Iron and outer membrane proteins in the susceptibility of Neisseria meningitidis to human serum

The proportion of carrier-isolated Neisseria meningitidis strains sensitive to human serum (37.2%) was found to be significantly higher than that of case-isolated ones (4.1%), although the difference is too low to consider serum-resistance responsible for invasion in this microorganism. Serum-susceptibility was not related to the existence of specific outer membrane proteins, as is the case of N. gonorrhoeae. Iron restriction induced iron-regulated outer membrane proteins in each strain (but not the same proteins in all strains) but without any detectable effect on serum-susceptibility. Iron excess was also unable to induce changes in the susceptibility of N. meningitidis to human serum.     

重金属在根际中的化学行为Ⅲ.土壤中铁形态转化的根际效应及其生态学意义

本文研究了两种熟化水稻土和两种新垦红壤植稻后根际中铁的形态转化。结果表明,水稻土植稻后根际中无定形铁、游离铁、络合态铁及铁的活化度均低于非根际;红壤中除络合态铁的根际内外分布趋势与水稻土相同外,其余均与水稻土相反。穆斯堡尔谱特征表明,根际中氧化铁的四极矩分裂较大,内磁场较低,即根际中氧化铁被活化。差热分析结果表明根际土的持水力强于非根际土。这对根际微生态系统的生态环境保护、物质流的调控及水分的保持和有效利用有重要作用。     

The status of cysteine residues in the extrinsic 33 kDa protein of spinach photosystem II complexes

Two cysteine residues of the extrinsic 33 kDa protein in the oxygen-evolving photosystemII (PS II) complexes were found to exist as cystine residues in situ. The 33 kDa protein, when reduced by 2-mercaptoethanol in either the presence or the absence of 6 M guanidine-HCl (Gdn-HCl), could not rebind with the CaCl₂-treated PS II complexes, from which the 33 kDa protein was removed, and evolve any oxygen. Two sulfhydryl (SH) groups of the 33 kDa protein were easily reoxidized to a disulfide (S-S) bond by stirring under aerobic conditions with the concomitant regaining of both the binding ability to the CaCl₂-treated PS II complexes and the oxygen-evolving activity.The molecular conformation of the 33 kDa protein was examined by circular dichroic (CD) spectrometry in the UV regions to reveal that the conformation in the reduced state was completely different from those of the untreated and reoxidized states. The disulfide (S-S) bond of the 33 kDa protein is thus essential to maintain the molecular conformation required to function.Abbreviations CD circular dichroism - Chl chlorophyll - DMQ 2,5-dimethyl-p-benzoquinone - DTNB 5,5-dithio-bis (2-nitrobenzoic acid) - EDTA ethylendiamine-tetraacetic acid - Gdn-HCl guanidine-hydrochloric acid - PS II photosystem II - SDS sodium dodecylsulfate This paper was presented at the U.S.-Japan Binational Seminar on Solar Energy Conversion, Okazaki, Japan, March 17–21, 1987     

Iron assimilation in plants: reduction of a ferriphytosiderophore by NADH:nitrate reductase from squash

NADH:nitrate reductase (EC 1.6.6.1) from squash (Cucurbita maxima Duch., cv. Buttercup) can catalyze the reduction of a ferriphytosiderophore from barley (Hordeum vulgare L. cv. Europa). Maximal activity occurs at pH 6, with an apparentK _m andV _max of 76 M and 21 nmol·min^-1·(mg protein)^-1, respectively. The ferriphytosiderophore strongly inhibits nitrate reduction catalyzed by nitrate reductase at the optimal pH for nitrate reduction, i.e. 7.5. On the contrary, nitrate is a poor inhibitor of ferriphytosiderophore reduction catalyzed by nitrate reductase at the optimal pH for this reaction, pH 6.0. Thus, squash has the potential to assimilate the iron from a ferriphytosiderophore synthesized by another plant.     

The toxicity of cadmium to barley plants as affected by complex formation with humic acid

An ‘alternating solution’ culture method was used to study the effects of chloride ions and humic acid (HA) on the uptake of cadmium by barley plants. The plants were transferred periodically between a nutrient solution and a test solution containing one of four levels of HA (0, 190, 569 or 1710 μg cm⁻³) and one of five levels of Cd (0, 0.5, 1.0, 2.5 or 5.0 μg cm⁻³) in either a 0.006M NaNO₃ or 0.006M NaCl medium. Harvest and analysis of shoots and roots was after nineteen days. The distribution of Cd in the test solutions between Cd²⁺, CdCl⁺ and HA-Cd was determined in a separate experiment by dialysis equilibrium. In the nitrate test solutions Cd uptake was clearly controlled by Cd²⁺ concentration and was therefore reduced by HA complex formation. In the absence of HA, chloride suppressed Cd uptake indicating that Cd²⁺ was the preferred species. However complex formation with Cl⁻ enhanced uptake when HA was present because of an increase in the concentration of inorganic Cd species relative to the nitrate system. The ratio root-Cd/shoot-Cd remained at about 10 across a wide range of shoot-Cd concentrations, from about 3 μg g⁻¹ (sub-toxic) up to 85 μg g⁻¹ (80% yield reduction). The ability of the barley plants to accumulate ‘non-toxic’ Cd in their roots was thus very limited. Humic acid also had no effect on Cd translocation within the plant and the root/shoot weight ratio did not vary with any treatment. At shoot-Cd concentrations in excess of 50 μg g⁻¹, K, Ca, Cu and Zn uptake was reduced, probably the result of root damage rather than a specific ion antagonism. The highest concentration of HA also lowered Fe and Zn uptake and there was a toxic effect with increasing HA concentration at Cd=0. However the lowest HA level, comparable with concentrations found in mineral soil solutions, only reduced yield (in the absence of Cd) by <5% while lowering Cd uptake across the range of Cd concentrations by 66%–25%.     

Transport of ferrous iron and lactate production inBifidobacterium bifidum var.pennsylvanicus

Initial rates of ferrous iron transport intoBifidobacterium bifidum var.pennsylvanicus were measured at low and high iron concentrations. The low affinity system (LAFIUS) had an apparent K_m of 167 μM, the high affinity system (HAFIUS) had a K_m of 50 μM. Iron removal from preloaded bifidobacteria revealed the existence of a labile and an inert iron pool in the bacterial cells. Iron uptake by the bifidobacteria was associated with lactate production, though lactate production could continue without iron uptake. Cessation of iron uptake and lactate production was not because of an exhaustion of any nutrient nor the accumulation of fermentation end products in the medium. It was apparently the result of an inactivation of the cellular enzyme machinery without replacing it through normal biosynthetic processes.     

Manganese complexes and the generation and scavenging of hydroxyl free radicals

In a wide variety of biological systems non-enzyme complexes of the metals copper (Cu) and iron (Fe) have been shown to enhance oxygen radical damage by increasing the production of an oxidative species generally believed to be the hydroxyl free radical (.OH) via "Fenton" and possibly "Haber-Weiss" type reactions. However, the behavior of the chemically and biologically similar transition metal manganese (Mn) with .OH is unknown. Unlike Fe and Cu, inorganic complexes of Mn are known to exist in high concentrations in certain cells. Three different oxygen free radical generating systems and four .OH detection methods were used to investigate the activity of biologically relevant inorganic Mn complexes. These complexes were compared to compounds reported to scavenge and generate .OH. The direct and indirect effects of Mn on the .OH flux were compared by attempting to distinguish the effects of hydrogen peroxide (H2O2), superoxide (O2-), and .OH through the use of selective scavengers and generators. Mn-EDTA and biologically relevant Mn-pyrophosphates and polyphosphates, in contrast to Fe-EDTA, do not generate .OH in these systems. The results suggest that Mn in various forms does, indeed, inhibit oxy-radical damage mediated by .OH, but only if the .OH production is dependent on the presence of O2- or H2O2. Thus, with .OH, as with O2- and H2O2, Mn complexes appear to behave in a fundamentally different fashion from Cu and Fe.     

Electronic interactions between iron and the bound semiquinones in bacterial photosynthesis. EPR spectroscopy of oriented cells of Rhodopseudomonas viridis

Electron paramagnetic resonance (EPR) spectroscopy of the iron-semiquinone complex in photosynthetic bacterial cells and chromatophores of Rhodopseudomonas viridis is reported. Magnetic fields are used to orient the prolate ellipsoidal-shaped cells which possess a highly ordered internal structure, consisting of concentric, nearly cylindrical membranes. The field-oriented suspension of cells exhibits a highly dichroic EPR signal for the iron-semiquinone complex, showing that the iron possesses a low-symmetry ligand field and exists in a preferred orientation within the native reaction-center membrane complex. The EPR spectrum is analyzed utilizing a spin hamiltonian formalism to extract physical information describing the electronic structure of the iron and the nature of its interaction with the semiquinones. Exact numerical solutions and analytical expressions for the transition frequencies and intensities derived from a perturbation theory expansion are presented, and a computer-simulated spectrum is given. It has been found that, for a model which assumes no preferred orientation within the plane of the membranes, the orientation of the Fe²⁺ ligand axis of largest zero-field splitting (Z, the principal magnetic axis) is titled 64±6° from the membrane normal. The ligand field for Fe²⁺ has low symmetry, with zero-field splitting parameters of |D₁|=7.0±1.3 cm^?1 and |E₁|=1.7±0.5 cm^?1 and $\text{|}\text{E}{}_1\text{D}{}_1\text{|=0.26}$ for the redox state Q₁^?Fe²⁺Q^2?. The rhombic character of the ligand field is increased in the redox state Q₁Fe²⁺Q^?₂, where $\text{0.33>|}\text{E}{}_2\text{D}{}_2\text{|>0.26}$. This indicates that the redox state of the quinones can influence the ligand field symmetry and splitting of the Fe²⁺. There exists an electron-spin exchange interaction between Fe²⁺ and Q^?₁ and Q^?₂, having magnitudes |J₁|=0.12±0.03 cm^?1 and $\text{|J}{}_2\text{|?0.06}\text{cm}{}^{\mathrm{?1}}$, respectively. Such weak interactions indicate that a proper electronic picture of the complex is as a pair of immobilized semiquinone radicals having very little orbital overlap (probably fostered by superexchange) with the Fe²⁺ orbitals. The exchange interaction is analyzed by comparison with model systems of paramagnetic metals and free radicals to indicate an absence of direct coordination between Fe²⁺ and Q^?₁ and Q^?₂. Selective line-broadening of some of the EPR transitions, involving Q^? coupling to the magnetic sublevels of the Fe²⁺ ground state, is interpreted as arising from an electron-electron dipolar interaction. Analysis of this line-broadening indicates a distance of 6.2–7.8 ? between Fe²⁺ and Q^?₁, thus placing Q₁ outside the immediate coordination shell of Fe²⁺.     

Effect of fertilization on exudation,dehydrogenase activity,iron-reducing populations and Fe++ formation in the rhizosphere of rice (Oryza sativa L.) in relation to iron toxicity

Summary To explain the mechanism of iron toxicity, greenhouse and growth chamber (¹⁴CO₂ atmosphere) experiments were carried out. In pot experiments (with a typical iron-toxic soil and a fertile clay) we studied the effect of N, P, K and Ca+Mg fertilization (alone or in combination) on dehydrogenase activity, Fe⁺⁺ formation, and the populations of iron-reducing bacteria in the rhizosphere of rice IR22 and IR42. Fe uptake by the plants was measured at regular intervals. Dehydrogenase activity, the number of N₂-fixing iron-reducing bacteria, and the formation and uptake of Fe⁺⁺ decreased with increased supply of K, Ca, and Mg. This effect was clearer with IR22 (susceptible to iron toxicity) than with IR42 (releatively tolerant). Increased exudation and Fe uptake by IR36 at low nutrient and high Fe supply were recorded in a growth chamber experiment. Nutritional conditions, exudation rate (a measure of metabolic root leakage), the iron-reducing activity of the rhizosphere, and Fe⁺⁺ uptake by wetland rice appear to be clearly related. Iron toxicity is considered a physiological disorder caused by multiple nutritional soil stress rather than by a low pH and high Fe supply per sé.     

Nitrite: a key compound in N loss processes under acid conditions?

Summary Nitrite is very important in N transformation processes because it is an intermediate product in the aerobic nitrification as well as in the anaerobic denitrification process. Under soil conditions whereby aerobic and anaerobic zones are close to each other, the mobile nitrite can be a link between both N transformation processes. Because of its low stability in acid conditions, nitrite can be a key compound in N loss processes.The results are presented in three sets of incubation experiments using soil+added nitrite before and after oxidation of organic matter; soil+added nitrite and various iron oxide minerals; nitrite solutions without soil but with added ferrous iron.It was found that under acid conditions, soil organic matter as well as the soil mineral phase have a stimulating effect on the nitrite decomposition. Conditions favouring the solubility of Fe(III)-compounds and promoting the formation of Fe²⁺ increase the nitrite decomposition, even under slightly acid conditions. Of the gaseous decomposition products, only trace amounts of NO₂ occur while NO is the major component. Conditions whereby NO and NO₂ cannot escape from the medium promote production of some nitrite.