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Rudenko G Henry L Henderson K Ichtchenko K Brown MS Goldstein JL Deisenhofer J 《Science (New York, N.Y.)》2002,298(5602):2353-2358
The low-density lipoprotein receptor mediates cholesterol homeostasis through endocytosis of lipoproteins. It discharges its ligand in the endosome at pH < 6. In the crystal structure at pH = 5.3, the ligand-binding domain (modules R2 to R7) folds back as an arc over the epidermal growth factor precursor homology domain (the modules A, B, beta propeller, and C). The modules R4 and R5, which are critical for lipoprotein binding, associate with the beta propeller via their calcium-binding loop. We propose a mechanism for lipoprotein release in the endosome whereby the beta propeller functions as an alternate substrate for the ligand-binding domain, binding in a calcium-dependent way and promoting lipoprotein release. 相似文献
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Konstantin Z. Budin† 《Genetic Resources and Crop Evolution》2002,49(3):229-235
The present paper provides a review of data from literature sources and results of research carried out at the Institute of PlantIndustry with the aim of identifying genetic centres of formation of late blight resistant genotypes. For the research results, speciesfrom the potato collection of VIR were used. 相似文献
46.
Rainer Gryschko Rainer Kuhnle Konstantin Terytze Jörn Breuer Karl Stahr 《Journal of Soils and Sediments》2005,5(2):101-106
Background, Aims and Scope The German DIN 19730 (1997) describes a method for the extraction of readily available trace elements from soil by shaking the soil with 1 M NH4NO3-solution. Based on this method trigger and action values for the transfer of heavy metals and arsenic from soils to plants have been published in the German Federal Soil Protection and Contaminated Sites Ordinance (BBodSchV 1999). The chemical mechanisms involved in this soil extraction procedure were evaluated in some detail to create requirements to improve environmental risk assessment for soil contaminations.Methods The chemical mechanisms involved when soil is extracted with 1 M NH4NO3-solution were evaluated. This was followed by a laboratory experiment to quantify the formation of soluble metal ammine complexes during the extraction.
Cd, Zn, Ni, Cu, Co and Hg were extracted from 16 soils by 1 M NH4NO3, 1 M KNO3 and water. pH was adjusted in 5 steps between 5.0 to 7.5. The potassium cation (K+) and the ammonium cation (NH4+) behave similarly when cations from soil surfaces are desorbed, because they have almost identical ion radii (e.g. 0.133 and 0.143 nm). K+ does not form ammine complexes with other ions, whereas, due to the increasing formation of NH3 from NH4 by dissociation with rising pH, metal ammine complex formation is an important process in soil extraction when using ammonium salt solutions. A difference in the extraction efficiency of 1 M NH4NO3- and 1 M KNO3-solution for a given soil can therefore be attributed to the formation of soluble metal ammine complexes. Conclusion Our experiments resulted in considerably higher extraction rates of Cu, Cd and Hg by 1 M NH4NO3-solution as compared to 1 M KNO3-solution. This effect, caused by the formation of soluble metal ammine complexes, was only evident in soils with higher readily soluble heavy metal contents and a soil pH above 6 – 6.5 for Cu and 7 – 7.5 for Cd. Further chemical mechanisms involved when soils are extracted with 1 M NH4NO3 are a moderate decrease in pH and an increase in ionic strength. Most of the colloids and parts of soluble metal-organic complexes are precipitated due to the high ionic strength. High ionic strength also decreases the activity of metal-OH+ species and the electrostatic potential of the particle surfaces, which in turn, increases the desorption of heavy metal cations from negatively charged soil surfaces. In contrast, the adsorption of anions like arsenate is favoured by the decreasing electrostatic potential.
The prediction of heavy metal uptake by plants from the results of the 1 M NH4NO3-solution extraction fits well for elements, which are mainly bound by low strength electrostatic forces to the soils. Such conditions are found in acidic soils for Cd and Tl, which have a low tendency for hydrolysis compared to other heavy metals. The correlation between 1 M NH4NO3 soil extraction and plant uptake is less significant for Ni and Zn. Only low positive correlation coefficients have been found for Pb, As, Hg and for the Cu-uptake by wheat. Imprecise prediction of plant uptake of heavy metals by the extraction with 1 M NH4NO3-solution is mainly caused by conditions leading to an overestimation of plant availability such as elements are strongly bound to soils, or low soluble trace element contents in soils. Neutral to alkaline soil pH can also lead to imprecise prediction due to increasing formation of soluble metal-organic (Cu, Pb, Hg) and metal ammine (Hg, Cu, Cd) complexes and less importantly due to the formation of colloids. Therefore, at low 1 M NH4NO3-extractable soil contents usually no high plant contents are to be expected. Recommendation and Outlook Extraction of soil with 1 M NH4NO3-solution is a suitable method for the determination of readily soluble and plant available trace element contents. The chemical soil extraction process may cause misleading predictions of the transfer of trace elements to plants for some soil properties. This knowledge should be used to improve risk assessment of soil contaminations. It has to be considered, that the processes involved in plant uptake of trace elements are too complex to expect that just one soil extraction method can always guarantee a correct prognosis of toxicological significant element contents in plants. Soil analyses may be used for the preliminary examination of suspicious areas and the demarcation of contaminated areas. The results of soil analyses should be checked additionally by plant analyses especially under conditions with a high probability for misleading results by the 1 M NH4NO3-extraction. Alternatively, extraction with 1 M KNO3-solution can be performed to exclude the effect of metal ammine complex formation. 相似文献
47.
Igor Popov 《水产资源保护:海洋与淡水生态系统》2020,30(9):1816-1820
- 1. The freshwater pearl mussel, Margaritifera margaritifera, is a threatened species. One of the threats faced by this species is direct extermination by poachers, and therefore some researchers conceal information on the locations of pearl mussel habitats. For example, some researchers do not publish the names of the rivers where pearl mussels occur, whereas other researchers do.
- 2. Concealing the names of rivers containing pearl mussels makes them unknown not only to potential offenders but also to administrators, nature conservation practitioners, and ordinary citizens. As a result, no protection measures are undertaken, which is all the more regrettable as damage to pearl mussel habitats can occur accidentally.
- 3. Concealing details of the location of pearl mussel habitats in publications has an adverse impact on the work of specialists, as they do not have access to enough information to study and analyse the biology and distribution of these molluscs.
- 4. One river with pearl mussels in the Gulf of Finland was initially described anonymously (as ‘River B’). Unfortunately, the river was subject to severe human impact, and the largest aggregation of pearl mussels was almost exterminated in the process of constructing a new electric supply line.
- 5. Concealing information on pearl mussel habitats in the Russian section of the Baltic Sea basin was also shown to have been a poor decision. Although the threat from potential poachers was insignificant because of the low commercial value of pearl mussels, the authorities remained uninformed about the existence of valuable habitats.
- 6. The question of whether the locations of pearl mussel habitats should be concealed or revealed in scientific publications does not have an unambiguous answer and should be considered on a case‐to‐case basis; however, the cases for which disclosure is reasonable seem to be more numerous.
48.
- 1. An internationally important population of the freshwater pearl mussel (Margaritifera margaritifera) was rediscovered in a small river in north‐west Russia.
- 2. A survey of visible mussels in 2008/2009 indicated an estimated population of 40 000 individuals. This is the largest population currently known in the Leningrad oblast and is comparable with the entire pearl mussel population of some western European countries.
- 3. The average density of visible mussels was 29.6 individuals m?2 in the middle part of the river. In the four largest mussel beds maximum densities of 1000+ individuals m?2 were recorded. Such densities are exceptional and have not been reported elsewhere in Europe during the last 100 years.
- 4. Live juvenile mussels were recorded, indicating that that this population is viable, although further investigation is required to establish its status.
- 5. Analysis of the population age structure, based on the measurement of empty shells, showed an age class distribution similar to those reported for other healthy Margaritifera populations.
- 6. Only two live juveniles were found. However, this is likely to be due to the survey being restricted to counts of visible mussels only, and the age structure being based on the analysis of dead shells.
- 7. The population's current status and possible reasons for its survival in this river are discussed. Conservation measures should include the construction of a fish ladder to make fish migration through the culvert possible, removal of a metal screen preventing fish migration from the upper reaches of the river to the lake, reduction of recreation activities, and providing the local children's camp with water treatment facilities. Copyright © 2011 John Wiley & Sons, Ltd.
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The exchangeable portion of the selectively sorbed 137Cs extractable by a 1 M ammonium acetate solution (α
Ex
) for soils, illite, bentonite, and tripolite was found to increase with the increasing concentration of the competitive cation
M+ (K+ or NH4+) and can be approximated by a logarithmic relationship. For clinoptilolite, the values of α
Ex
did not depend on the concentration of M+. The expression 1 − α
Ex
(C
M=n
)/α
Ex
(C
M
= 16) as a function of the M+ concentration (where α
Ex
(C
M=n
) is the α
Ex
value at the competitive cation concentration equal to 16 mmol/dm3) was proposed to compare the dependence of α
Ex
on the concentration of K+ or NH4+in different sorbents. For soils and illite, these dependences almost coincided, which indicated that the selective sorption
of 137Cs in soils is determined by the presence of illite-group minerals. 相似文献