or contrast. The results suggest that brief stimuli are processed by a different subset of processes or channels than are longer stimuli.

Adaptation, Ocular↗

PubMed1989

Optic nerve blockade influences the retinal responses to flash in rabbits.

The presence of retinopetal fibers in mammals has been debated many times in the past two decades. Do rabbits have a retinopetal system? This question is addressed with the present investigations. In anesthetized and paralysed rabbits the b-wave and the oscillatory potentials (OP) are recorded at the cornea. The optic nerve is isolated retrobulbarly and is gently hooked to a curved injecting capillary. Through the latter lidocaine hydrochloride is pressure injected. This drug interrupts the neuronal flow travelling along the nerve. A steel electrode is positioned in the optic chiasm allowing us to monitor the evoked field potentials from the tested and untested eyes. The optic nerve blockade produces the following observations: (1) the amplitude of the b-wave is not significantly altered; and (2) the amplitudes of the long latency OP are significantly increased. The retinal capacity to respond to a second flash after the application of an initial light pulse was evaluated by varying the interval between the two flashes. After optic nerve blockade the recovery of the retinal responsiveness is considerably slower. Fourier analysis indicated that the highest power increases occurred around 200 Hz. It is difficult to escape the suggestion that rabbits possess retinopetal fibers.

Animals↗

PubMed1991

Visual evoked magnetic fields to flash and pattern in 100 normal subjects.

The practicality of recording visual evoked magnetic fields in 100 subjects 15-87 yr of age using a single channel d.c. SQUID second order gradiometer in an unshielded environment was investigated. The pattern reversal response showed a major positive component between 90 and 120 msec (P100M) while the response to flash produced a major positive component between 90 and 140 msec (P2M). Latency norms of the P100M were more variable than the corresponding P100 and P2 visual evoked potentials. The latency of the P100M may show a steep increase with age in most subjects after about 55 yr whereas only a small trend of latency with age was detected for the flash P2M.

Adolescent↗

PubMed1995

Adaptation mechanisms in spatial vision--II. Flash thresholds and background adaptation.

To examine how the mechanisms of light adaptation affect spatial pattern vision, contrast detection thresholds were measured for sinusoidal (increment-Gabor) probes on flashed backgrounds in the presence of steady adapting backgrounds. The thresholds for all spatial frequencies (1-12 c/deg), flashed-background intensities (dark to 4 log td) and adapting-background intensities (dark to 4 log td) were adequately described by a simple model consisting of a compressive nonlinearity (a modified Naka-Rushton function), a subtractive adaptation factor, and a multiplicative adaptation factor. For all five subjects the compressive nonlinearity was found to vary systematically with spatial frequency; for all but one subject, the subtractive and multiplicative factors were found to be relatively constant.

Adaptation, Ocular↗

PubMed1977

Flash evoked potentials from rat superior colliculus.

In view of reports that the superior colliculus evoked potential from rats is uniquely sensitive to toxic gases, the present study characterized normal flash evoked potentials from unanesthetized rats. The waveform was complex, with at least 5 positive and 5 negative peaks. The waveform originated in the SGS layer, and some components were stable over time if conditions of light intensity, stimulus frequency and dark adaptation were held constant. The greater complexity of the waveforms reported here compared to those described by others can be attributed to both an intense flash stimulus and unanesthetized preparation.

Animals↗