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 Mammalian metallothioneins (MT) are known to maximally bind 12 copper ions in two six-Cu(I) ion clusters. Using electrospray ionization mass spectrometry of MT at pH 4.5, a four-Cu(I) ion cluster was observed intermediate to a fully formed six Cu(I) in a single domain or a fully formed Cu12MT species. The four-Cu(I) cluster was observed in both MT1 and MT3 isoforms. Addition of increasing amounts of Cu(I) to MT at pH 4.5 resulted in prominent ions whoses masses were consistent with apo-MT, Cu4MT, Cu6MT, and Cu12MT. The cooperativity of cluster formation was reduced at pH 2.5. Addition of Cu(I) to apo-MT at a reduced pH resulted in a series of ions consistent with Cu4 to Cu12MT species. However, formation of the tetracopper MT species remained cooperative at low pH, suggesting that this species is very stable. To determine whether the tetracopper cluster was formed in either the α or β domain, domain peptides of MT3 were used. Addition of Cu(I) to the apo β domain resulted in a peak consistent with the formation of a four-Cu(I) cluster. This is consistent with reports that Cu(I) ions bind preferentially to the β domain of MTs. Received: 2 June 1998 / Accepted: 21 August 1998  相似文献   
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Rey P  Peltier G 《Plant physiology》1989,89(3):762-767
The photorespiratory activity of mesophyll protoplasts of Nicotiana plumbaginifolia has been clearly demonstrated by the presence of a Warburg-effect, the occurrence of an important CO2-sensitive O2 uptake and the effect of some photorespiratory inhibitors on photosynthetic activity. At a nonsaturating dissolved inorganic carbon (DIC) concentration (0.1 millimolar), we observed that the rate of CO2 fixation was 60% lower at 50% O2 compared to that measured at 2% O2. Using 18O2 and mass spectrometry, we measured O2 exchange as a function of light intensity and of DIC concentration. Oxygen uptake measured at the CO2 compensation point (47.4 micromoles O2 per hour per milligram chlorophyll) was three-fold higher than that measured at a saturating CO2 concentration. Cyanide or iodoacetamide, inhibitors of the Calvin cycle, were found to reduce the O2 uptake to the same extent as CO2 saturation. We conclude from these results that the major part of the CO2-sensitive O2 uptake is due to photorespiration. Further, we investigated the effect on net photosynthesis of some inhibitors of the glycolate pathway. At CO2 saturation (10 millimolar DIC), 5 millimolar aminoacetonitrile (AAN), and 1 millimolar aminooxyacetate (AOA) did not cause any significant decrease in net photosynthesis. However, when these two inhibitors were added under a period of active photorespiration (10 minutes at the CO2 compensation point at 20% O2), we observed a decrease in the rate of net photosynthesis at 10 millimolar DIC measured afterward (respectively, 18 and 29%). This inhibition did not appear at 2% O2, but was stronger at 50% O2 (40% for AAN and 47% for AOA). With 0.05 millimolar butyl 2-hydroxy-3-butynoate (BHB) or 0.5 millimolar l-methionine-dl-sulfoximine (l-MSO), rates of net photosynthesis at 10 millimolar DIC were decreased by 10 to 15%. Additional decreases were observed after a period at the CO2 compensation point at 20% O2 (30% for BHB and 20% for l-MSO). From the sites of action of the four inhibitors tested, we suggest the inhibition of photosynthesis occurring after a period of active photorespiration to be due to the toxic accumulation of nonmetabolized phosphoglycolate.  相似文献   
5.
SUMMARY. 1. The overlying water of intact sediment cores was constantly stirred with an impeller at a rate sufficient to mix turbulently the water column and maintain the diffusive boundary layer at a determined thickness. The system allowed standardization of water circulation in laboratory sediment core experiments.
2. Both oxygen concentration and oxygen penetration depth in the sediments decreased, the former by 70% and the latter from 4.2 mm to 2.0 mm, when the overlying water was not stirred for 24 h, as measured with oxygen microelectrodes in a lake sediment core.
3. Oxygen profiles measured in sediment cores in the laboratory were similar to those measured in situ when the overlying water was stirred with an impeller at such a rate that a similar thickness of the diffusive boundary layer at the sediment-water interface developed in the laboratory as that in situ.
4. Sediment oxygen consumption was calculated from: (1) measured oxygen profiles in the diffusive boundary layer and the molecular diffusion coefficient for oxygen in water; (2) the measured oxygen decrease in the top of the sediments and the estimated diffusion coefficient in the sediment; and (3) by oxygen differences in the overlying water after incubation of sediment cores.  相似文献   
6.
Atrazine and diuron resistant clones were isolated from diploid photoautotrophic protoplastderived colonies of Nicotiana plumbaginifolia. Protoplasts were mutagenised with 0.1 mM N-ethyl-N-nitrosourea and colonies were screened for resistance after plating. Selection of calli was carried out on their ability to grow and green on a selective medium containing either atrazine or diuron. Plants were regenerated from most tolerant calli. Herbicide spray showed that plants of 6 and 4 clones were resistant to atrazine and diuron, respectively.Abbreviations Atrazine 2-chloro-4-ethylamino-6-isopropyl-amino-s-triazine - diuron 3-(3,4-dichlorophenyl)-1,1-dimethylurea - NEU N-ethyl-N-nitrosourea - PSII photosystem II  相似文献   
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Living Vessel Elements in the Late Metaxylem of Sheathed Maize Roots   总被引:6,自引:0,他引:6  
The two types of nodal roots of field-grown maize, sheathedand bare, were found to have such different water conductivitiesthat an investigation of the anatomy of their large metaxylemvessels was made. While the vessels of the bare roots were openfor scores of centimetres, those of the sheathed roots werefound to be not vessels but developing vessel elements, withcross walls at 1 mm intervals, and protoplasts. The cross wallsbetween the elements had several unique histochemical properties.Previous investigators have often failed to find the cross wallsbecause they are very easily dislodged during the usual methodsof tissue preparation. They are best identified by microdissectionof fresh xylem. The living elements persist in the late metaxylemup to 20 – 30 cm from the tip. As the roots become longerthan this both the cross walls and the soil sheaths disappearand there is a transition to a bare root with open vessels inthe proximal region. The soil sheath persists a little longerthan the cross walls. The two types are thus stages in a developmentalsequence through which all nodal roots pass. A fundamental differencebetween the two types is in their water status, since the estimatedconductive capacity of a bare root is about 100 times greaterthan that of a sheathed root. These observations point to theneed for a reassessment of the published work on transport ofions into the xylem of grass roots through a reinvestigationof the ‘maturity’ of their xylem vessels. Grass roots, dimorphic roots, ion secretion to xylem, soil sheaths, xylem vessels, xylem differentiation, water conduction, Zea mays L  相似文献   
9.
Ammonia exchange and photorespiration in chlamydomonas   总被引:2,自引:2,他引:0  
Two hours after the addition of l-methionine-dl-sulfoximine to the cell suspension, glutamine synthetase activity was inhibited by more than 90% in air-grown Chlamydomonas reinhardii. Cells continued to take up NH3 from the medium provided that the concentration of dissolved CO2 was high (equilibrated with 4% CO2 in air). This NH3 uptake, about 30% of the control, is discussed in terms of glutamate dehydrogenase activity. Without CO2, or with a low CO2 level, a NH3 excretion was observed, the rate of which depended on the actual concentration of the dissolved CO2. Experiments with 15NH3 demonstrated that no NH3 uptake was masked by this excretion and inversely that no excretion occurred during the uptake in the conditions where it took place. Furthermore, the NH3 excretion observed in the absence of CO2 increased when O2 concentration rose to 15% and was inhibited when 10 millimolar isonicotinic acid hydrazide was supplied to the algal suspension. Thus, NH3 excretion in the presence of l-methionine-dl-sulfoximine seems to be related to a photorespiratory process inasmuch as it presents the same properties with regard to the O2 and the isonicotinic acid hydrazide effects. These results favor the hypothesis that NH3 produced in the medium originates from the glycine to serine reaction. On the other hand, partial inhibition (50%) of photosynthesis by l-methionine-dl-sulfoximine was attributed to uncoupling between electron transfer and photophosphorylation due to NH3 accumulation into the cell.  相似文献   
10.
DURING each step of peptide chain elongation the ribosome shifts up one triplet along the messenger RNA with concomitant movement of the peptidyl-transfer RNA from the donor to the acceptor site. This process, commonly known as translocation, is triggered by a supernatant protein, factor G, which in association with the ribosome cleaves GTP into GDP and inorganic phosphate1,2 and it has been argued that the energy liberated in this reaction is used “to carry the complex one triplet forward”3.  相似文献   
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