Biomedical subjects

S Malkin

Publications and source records attributed to S Malkin.

At least 19 recordsLinked to original sources

PubMed2001

Thermal and structural changes of photosynthetic reaction centers characterized by photoacoustic detection with a broad frequency band hydrophone.

Photoacoustic measurements using a broad frequency band hydrophone were carried out in photosynthetic reaction centers (RC) isolated from Rhodobacter sphaeroides R-26 purple bacteria. Data were extracted on enthalpy and volume changes accompanying the primary steps after excitation in the range of 0-500 microseconds aimed at further characterizing the thermodynamic properties of the RC. Quinone titration showed that the volume contraction accompanying the electron transport is sensitive to the molecular species occupying the secondary quinone site. delta VM = 14.4, 7.7 and 4.3 cm3 molar volume contractions were calculated from the measured parameters for 1, 2 and 0.07 quinone/RC after light excitation. Comparing the enthalpy changes (delta H) to the Gibbs free energy data in the literature, a rather large (26%) entropic contribution to the free energy changes (delta G) is estimated for the P*QAQB-->P+QA-QB electron transport (where QA and QB represent primary and secondary quinones, respectively). This is in contrast to previous estimations that delta G = delta H in these processes. On the other hand, only a small (4%) entropic contribution to the delta G of the P*QAQB-->P+QAQB- process is estimated, in agreement with the literature data. Our results are in good agreement with the data obtained earlier (Edens et al. [2000] J. Am. Chem. Soc. 122, 1479-1485).

Electron Transport↗

PubMed2001

Light-induced carbon dioxide and oxygen uptake in plant leaves measured by the photoacoustic method.

Leaf discs, enclosed in a photoacoustic (PA) chamber, generate two types of PA gas-uptake signals under certain conditions. Type I is manifested by a severe signal decrease that develops slowly under very low-light intensity and often reaches negative values. It is partially reversed by low-intensity far-red light. Type II occurs transiently in modulated far-red light. It is manifested by a rapid and dramatic decrease of the PA signal, upon the addition of short-wave background light, which is subsequently reversed. It differs from type-I uptake in that it occurs at much higher total light intensities. A thorough study, including modulation frequency and atmospheric composition dependencies, indicates different mechanisms for the two types of uptakes. Type-I uptake results from CO2 accumulation in the PA cell by leaf respiration and reflects modulations in CO2 solubilization. Type-II uptake likely reflects oxygen photoreduction in photosystem I, occurring prior to the activation of photosynthesis (i.e. during photosynthesis induction). This is supported by the complete suppression of type-II uptake when O2 was removed. Also, type-II uptake was only mildly sensitive to CO2 elimination, whereas type-I uptake was totally dependent on the presence of CO2. Type-II uptake consists usually of two uptake waves. Fluorescence transients measured in parallel give further support to the reality and interpretation of these two uptake waves. PA could thus provide a unique opportunity to monitor oxygen photoreduction in vivo with high sensitivity and time resolution.

Arabidopsis↗

PubMed2000

Excitation energy transfer in aggregates of Photosystem I and Photosystem II of the cyanobacterium Synechocystis sp. PCC 6803: Can assembly of the pigment-protein complexes control the extent of spillover?

The fluorescence profile of Photosystem I/Photosystem II mixtures in different solvent systems shows that both non-hydrophobic and hydrophobic interactions govern their association and control energy transfer from Photosystem II to Photosystem I. The non-hydrophobic interactions lead to a highly efficient excitation energy transfer from Photosystem II to Photosystem I. In view of this, we propose that similar non-hydrophobic interactions, between the Photosystem II and Photosystem I peripheral proteins, also play a significant role in their association in thylakoids that control state transitions in cyanobacteria.

Journal Article↗

PubMed2000

Oxygen uptake photosensitized by disorganized chlorophyll in model systems and thylakoids of greening barley.

Light-dependent oxygen uptake was observed and studied in thylakoids from early greening barley in comparison to oxygen uptake in chlorophyll solutions and in thylakoids from fully green leaves. Substantial oxygen uptake was observed in chlorophyll solutions supplemented with tryptophan, histidine, ascorbic acid or linoleic acid. This uptake was diminished by adding azide, beta-carotene and alpha-tocopherol, which are specific singlet-oxygen quenchers. Illuminated thylakoids from greening barley also exhibited marked oxygen uptake that, likewise, was strongly quenched by azide. In comparison, azide was found not to affect oxygen uptake that is associated with the methyl viologen-catalyzed Mehler reaction. It is reasoned that in the first two cases the oxygen uptake arises from chlorophyll-photosensitized activation of oxygen to the singlet state and its consumption by exogenous or endogenous substrates. In greening, we propose that disorganized chlorophyll photo-sensitizes the oxygen uptake.

Chlorophyll↗

PubMed1999

Differential responses to different light spectral ranges of violaxanthin de-epoxidation and accumulation of Cbr, an algal homologue of plant early light inducible proteins, in two strains of Dunaliella.

Unicellular green algae of the genus Dunaliella, similar to higher plants, respond to light stress by enhanced de-epoxidation of violaxanthin and accumulation of Cbr, a protein homologous to early light inducible proteins (Elips) in plants. These proteins belong to the superfamily of chlorophyll a/b binding proteins. Two Dunaliella strains, D. bardawil and D. salina, were compared for these two responses under light in the UVA, blue, green and red spectral ranges. In D. bardawil, the two stress responses were similarly induced under UVA, blue or red light and to a lesser extent under green light. In D. salina, a similar spectral range dependence was exhibited for violaxanthin de-epoxidation. However, Cbr accumulated only under UVA or blue light but not under green or red light. A strong synergistic effect of a low dose of blue light superimposed on red light resulted in Cbr accumulation. These results reveal strain-specific differences in spectral range requirements of the two light-stress responses. In the two strains, violaxanthin de-epoxidation is triggered under photosynthetically-active spectral ranges but at least in D. salina, Cbr accumulation appears to require a specific light signal additionally to a signal(s) generated by light stress.

Journal Article↗

PubMed1994

The D-E region of the D1 protein is involved in multiple quinone and herbicide interactions in photosystem II.

The region between helices D and E (D-E region) of the D1 protein of photosystem II (PSII) is exposed at the stromal side of the photosynthetic membrane, contains the secondary plastoquinone (QB) binding niche, and is involved in processes at the reducing side of PSII. The role of the D-E region was studied in 27 site-directed mutants generated in the psbAII gene of the cyanobacterium Synechocystis sp. PCC 6803. The photochemical performance of the modified PSII reaction centers was assessed with respect to photoautotrophic growth, oxygen evolution, fluorescence induction, and herbicide inhibition. A few mutations, located at positions presumably involved in essential interactions in the QB binding niche, greatly interfered with PSII performance. On the other hand, mutations in the presumptive loop region between helices D and de resulted in relatively minor effects, indicating a flexible region not critical for photochemical function. Indeed, although more than 80% of the D-E region is phylogenetically invariant, the bulk of the mutations affected the measured parameters only moderately. The significance of the conserved residues appears to be in subtle interactions that optimize the thermodynamic balance between some of the redox components of PSII, as indicated by mild changes in the steady state fluorescence. Many mutations modified tolerances to PSII herbicides. The dispersion of these mutations throughout the D-E region indicates the complex nature of the interactions, direct and indirect, affecting herbicide binding in the QB niche. Mutation of codons Ser221 and Ser222 to Leu221 and Ala222 revealed a new location coordinating the herbicide diuron in the D1 protein.(ABSTRACT TRUNCATED AT 250 WORDS)

Amino Acid Sequence↗

PubMed1993

Measurement of red blood cell-vitamin B12: a study of the correlation between intracellular B12 content and concentrations of plasma holotranscobalamin II.

We have recently reported a new and rapid assay to measure plasma holotranscobalamin II (holo TC II) as a means of exploring vitamin B12 status. In order to further evaluate the significance of plasma holoTC II in determining tissue cobalamin, we have chosen the red blood cell-vitamin B12 (RBC-B12) assay as a measure of tissue vitamin B12 content and studied the relationship between RBC-B12 and plasma holoTC II levels. Plasma holoTC II and RBC-B12 concentrations were concomitantly assayed in 20 hematologically normal controls and cancer patients. In our groups of controls, the mean value of RBC-B12 was determined as 241 +/- 51 pg/ml of packed erythrocytes (PE) with a range varying from 180 to 355 pg/ml PE. Preliminary results obtained in 32 cancer patients revealed lower holoTC II and RBC-B12 levels than the control group and a required threshold value of 70 pg/ml of holoTC II in order to maintain a normal RBC-B12 greater than 180 pg/ml PE.

Erythrocytes↗

PubMed1992

A new way to monitor by-pass restorations of electron transport in inhibited chloroplasts by cyclic electron flow cofactors--a study by modulated fluorimetry.

Inhibition of electron transport in broken chloroplasts by DBMIB, under light-limiting conditions, is shown to be bypassed by PMS in a manner similar to the known effects of the phenylenediamine derivatives TMPD and DAD. These bypasses were demonstrated and further studied by modulated fluorimetry, monitoring DBMIB inhibition by the shift of the steady-state fluorescence towards the Fm level and the release of inhibition by a reverse shift together with establishment of a quenching effect by background far-red light. Comparative studies were also made with electron transport blocked by DCMU or BNT. A weak bypass by TMPD and a weaker one by PMS of the block created by DCMU was observed by modulated fluorimetry. The block created by BNT is similarly shown to be bypassed by TMPD but hardly or not at all by PMS. Bypass effects persisted even in the presence of ascorbate. It appears that, following reduction of the different cofactors by ascorbate in the stroma side, illumination caused the accumulation of a pool of oxidized cofactor molecules in the lumen, which is able to mediate electron transport between reduced plastoquinone and plastocyanin or P-700. The existence and the size of this pool were found to depend largely on the internal pH at the lumen, presenting an artificial system in which electron flow is controlled by the lumenal pH. The bypassing electron transport in the presence of DBMIB presumably avoids the participation of the cytochrome b6f complex. During its occurrence, there is also a strong imbalance in the activities of the two photosystems for linear electron flow, in favor of PS II. These experiments may thus serve to establish an in vitro model system for a future investigation of effects related to changes in the imbalance between the two photosystems and its regulation. Furthermore, this experimental system may also be utilized to study the role of the internal lumenal pH in control of photosynthesis.

Ascorbic Acid↗

PubMed1991

Proton flow through the ATP synthase in chloroplasts regulates the distribution of light energy between PS I and PS II.

The involvement of ATP synthase in the imbalance between the photoactivities of PS I and PS II under light-limiting conditions, was examined in broken lettuce chloroplasts using modulated fluorimetry. The imbalance, in favor of PS II, was minimal and roughly constant between pH 6.5-7.3 (ratio of PS II/PS I activities about 1.1), and maximal at pH 8.5 (ratio of PS II/PS I activities about 1.4). This increase was strongly inhibited by a treatment of the chloroplasts with the CF0 ATP synthase inhibitor DCCD, but unaffected by the CF1 ATPase inhibitor, tentoxin. However, tentoxin plus ADP-P1 did inhibit the high pH-induced increased imbalance. These results, when considered with the previous results on the effect of high pH on proton flux through the ATP synthase, suggest that the rate of such proton flow controls the imbalance between the two photo-systems. It is possible that there is an in vivo fine-tuning regulating mechanism of the photosystems imbalance via the opening and closing of proton gradient dissipation through the ATP synthase. This mechanism may help alleviate photoinhibitory damage.

Adenosine Diphosphate↗

PubMed1991

Light Energy Distribution in the Brown Alga Macrocystis pyrifera (Giant Kelp).

The brown alga Macrocystis pyrifera (giant kelp) was studied by a combination of fluorescence spectroscopy at 77 kelvin, room temperature modulated fluorimetry, and photoacoustic techniques to determine how light energy is partitioned between photosystems I and II in states 1 and 2. Preillumination with farred light induced the high fluorescence state (state 1) as determined by fluorescence emission spectra measured at 77K and preillumination with green light produced a low fluorescence state (state 2). Upon transition from state 1 to state 2, there was an almost parallel decrease of all of the fluorescence bands at 693, 705, and 750 nanometers and not the expected decrease of fluorescence of photosystem II and increase of fluorescence in photosystem I. The momentary level of room temperature fluorescence (fluorescence in the steady state, F(s)), as well as the fluorescence levels corresponding to all closed (F(m)) or all open (F(o)) reaction-center states were measured following the kinetics of the transition between states 1 and 2. Calculation of the distribution of light 2 (540 nanometers) between the two photosystems was done assuming both the ;separate package' and ;spill-over' models. Unlike green plants, red algae, and cyanobacteria, the changes here of the light distribution were rather small in Macrocystis so that there was approximately an even distribution of the photosystem II light at 540 nanometers to photosystem I and photosystem II in both states 1 and 2. Photoacoustic measurements confirmed the conclusions reached as a result of fluorescence measurements, i.e. an almost equal distribution of light-2 quanta to both photosystems in each state. This conclusion was reached by analyzing the enhancement phenomenon by light 2 of the energy storage measured in far red light. The effect of light 1 in decreasing the energy storage measured in light 2 is also consistent with this conclusion. The photoacoustic experiments showed that there was a significant energy storage in light 1 which could be explained by cyclic electron transport around photosystem I. From a quantitative analysis of the enhancement effect of background light 2 (maximum enhancement of 1.4-1.5) it was shown that around 70% of light 1 was distributed to this cyclic photosystem I transport.

Journal Article↗

PubMed1990

Photoacoustic measurements in vivo of energy storage by cyclic electron flow in algae and higher plants.

Energy storage by cyclic electron flow through photosystem I (PSI) was measured in vivo using the photoacoustic technique. A wide variety of photosynthetic organisms were considered and all showed measurable energy storage by PSI-cyclic electron flow except for higher plants using the C-3 carbon fixation pathway. The capacity for energy storage by PSI-cyclic electron flow alone was found to be small in comparison to that of linear and cyclic electron flows combined but may be significant, nonetheless, under conditions when photosystem II is damaged, particularly in cyanobacteria. Light-induced dynamics of energy storage by PSI-cyclic electron flow were evident, demonstrating regulation under changing environmental conditions.

Journal Article↗

PubMed1989

Separate photosensitizers mediate degradation of the 32-kDa photosystem II reaction center protein in the visible and UV spectral regions.

A component of the photosystem II reaction center, the 32-kDa protein, is rapidly turned over in the light. The mechanism of its light-dependent metabolism is largely unknown. We quantified the rate of 32-kDa protein degradation over a broad spectral range (UV, visible, and far red). The quantum yield for degradation was highest in the UVB (280-320 nm) region. Spectral evidence demonstrates two distinctly different photosensitizers for 32-kDa protein degradation. The data implicate the bulk photosynthetic pigments (primarily chlorophyll) in the visible and far red regions, and plastoquinone (in one or more of its redox states) in the UV region. A significant portion of 32-kDa protein degradation in sunlight is attributed to UVB irradiance.

Chlorophyll↗

PubMed1989

Increased Tolerance to Photoinhibitory Light in Paraquat-Resistant Conyza bonariensis Measured by Photoacoustic Spectroscopy and CO(2)-Fixation.

Tolerance to photoinhibition was compared between a paraquat-resistant and a sensitive biotype of Conyza bonariensis (L.). Cronq. Photoinhibitory damage was measured as a decrease in oxygen evolution or energy storage using photoacoustic spectroscopy, or as a decrease of (14)CO(2)-fixation. Prior to exposure to high fluence rates, both biotypes had similar quantum yields of oxygen evolution and energy storage. After exposure to high intensity light, the resistant biotype continued to evolve oxygen and to store energy with a high quantum yield while both energy storage and oxygen evolution were severely reduced in the sensitive biotype. CO(2)-fixation was less rapidly inhibited in the resistant biotype compared to the sensitive one. The data show that the paraquat resistant biotype with its high constitutive levels of the chloroplast localized enzymes of the oxygen detoxification pathway, is also partially protected from photoinhibition. This supports the theory that an enhanced radical scavenging system can give temporary protection against photooxidative damage from a variety of sources.

Journal Article↗

PubMed1989

Safety and effectiveness of cranial electrotherapy in the treatment of tension headache.

One hundred patients were enrolled in a multicenter double-blind study to evaluate the safety and effectiveness of the Pain Suppressor Unit, a cranial electrotherapy stimulator for the symptomatic treatment of tension headaches. Treatment consisted of extremely low level, high frequency current applied transcranially. Pain scores before and after 20 minute treatments of individual headaches as well as patient and physician global evaluations were the primary efficacy variables. Following use of the active unit, patients reported an average reduction in pain intensity of approximately 35%. Placebo patients reported a reduction of approximately 18%. The difference was statistically significant (p = 0.01). The active unit was rated as moderately or highly effective in 40% by physicians, and in 36% by patients. Both physicians and patients scored the placebo unit moderately or highly effective for only 16%. The difference in ordered outcomes was statistically significant (p = 0.004). Approximately 10% of patients in each group reported at least one minor adverse experience. Cranial electrotherapy stimulation is distinct from TENS, and is safe and often effective in ameliorating the pain intensity of tension headaches. It should be considered as an alternative to the chronic usage of analgesics.

Adult↗

PubMed1988

Pulsed photoacoustic detection of flash-induced oxygen evolution from intact leaves and its oscillations.

Photoacoustic signals from intact leaves, produced upon excitation with single-turnover flashes, were shown to be dependent on their position in the flash sequence. Compared to the signal obtained from the first flash, all the others were time-shifted and had increased amplitudes. The signal from the third flash had the largest deviation, whereas that from the second flash deviated only minimally. The amplitude difference of the signals relative to that from the first flash was measured at a convenient time point (5 ms) and showed oscillations of period 4, similar to the O(2)-evolution pattern from algae. These oscillations were strongly damped, tending to a steady state from about the seventh flash on. The extra photoacoustic signal (relative to the first flash) was shown to be inhibited by 3-(3,4-dichlorophenyl)-1,1-dimethylurea, heat treatment, or water infiltration. Its change with flash number, its saturation with increasing flash energy, and the above inhibition criteria indicate that it originates in pulsed O(2) evolution. The sound wave produced by the first flash, however, arose by a photothermal mechanism only, as shown by its linear dependence on the flash intensity and insensitivity to the above treatments. The above flash pattern demonstrates that the photocycle of the S states (i.e., positive charge accumulation before two water molecules can be oxidized in a concerted way to produce molecular oxygen) occurs in intact leaves. It proves the applicability of the photoacoustic method for mechanistic studies of O(2) evolution in leaves under physiological conditions. Water content of leaves is readily measured by this method.

Journal Article↗

PubMed1986

Photosynthetic responses of leaves to water stress, expressed by photoacoustics and related methods : I. Probing the photoacoustic method as an indicator for water stress in vivo.

The effect of leaf desiccation on the photosynthetic activities in vivo was probed by the photoacoustic method. The aim of this research was: (a) To study the photoacoustic signal per se in varied conditions in order to develop this tool as a probe for stress conditions in vivo. (b) To obtain results pertaining to electron transport activities in vivo, and confirm conclusions based on work with isolated chloroplasts, which could otherwise be the result of nonspecific damage occurring during their isolation. Leaf discs from tobacco (Nicotiana tabacum L.) were routinely used, with other species tested also for comparison. Rapid leaf desiccation caused changes in the low frequency photoacoustic signal, attributed both to the mechanism of signal transduction, influenced by changes in the structural parameters of the leaf, and to the direct (nonstomatal) inhibition of gross photosynthesis. The dependence of the photothermal part of the signal on the frequency indicated the presence of two photothermal components, one of which persisted only at low modulation frequencies (below about 100 Hz) and which largely increased with the desiccation treatment. This component was ascribed to a thermal wave which reaches the leaf surface. The other nonvariable photothermal component was ascribed to a thermal wave propagating from the chloroplasts to the surface of the mesophyll cell. Only this component is considered in the ratio of the O(2) signal to the photothermal signal, which is used to estimate the quantum yield of photosynthesis. The specific dependence of the latter ratio on the frequency yielded a comparative quantum yield parameter from its extrapolation to zero frequency, and also indicated stress induced changes in the diffusion of O(2) through the mesophyll cell, reflected by changes in its characteristic slope. The (zero frequency extrapolated) quantum yield was markedly reduced with the progression of the water stress, indicating the inhibition of (gross) phototosynthetic electron transport in vivo. This result was expressed even more emphatically by the stronger inhibition of the photochemical energy storage, obtained by photoacoustic measurements at a high modulation frequency.

Journal Article↗

PubMed1986

Photosynthetic Responses of Leaves to Water Stress, Expressed by Photoacoustics and Related Methods : II. The Effect of Rapid Drought on the Electron Transport and the Relative Activities of the Two Photosystems.

The effect of rapid dehydration of detached tobacco leaves (Nicotiana tabacum L.) on the photochemical apparatus of photosynthesis was studied in vivo by a combination of methods: photoacoustics, chlorophyll a fluorescence, and cytochrome f difference spectroscopy. It was shown that the inhibition of gross O(2) evolution was mainly caused by inactivation of PSII: (a) The saturation curve of cytochrome-f photooxidation by farred (>710 nanometers) light was resistant to the stress, leading to the conclusion that photosystem I (PSI) was largely unaffected by the stress. (b) The extent of the chlorophyll a variable fluorescence arising from photosystem II (PSII) decreased with the progression of the stress, but was largely unaffected when the leaf was preincubated with electron donors to PSII, such as hydroxylamine. It is concluded that the drought damage to PSII occurred on the photooxidative side. Despite the extensive inhibition of PSII and the relative preservation of PSI, the apparent PSII/PSI activity balance was somewhat larger in stressed leaves than in the control, as indicated by photoacoustic measurements of Emerson enhancement. These measurements were performed continuously under conditions which favor transitions to either state 1 or 2, showing that the transition to state 2 was considerably inhibited. Simultaneous measurements of chlorophyll fluorescence induction at 680 and 730 mm at room temperature were also used to probe changes in energy distribution between PSII and PSI and indicated that the transition from a dark adapted state to state 2 was also affected in water-stressed leaves. The saturation curve of the far-red light effect in Emerson enhancement was not changed by the stress, giving another independent evidence for the drought resistance of PSI activity. This apparent preservation of the imbalance in photochemical activities in favor of PSII, despite the fact that PSII is strongly inhibited, and PSI is not, supports a previous suggestion that the electron transfer between the two photosystems is not random but that a large extent of PSII and PSI units are specifically linked.

Journal Article↗

PubMed1985

Modified cryptochrome in vivo absorption in dim photosporulation mutants of Trichoderma.

Mutants of Trichoderma harzianum that are defective in blue-light-induced sporulation were induced by nutritional stresses as an alternative to light. These mutants may be modified in the putative photoreceptor pigment "cryptochrome" or in an early transduction component. dim (dimsighted) mutants induced by a short transient stress were mapped into four complementation groups and were chosen for study of pigment deficiencies by in vivo absorption spectroscopy. Mutants rib(-)10 and lys(-)44 in the dimY complementation group had altered in vivo absorption spectra in the blue region. Difference spectra obtained by subtracting dimY spectra from that of the wild type had difference bands with peaks at 455 and 480 nm. The similarity between the in vivo difference spectra and the action spectrum for sporulation in wild-type Trichoderma suggests that the mutants lack cryptochrome or have a defective cryptochrome. The decrease in photoresponse as well as the modification of the action spectrum near 480 nm in a dimY mutant support these suggestions. Both dimY mutants pleiotropically accumulate a yellow water-soluble pigment absorbing at wavelengths lower than the blue maxima of cryptochrome; this yellow pigment may be related to cryptochrome.

Journal Article↗