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排序方式: 共有319条查询结果,搜索用时 15 毫秒
261.
Marina Kozuleva Anastasia Petrova Yuval Milrad Alexey Semenov Boris Ivanov Kevin E Redding Iftach Yacoby 《Plant physiology》2021,186(4):1848
Photosynthesis is a vital process, responsible for fixing carbon dioxide, and producing most of the organic matter on the planet. However, photosynthesis has some inherent limitations in utilizing solar energy, and a part of the energy absorbed is lost in the reduction of O2 to produce the superoxide radical () via the Mehler reaction, which occurs principally within photosystem I (PSI). For decades, O2 reduction within PSI was assumed to take place solely in the distal iron–sulfur clusters rather than within the two asymmetrical cofactor branches. Here, we demonstrate that under high irradiance, O2 photoreduction by PSI primarily takes place at the phylloquinone of one of the branches (the A-branch). This conclusion derives from the light dependency of the O2 photoreduction rate constant in fully mature wild-type PSI from Chlamydomonas reinhardtii, complexes lacking iron–sulfur clusters, and a mutant PSI, in which phyllosemiquinone at the A-branch has a significantly longer lifetime. We suggest that the Mehler reaction at the phylloquinone site serves as a release valve under conditions where both the iron–sulfur clusters of PSI and the mobile ferredoxin pool are highly reduced.In high light, O2 reduction to superoxide radical primarily occurs at the phylloquinone cofactors of photosystem I rather than its terminal [4Fe–4S] clusters. 相似文献
262.
Complex formation of platelet membrane glycoproteins IIb and IIIa with the fibrinogen D domain 总被引:2,自引:0,他引:2
R L Nachman L L Leung M Kloczewiak J Hawiger 《The Journal of biological chemistry》1984,259(13):8584-8588
Glycoprotein IIb (GPIIb) and glycoprotein IIIa (GPIIIa) form a macromolecular complex on the activated platelet surface which contains the fibrinogen-binding site necessary for normal platelet aggregation. To identify the specific region of the fibrinogen molecule responsible for its interaction with the GPIIb-GPIIIa complex, purified fragment D1 (Mr = 100,000) and fragment E (Mr = 50,000) were prepared from plasmin digests of purified human fibrinogen. In addition, the polypeptide chain subunits A alpha, B beta, and gamma of fibrinogen were prepared. Using an enzyme-linked immunosorbent assay we have demonstrated that isolated fragment D1 in a solid phase system forms a complex with a mixture of GPIIb and GPIIIa. The binding of the GPIIb-GPIIIa mixture to fragment D1-coated plates reached saturation at 8 nM and to fibrinogen-coated plates at 24 nM. Isolated A alpha, B beta, and gamma chains were not reactive with added glycoproteins. Fragment E coated directly on plastic plates or immobilized on antibody-coated plastic plates did not form a complex with GPIIb-GPIIIa. Only fluid phase fibrinogen and fragment D1 but not fragment E were inhibitory toward formation of a complex between solid phase fibrinogen and GPIIb-GPIIIa. Isolated A alpha, B beta, and gamma chains at concentrations equivalent to fluid phase fibrinogen were inactive. Binding of fragment D1 but not fragment E to the GPIIb-GPIIIa complex was also demonstrated by rocket immunoelectrophoresis of the membrane glycoprotein mixture through a gel containing the individual fragments and subsequent autoradiography of the complex following exposure to 125I-anti-fibrinogen. These observations with isolated platelet membrane glycoproteins provide strong evidence that each of the D domains of the fibrinogen molecule interacts directly with the GPIIb-GPIIIa complex on the activated platelet surface, thus allowing formation of a tertiary molecular "bridge" across the surface of two adjacent activated platelets. 相似文献
263.
Subcellular membrane and granule fractions derived from human platelets contain immunologically identifiable alpha2-macroglobulin and alpha1-antitrypsin. These platelet-derived inhibitors show a reaction of immunologic identity when compared to alpha2-macroglobulin and alpha1-antitrypsin purified from human plasma. Further, the platelet protease inhibitors possessed a similar subunit polypeptide chain structure to their plasma counterparts as determined by sodium dodecyl sulfate-polyacrylamide gel electrophoretic analysis. Studies of the binding of radiolabeled trypsin to the various solubilized platelet subcellular fractions suggest that the granule-associated alpha2-macroglobulin and alpha1-antitrypsin, as well as membrane-associated alpha2-macroglobulin were functionally active. Quantitatively, circulating platelets contain relatively small concentrations of these inhibitors as compared to platelet-associated fibrinogen and factor VIIIAGN. Platelet protease inhibitors may modulate the protease-mediated events involved in the formation of hemostatic plugs and thrombi. 相似文献
264.
Jonna Tomkiewicz Henrik Skovgård Gösta Nachman Mikael Münster-Swendsen 《Experimental & applied acarology》1993,17(1-2):29-40
A relative scale of Leaf Damage Indices (LDI) from 0 to 5 describes the visible injury to leaves of cassava, Manihot esculenta Crantz caused by the cassava green mite, Mononychellus tanajoa (Bondar). As the scale is ordinal and thus not quantitative, the observed LDIs are converted individually to relative loss of chlorophyll on a ratio scale before an average injury is determined. This calibration is required because the ordinal and the ratio scales are not linearly related. A calibration curve was established on the basis of laboratory experiments to determine the chlorophyll content, c, of leaves representing various leaf damage indices. Several monotonously decreasing functions were fitted to the experimental data yielding the following relation
, where c
o
is the chlorophyll content of unijured leaf tissue and a is a constant describing the steepness of the curvilinear relation. This means that LDIs could be converted to relative loss of chlorophyll, d, where
. The photosynthetically active leaf area of plants can be estimated by combining the relative loss of chlorophyll with leaf area assessments and adds the effects of defoliation and suspended growth to the chlorophyll depletion. The difference in photosynthetically active area that arises between uninjured and injured plants over a period of time provides a measure of spider mite injury that can be related to growth and yield. The method integrates the injury inflicted over a period of time, allows successive observations of the same plants, and is rapid and reasonably precise considering the time savings. 相似文献
265.
Locustamyotropin-like immunoreactivity was visualized in the nervous system of Locusta migratoria by means of the peroxidase antiperoxidase method. Highly specific antibodies to the carboxy-terminus of the locustamyotropins were obtained by elution through an affinity column to which Lom-MT II was covalently bound. Specific cells in the nervous system of Locusta migratoria contain substances immunoreactive to anti-locustamyotropin. In total, about 100 cells immunoreactive to the Lom-MT-II antiserum were detected in the head ganglia, in the abdominal neuromeres of the metathoracic ganglion, and in the five free abdominal ganglia. In the brain, immunoreactive cell groups were situated in the inner and outer edge of the tritocerebrum. Prominent axon bundles tightly surround the tractus I to the corpora cardiaca. The corpora allata were innervated by the nervus corporis allati I coming from the corpora cardiaca and by fibers in the nervus corporis allati II originating from cell bodies in the suboesophageal ganglion. Immunoreactive cell bodies in the suboesophageal and abdominal ganglia are distributed along the anterior posterior midline axis, both dorsally and ventrally. The processes of the cell bodies in the abdominal ganglia leave the ganglia and were traced in the respective median nerves into the neurohaemal organs. Since the Lom-MT-II antiserum cross-reacts with all peptides of the locustamyotropin family that have a carboxy-terminus in common, these cells may contain one or several locustamyotropins. The Lom-MT antiserum also recognizes pheromone biosynthesis activating neurohormone, as was revealed by the intensive labeling of suboesophageal cell bodies in Bombyx mori. 相似文献
266.
B A Konkle S S Shapiro A S Asch R L Nachman 《The Journal of biological chemistry》1990,265(32):19833-19838
Platelet glycoprotein Ib is a major platelet membrane protein composed of two disulfide-linked chains, termed the alpha and beta chains. The larger alpha chain (GpIb alpha), a platelet receptor for von Willebrand factor, plays a major role in mediating platelet adhesion to the subendothelium. Our laboratories have previously reported synthesis of a protein in human endothelial cells that is immunoprecipitated with polyclonal and monoclonal antibodies to platelet GpIb alpha. Lopez et al. (Lopez, J. A., Chung, D. W., Fujikawa, K., Hagan, F. S., Papayannopoulou, T., and Roth, G. J. (1987) Proc. Natl. Acad. Sci. U.S.A. 84, 5615-5619) have reported the cloning of GpIb alpha from a human erythroleukemia (HEL) cell cDNA library. Using this clone as probe, we have isolated two partial GpIb alpha clones from a human umbilical vein endothelial cell lambda gt11 cDNA library. These clones were localized within HEL-derived GpIb alpha cDNA by sequence and restriction enzyme analysis. Additionally, they detected the same message species in HEL and tonsilar RNA that was detected with the HEL GpIb alpha cDNA. Low level GpIb alpha mRNA expression was detected in cultured human umbilical vein endothelial cells, which was increased by treatment of the cells with tumor necrosis factor-alpha. This effect was enhanced by pretreatment with interferon-gamma. Additionally, localization of GpIb alpha in endothelium of fresh tonsilar tissue was demonstrated by immunohistochemistry and in situ hybridization. GpIb alpha may play a role in mediating platelet or other effector cell adhesion to activated endothelium. 相似文献
267.
Isolation, identification and synthesis of locustamyotropin (Lom-MT), a novel biologically active insect peptide 总被引:1,自引:0,他引:1
L Schoofs G M Holman T K Hayes A Tips R J Nachman F Vandesande A De Loof 《Peptides》1990,11(3):427-433
A peptide that stimulates the spontaneous contractions of the hindgut of Leucophaea maderae has been purified from extracts of brain-corpora cardiaca/corpora allata-subesophageal ganglion complexes of 9000 adult Locusta migratoria and was designated locustamyotropin or Lom-MT. The primary structure of this 12 residue peptide has been determined Gly-Ala-Val-Pro-Ala-Ala-Gln-Phe-Ser-Pro-Arg-Leu-NH2. The C-terminal sequence (Phe-Ser-Pro-Arg-Leu-NH2) is identical to the C-terminal pentapeptide of the pheromone biosynthesis activating neuropeptide, recently isolated from Heliothis zea, and is also similar to the C-terminal of leucopyrokinin of Leucophaea. Synthetic Lom-MT showed biological as well as chemical characteristics, indistinguishable from those of native Lom-MT. In locust preparations, Lom-MT provoked an increase in frequency, amplitude and tonus of contractions of the oviduct, but was inactive in the same conditions on the locust hindgut preparation. 相似文献
268.
269.
Statistical analyses of DNA sequences have revealed patterns of nonneutral evolution in mitochondrial DNA of mice, humans, and Drosophila. Here we report patterns of mitochondrial sequence evolution in South American marsh rats (genus Holochilus). We sequenced the complete mitochondrial ND3 gene in 82 Holochilus brasiliensis and 21 H. vulpinus to test the neutral prediction that the ratio of nonsynonymous to synonymous nucleotide changes is the same within and between species. Within H. brasiliensis we observed a greater number of amino acid polymorphisms than expected based on interspecific comparisons. This contingency table analysis suggests that many amino acid polymorphisms are mildly deleterious. Several tests of the frequency distribution also revealed departures from a neutral, equilibrium model, and these departures were observed for both nonsynonymous and synonymous sites. In general, an excess of rare sites was observed, consistent with either a recent selective sweep or with populations not at mutation-drift equilibrium. 相似文献
270.
Y chromosome variation of mice and men 总被引:7,自引:5,他引:2
DNA sequences from the nonrecombining portion of the Y chromosome were
compared with autosomal and X-linked sequences from mice and humans to test
the neutral prediction that ratios of polymorphism to divergence are the
same for different genes. Intraspecific variation within Mus domesticus was
compared with divergence between M. domesticus and Mus caroli for Sry, a
region 5' to Sry, and four X-linked genes, Hprt, Plp, Amg, and Glra2. None
of these comparisons revealed significantly reduced variation on the Y
chromosome. Intraspecific variation within humans was compared with
divergence between humans and chimpanzees for three Y-linked loci (Zfy, the
YAP region, and the Sry region), seven X- linked loci (Il2rg, Plp, Hprt,
Gk, Ids, Pdhal, and Dmd), and the beta- globin locus on chromosome 11. In
these comparisons, the observed level of variation on the human Y
chromosome was slightly lower than expected, but was significantly lower in
only one case (Sry region vs. Dmd). These results suggest that the levels
of variability on the Y chromosome in mice and humans are close to expected
values given the effective population size and mutation rates for these
loci. There is at most only a modest reduction in variability that may be
attributed to natural selection (either genetic hitchhiking or background
selection).
相似文献