Abbreviations: DCMU, 3-(3,4-dichlorophenyl)-1, 1-dimethylurea 相似文献
2. Phenazine methosulfate (PMS) (in the presence or in the absence of ascorbate) reduction appeared to be coupled to P700 photooxidation, as shown by the corresponding transients at 430 and 388 mμ. The absorbance changes at these two wavelengths indicate that the amount of PMS photoreduced was equivalent to that of P700 photooxidized. Higher PMS concentrations accelerate the dark decay of the P700 signal. When PMS alone is present, anaerobiosis caused the dark decay to become more rapid than in the presence of ascorbate.
3. Unlike PMS, other redox agents such as 2,6-dichlorophenolindophenol, N,N,N′,N′-tetramethyl-p-phenylenediamine or diaminodurol in the presence of excess ascorbate, did not noticeably affect the kinetics of the dark decay at 430 or 703 mμ, suggesting that these reduced species are not efficiently coupled to photooxidized P700.
4. The onset and decay rates of the P700 transient in the presence of PMS and excess ascorbate was insensitive to temperature between 25° and o°. However, when the chloroplast sample was frozen at temperatures ranging from −5° to −196°, all reactions ceased. When the frozen (−196°) sample was brought back to the room temperature, the reaction was restored completely. Fresh broken chloroplasts behave similarly. Digitonin-treated chloroplasts persisted down to about −25° but with diminishing magnitude and slower decay. 相似文献
2. The distribution of finger skin temperatures was quite similar at ambient temperature ranges 0 to −10 °C and −10 to −20 °C, while at −20 to −30 °C the finger temperatures were clearly lower.
3. At different ambient temperature ranges, 20–69% of finger temperatures were low enough to cause cold thermal sensations.
4. Sensation of cold was experienced at a finger temperature of 11.6±3.7 °C (mean±SD). 相似文献
At − 196 °C, continuous illumination results in a parallel reduction of C-550 and oxidation of cytochrome b559 high potential. With flash excitation, C-550 is reduced, but only a small fraction of cytochrome b559 is oxidized. The specific effect of flash illumination is suppressed if the chloroplasts are preilluminated by one flash at 0 °C.
At − 50 °C, continuous illumination results in the reduction of C-550 but little oxidation of cytochrome b559. However, complete oxidation is obtained if the chloroplasts have been preilluminated by one flash at 0 °C. The effect of preillumination is not observed in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea.
A model is discussed for the reaction center, with two electron donors, cytochrome b559 and Z, acting in competition. Their respective efficiency is dependent on temperature and on their states of oxidation. The specific effect of flash excitation is attributed to a two-photon reaction, possibly based on energy-trapping properties of the oxidized trap chlorophyll. 相似文献
(2) The response of the barnacle larvae (naupliar and cyprid stages) to elevated temperature was dependent on exposure time and their stage of development.
(3) Among the stages tested, N-6 larvae showed maximum tolerance. Exposure to 37°C did not affect larval survival, but delayed development of N-2 larva to cypris by one day.
(4) Exposure at 40°C delayed, hastened or did not affect the development time of N-2 and N-4 larvae through cypris, depending on exposure time.
(5) Ten mins exposure at 43°C proved lethal to all larval stages with mortality ranging from 20 to 86%.
(6) Development success of the surviving larvae, measured in terms of cypris yield, showed no significant difference from controls, at temperatures below 40°C.
(7) Settlement activity was significantly affected in only those cyprid larvae which were exposed to 43°C for 10 min.
(8) Results of the present study indicate that thermal stress experienced in the once-through cooling system does not have significant impact on survival and development of the barnacle larvae at temperatures of 37–40°C. 相似文献
A specific effort was made to measure the difference spectrum for the photooxidation of P680 under conditions (chloroplasts frozen to −196 °C in the presence of ferricyanide) where a stable, Photosystem II mediated EPR signal, attributed to P680+ has been reported. The difference spectra, however, did not show that P680+ was stable at −196 °C under any conditions tested. Absorbance measurements induced by saturating flashes at −196 °C (in the presence or absence of ferricyanide) indicated that all of the P680+ formed by the flash was reduced in the dark either by a secondary electron donor or by a backreaction with the primary electron acceptor. We conclude that P680+ is not stable in the dark at −196 °C: if the normal secondary donor at −196 °C is oxidized by ferricyanide prior to freezing, P680+ will oxidize other substances. 相似文献
(2) The lower SCP in mummies reared at 25 °C may be partially explained by their smaller size, a negative relationship being observed between SCP and size.
(3) A bimodality was observed in SCP distributions, with two modes around −26 and −22 °C, likely because of presence/absence of gut content.
(4) The type of exposure had a striking impact, mortality being considerably lower under fluctuating regime.
(5) While energy storage is an important factor, vulnerability to chill-injury is supposed to be the primary factor regulating survival at low temperature. 相似文献
At temperatures below −100 °C, the primary reaction of Photosystem II is irreversible. However, at temperatures between −100 and −20 °C a back reaction that is insensitive to 3-(3′,4′-dichlorophenyl)-1,1′-dimethylurea (DCMU) occurs between P680+ and the reduced acceptor.
The amount of reduced acceptor and P680+ present under steady-state illumination at temperatures between −100 and −20 °C is small unless high light intensity is used to overcome the competing back reaction. The amount of reduced acceptor present at low light intensity can be increased by adjusting the oxidation-reduction potential so that P680+ is reduced by a secondary electron donor (cytochrome b559) before P680+ can reoxidize the reduced primary acceptor. The photooxidation of cytochrome b559 and the accompanying photoreduction of C-550 are inhibited by DCMU. The inhibition of C-550 photoreduction by DCMU, the dependence of P680 photooxidation and C-550 photoreduction on light intensity, and the effect of the availability of reduced cytochrome b559 on C-550 photoreduction are unique to the temperature range where the Photosystem II primary reaction is reversible and are not observed at lower temperatures. 相似文献
2. The kinetics of the disappearance of the 648-nm band of cytochrome d with excess cyanide deviates from first-order kinetics at lower temperatures (22 °C) indicating that at least two conformations of the enzyme are involved. At higher temperatures (32 °C) the observed kinetics of the cyanide reaction are first order with a kon = 0.7 M−1·s−1 and with an estimated koff of approximately 5·10−5 s−1.
3. The value of the koff (7·10−4−14·10−4 s−1 at 32 °C) determined from the rate of reduction of cyanocytochrome d by Na2S2O4 or NADH is one order of magnitude larger than the koff value found when the enzyme is in its oxidized state.
4. No effect of cyanide is found on the spectrum of cytochrome a1. 相似文献
1. 1. Spinach chloroplasts were stored in the dark for at least 1 h, rapidly cooled to −40 °C, and illuminated with continuous light or short saturating flashes. In agreement with the measurements of Joliot and Joliot, chloroplasts that had been preilluminated with one or two flashes just before cooling showed a less efficient increase in the yield of chlorophyll a fluorescence upon illumination at −40 °C than dark-adapted chloroplasts. The effect disappeared below −150 °C, but reappeared again upon warming to −40 °C. Little effect was seen at room temperature in the presence of 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU), added after the preillumination.
2. 2. Light-induced absorbance difference spectra at −40 °C in the region 500–560 nm indicated the participation of two components, the socalled 518-nm change (P518) and C-550. After preillumination with two flashes the absorbance change at 518 nm was smaller, and almost no C-550 was observed. After four flashes, the bands of C-550 were clearly visible again.
3. 3. The fluorescence increase and the absorbance change at 518 nm showed the same type of flash pattern with a minimum after the second and a maximum at the fourth flash. In the presence of 100 μM hydroxylamine, the fluorescence response was low after the fourth and high again after the sixth flash, which confirmed the hypothesis that the flash effect was related to the so-called S-state of the electron transport pathway from water to Photosystem 2.
4. 4. The kinetics of the light-induced absorbance changes were the same at each wavelength, and, apart from the size of the deflection, they were independent of preillumination. Flash experiments indicated that the absorbance changes were a one-quantum reaction. This was also true for the fluorescence increase in dark-adapted chloroplasts, but with preilluminated chloroplasts several flashes were needed to approximately saturate the fluorescence yield.
5. 5. The results are discussed in terms of a mechanism involving two electron donors and two electron acceptors for System 2 of photosynthesis.
The results can be explained by the following hypotheses: (a) The primary donor of Photosystem II in its oxidized state, P+, is a fluorescence quencher. (b) Hydroxylamine prevents the secondary electron donor Z from reducing the oxidized reaction center pigment P+ rapidly. This inhibition is dependent on hydroxylamine concentration and is complete at a concentration of 10−2 M. (c) A second donor (not transporting electrons from water) transfers electrons to P+ with a half time of roughly 25 μs. 相似文献
2. A highly active membrane-bound electron transport system has been demonstrated, and functional roles suggested for ubiquinone, two c-type cytochromes ( peaks at 549 and 553 nm at — 196°), and two b-type cytochromes ( peaks at 558 and 564 nm at — 196°).
3. Evidence is presented suggesting that both the b-type cytochromes may be terminal oxidases of the cytochrome o type, and that cytochrome o (558) has an O2 affinity approx. 10 times greater than cytochrome o (565), and a CO affinity only half as great. 相似文献
2. Fluoride shifts the γ-band of the enzyme from 423 to 421 nm and the -band from 597 to 598 nm. The difference spectrum (oxidized enzyme in the presence of fluoride minus oxidized enzyme) has peaks at 400, 453, 482, 605 and 638 nm and troughs at 430, 520, 552 and 674 nm. The changes in absorbance are small (about 3% at absorbance maxima) with respect to those of other hemoproteins.
3. On addition of fluoride to isolated cytochrome c oxidase 3 reactions can be distinguished: (I) a bimolecular binding reaction (Kon = 4 M−1 · s−1 and koff = 2.9 · 10−2s−1 at 25 °C, pH 7.4) contributing at 638 nm and 430 nm; (II) a first-order reaction (k = 2.4 · 10−2) s−1 at 22 °C, pH 7.2) visible mainly at 430 nm and (III) a very slow reaction with a half-time in the order of 10 min.
4. The spectroscopic dissociation constants for the fluoride binding, determined from Hill plots using the absorbance changes at 638 and 430 nm, are similar (7 and 10 mM, respectively, at 22 °C, pH 7.2).
5. A mechanism for the reaction is discussed in which the bimolecular binding reaction is followed by a conformational change of the enzyme-fluoride complex. 相似文献
1. (1) In dark-adapted chloroplasts (i.e. in States S0+S1 according to Kok, B., Forbush, B. and McGloin, M. (1970) Photochem. Photobiol. 11, 457–475), Q, reduced by a flash at low temperature, is reoxidized by a secondary acceptor and the positive charge is stabilized on the Photosystem II donor Z. Although this reaction is strongly temperature dependent, it still occurs very slowly at −60°C.
2. (2) When chloroplasts are placed in the S2+S3 states by a two-flash preillumination at room temperature, the reoxidation of Q− after a flash at low temperature is mainly due to a temperature-independent back reaction which occurs with non-exponential kinetics.
3. (3) Long continuous illumination of a frozen sample at −30°C causes 6–7 reducing equivalents to be transferred to the pool. Thus, a sufficient number of oxidizing equivalents should have been generated to produce at least one O2 molecule.
4. (4) A study of the back reaction in the presence of 3(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU) shows the superposition of two distinct non-exponential reactions one temperature dependent, the other temperature independent.
Abbreviations: DCMU; 3(3; 4-dichlorophenyl)-1; 1-dimethylurea 相似文献
2. Hydrated electrons do not readily reduce the heme of cytochrome c oxidase. This observation supports our previous conclusion that heme a is not directly exposed to the solvent.
3. In a mixture of cytochrome c and cytochrome c oxidase, cytochrome c is first reduced by hydrated electrons (k = 4 · 1010 M−1 · s−1 at 22 °C and pH 7.2) after which it transfers electrons to cytochrome c oxidase with a rate constant of 6 · 107 M−1 · s−1 at 22 °C and pH 7.2.
4. It was found that two equivalents of cytochrome c are oxidized initially per equivalent of heme a reduced, showing that one electron is accepted by a second electron acceptor, probably one of the copper atoms of cytochrome c oxidase.
5. After the initial reduction, redistribution of electrons takes place until an equilibrium is reached similar to that found in redox experiments of Tiesjema, R. H., Muijsers, A. O. and Van Gelder, B. F. (1973) Biochim. Biophys. Acta 305, 19–28. 相似文献