A biogeochemical cycle for aluminium?

The elaboration of biogeochemical cycles for elements which are known to be essential for life has enabled a broad appreciation of the homeostatic mechanisms which underlie element essentiality. In particular they can be used effectively to identify any part played by human activities in element cycling and to predict how such activities might impact upon the lithospheric and biospheric availability of an element in the future. The same criteria were the driving force behind the construction of a biogeochemical cycle for aluminium, a non-essential element which is a known ecotoxicant and a suspected health risk in humans. The purpose of this exercise was to examine the concept of a biogeochemical cycle for aluminium and not to review the biogeochemistry of this element. The cycle as presented is rudimentary and qualitative though, even in this nascent form, it is informative and predictive and, for these reasons alone, it is deserving of future quantification. A fully fledged biogeochemical cycle for aluminium should explain the biospheric abundance of this element and whether we should expect its (continued) active involvement in biochemical evolution.     

Does neurotransmission impairment accompany aluminium neurotoxicity?

Neurobehavioral disorders, except their most overt form, tend to lie beyond the reach of clinicians. Presently, the use of molecular data in the decision-making processes is limited. However, as details of the mechanisms of neurotoxic action of aluminium become clearer, a more complete picture of possible molecular targets of aluminium can be anticipated, which promises better prediction of the neurotoxicological potential of aluminium exposure. In practical terms, a critical analysis of current data on the effects of aluminium on neurotransmission can be of great benefit due to the rapidly expanding knowledge of the neurotoxicological potential of aluminium. This review concludes that impairment of neurotransmission is a strong predictor of outcome in neurobehavioral disorders. Key questions and challenges for future research into aluminium neurotoxicity are also identified.     

How can aluminium(III) generate fluorescence?

In a previous paper [F. Launay, V. Alain, E. Destandau, N. Ramos, E. Bardez, P. Baret, J. L. Pierre, New J. Chem. 25 (2001) 1269-1280] [New J. Chem. 25 (2001) 1269], we showed that the hexadentate tripodal ligand O-TRENSOX (O-TR), incorporating three 8-hydroxy-5-sulfoquinoline subunits, was an efficient chelator of Al(III), quantitatively giving the 1:1 chelate in stoichiometric conditions even at the 10(-5) mol L(-1) concentration scale. However, the 1:1 Al:O-TR chelate turned out to be not significantly more fluorescent than the free ligand, whereas fluorescence enhancement by factors of at least 100 occurred either with the 3:1 Al:O-TR chelate, or with the 1:1 complex obtained with n-BUSOX, a ligand similar to one arm of O-TRENSOX. The present paper addresses the unresolved question of the magnitude of the fluorescence enhancement. Time-resolved fluorescence measurements, and additional complexation experiments carried out with the tripod TRENSOXCAMS2 (one 8-HQS and two 5-sulfocatechol subunits) and with n-BUCAMS analogous to one catechol arm of TRENSOXCAMS2, show that stoichiometry between Al(III) and the bound bidentate subunits is the key factor of fluorescence enhancement. The charge density on Al(III), tuned by the number of chelating groups and by their formal charges, influences the photoinduced charge transfer which tends to quench the fluorescence emission of the 8-hydroxyquinoline ligand. Transposition can be done to other bifunctional amphoterous ligands such as morin.     

Complexation of aluminium with (−) -epigallocathechin gallate studied by spectrophotometry

Complexation equilibria between Al(III) and (−)-epigallocathechin gallate (EGCG) in the presence of acetate buffer have been studied by spectrophotometry. The method is based on the competition between EGCG and buffer ligands for Al(III) ions. The apparent formation constant of the EGCG complex for Al(III), which could be determined by measuring the absorbance of the free EGCG, decreased with increasing acetate ion concentration at a fixed pH. This phenomenon has been quantitatively investigated and both types of complexes (EGCG and acetate) could be analyzed. The apparent formation constant of Al(III) complex with EGCG also decreased with decreasing pH at a fixed acetate ion concentration. The pH dependence of the apparent formation constant indicates the 1:1 competition between metal ions and hydrogen ions for the binding site of EGCG. The intrinsic formation constant of Al-EGCG complex, the proton association constant of EGCG and the formation constant of Al-acetate complex are found to be log K_Al-EGCG = 7.6 ± 0.1, log K_H-EGCG = 7.65 ± 0.03 and log K_Al-acetate = 2.07 ± 0.05 by a graphical analysis.     

A role for cyclic hydroxamates in aluminium resistance in maize?

Hydroxamate siderophores have been found to alleviate Al toxicity in bacteria. In Poaceae plants cyclic hydroxamates, like DIMBOA (2,4-dihydroxy-7-methoxy-1,4-benzoxazin-3-one) and its derivatives have mostly been studied in relation to either defence against insects or allelopathy. In this study the influence of Al on concentrations of these benzoxazinoids (Bx) in root tips, whole roots and root xylem exudates of Zea mays L. varieties differing in Al resistance was analyzed by HPLC-MS. Aluminium resistant maize variety Sikuani maintained considerably higher Bx levels in root tips than the Al sensitive variety Bakero. In vitro binding of Al to DIMBOA was shown by fluorescence quenching. Addition of DIMBOA to Al-containing nutrient solution protected the sensitive maize against Al toxicity as shown by bioassays using callose and haematoxylin staining of root tips as stress indicators. This is the first study showing that Bx can detoxify Al in solution. Tissue analysis data provide first, circumstantial, support for a role of Bx in defence against Al toxicity also in planta.     

Speciation studies of aluminium(III)–acetate complexes under physiological conditions

Abstract

The aluminium complexes of acetic acid (ACT) have been studied using Potentiometric titrations under physiological conditions of temperature (37°C) and ionic strength (0.15 dm^?3 dm^?3 NaCI) and at different ligand to metal ratios. The variations of pH were measured with the help of a glass electrode calibrated daily in hydrogen ion concentrations. Results obtained within the pH range of 2.6–4.2 were analysed to determine stability constants using the SUPERQUAD program. Different complex combinations were considered during the calculation procedure, and evidence was found for ML₂ mononuclear species beside binuclear hydroxo-complexes M₂L(OH)₂ and M₂L(OH)₃ and metal ion hydroxides. Speciation calculations based on the corresponding constants were then used to simulate species distributions.     

Does either the gastrointestinal peptide PYY or the neuropeptide NPY bind aluminium?

Peptide YY and neuropeptide Y are common peptides with a high degree of primary and tertiary structural homology. They are multifunctional and participate in a diverse array of distinct activities including regulation of gastrointestinal function and neural regulation of satiety. Recently both have been implicated in aluminium chemistry in vivo although their modus opperandi have not been determined. We have used molecular fluorescence, RP-HPLC, ESMS and equilibrium dialysis to identify if either peptide YY or neuropeptide Y will bind aluminium in vitro under near-physiological conditions. We were unable to demonstrate any direct interaction between either peptide and aluminium although we have speculated upon an in vivo mechanism whereby PYY, in particular, might form a stable complex with aluminium.     

Influence of aluminium on the immune system – an experimental study on volunteers

The purpose of this study was to examine whether oral exposure to aluminum (Al) can affect the human immune system. Eighteen healthy volunteers (mean age 42, 28–57 yr) were divided into a test group (9 females, 4 males) and a referent group (3 females, 2 males). Over 6 weeks, the test subjects ingested 10 ml of antacid (aluminum hydroxide, 59 mg Al/ml) three times daily. Aluminum was analyzed in urine before and during the exposure period (ICP-MS). Blood samples were used for analysis of lymphocyte subpopulations, mitogen-induced lymphocyte proliferation and in vitro production and circulating plasma concentrations of immunoglobulin (Ig) A, IgG, IgM, interleukin (IL) -2 and IL-4. Urinary Al concentration in the test subjects was approximately 10- to 20-fold higher than in the referent group during exposure. This indicates that ingestion of an Al-containing antacid is associated with an Al absorption far above that originating from food and drinking water. In both referents and test subjects the lymphocyte subpopulations, lymphocyte proliferation and the in vitro Ig and IL production showed similar, time-dependent changes before as well as during the exposure period. No major differences were seen between the referent and test groups regarding the immune parameters, except for a slightly smaller CD8+CD45R0+ population (primed cytotoxic T-cells), in the exposed individuals as compared to the referents. The results also show that subjects on antacid therapy may constitute a suitable population for studying biological effects of high-dose oral exposure to Al.     

Does the degree of pectin esterification influence aluminium sorption by the root apoplast?

This study investigates the influence of the degree of pectin esterification (DE) on the sorption of aluminium (Al) by plant roots. Ca-pectates, with varying degrees of esterification, are major constituents of the soil–root interface and of the root apoplast. Ca-pectate networks (Ca–PG and Ca–Al–PG) were formed at three DEs (0%, 26%, 65%) with custom-made cells and used as a model system for the root cell wall. Sorption of Al was conducted for 24 h at a range of oxalic acid concentrations (0–500 μM) at pH 4.50 to examine two different metal resistance mechanisms of plants. In fact, plants release organic acids either to desorb or to complex metals to prevent their sorption by plant roots.Thermal analysis showed that Al sorption did not seem to affect the stability of the pectate gels and the presence of hydrophobic groups (–CH₃) at DE?>?0% seemed to even increase the stability of the gels decreasing thermal decomposition. Results suggest two potential Al tolerance mechanisms: (a) high oxalic acid concentrations (500 μM) were able to desorb almost 100% and 72% at DE 65 and 0%, respectively; (b) high oxalic acid concentrations (500 μM) and thus molar ratios of 5:1 (oxalate/Al) reduced Al sorption by 98% and 86% at DE 65 and 0%, respectively. In conclusion, both mechanisms indicate that high degrees of esterification as 65% are much more efficient in excluding Al from the apoplast and might therefore contribute to Al resistance in plants.     

A β-diketiminato magnesium acetylide and formation of an imido aluminium magnesium hydride compound

The reaction of LiCCMe with [(^Dipnacnac)MgI(OEt₂)] (^Dipnacnac = [(DipNCMe)₂CH], Dip = 2,6-diisopropylphenyl) afforded [{(^Dipnacnac)MgCCMe}₂] 1 in good yield. Treatment of 1 with AlH₃·NMe₃ in toluene under reflux afforded a low yield of the novel imido aluminium magnesium hydride compound [{(AlH)(AlH₂)₃(NDip)₃}₂Mg] 2. The latter compound features a central Mg²⁺ ion that is coordinated via hydride ligands to two monoanionic [(AlH)(AlH₂)₃(NDip)₃]⁻ fragments. The compounds have been characterised by ¹H and ¹³C NMR spectroscopy, IR spectroscopy and the crystal structures of 1 and 2·4C₇H₈ have been determined.     

What is wrong with aluminium? The J.D. Birchall Memorial Lecture

Aluminium chemistry has features in common with two other groups of elements: (1) divalent magnesium and calcium, and (2) trivalent chromium and iron. The essential differences between the first group and aluminium are explored and it is shown that the much higher acidity of aluminium makes it such a powerful competitor for oxygen-donor ligands, opposite functions of both magnesium and calcium, in cells that its presence is damaging. By way of contrast aluminium is a weaker acid than ferric ions but it is more available. It was necessary for iron to be utilised in the presence of aluminium so special methods had to be devised to distinguish between them. In essence aluminium has always, throughout evolution, been a threat to the biological chemistry of all these three elements. We shall examine this chemistry and then explore the relationship of calcium and aluminium under acid rain conditions.     

The relationship between ‘labile soil phosphorus’ and the aluminium and iron-bound phosphorus in tropical soils

Summary When millet was grown in short-term pot experiments in the greenhouse, in the red-clay and sand-veld soils of Southern Rhodesia and the red clay loams of Uganda, a very close correlation was obtained between labile soil phosphorus and the aluminium- and iron-bound phosphate in these soils. A large increase in labile soil phosphorus was obtained when millet was grown in Uganda soils that had been stored in the air-dry condition for 15 weeks, and a greater increase after 24 weeks. The increase is attributed to the mineralisation of soil organic phosphorus. The maximum temperature used to dry the soils had no effect on the labile soil phosphorus values obtained.     

The amelioration of aluminium toxicity by silicon in higher plants: Solution chemistry or an in planta mechanism?

Aluminium (Al) toxicity is a very important factor limiting the growth of plants on acidic soils. Recently, a number of workers have shown that, under certain conditions, silicon (Si) can ameliorate the toxic effects of A1 in hydroponic culture. The mechanism of the amelioration is unclear, but three suggestions have been put forward: Si‐induced increase in solution pH during the preparation of hydroponic solutions; reduced availability of Al due to the formation of hydroxyaluminosilicate (HAS) species in those solutions during plant growth; or in planta detoxification. It is now known that it is possible to make up Al and Si solutions in an order in which pH is lowered prior to Al addition; in these cases amelioration has still been observed. Amelioration has also been noted in experiments where HAS formation is minimal. These observations would suggest that, at least under some circumstances, there is an in planta component to the amelioration phenomenon. Several microanalytical investigations have noted codeposition of Al and Si in root cell walls. We propose a model in which root cell walls are the main internal sites of aluminosilicate (AS) and/or HAS formation and of Al detoxification. Factors promoting AS/HAS formation in this compartment include: high apoplastic pH; the presence of organic substances (e.g. malate); and the presence of suitable local concentrations of reactive forms of Al and Si, on or within the surfaces of the wall matrix. All these are likely to be important in the amelioration of Al toxicity.     

Hydrogénolyse de 3,6,8-trioxabicyclo[3.2.1]octanes par le complexe aluminohydrure de lithium-chlorure d'aluminium

Hydrogenolysis of 3,6,8-trioxabicyclo[3.2.1]octanes by the 1:1 lithium aluminum hydride-aluminum chloride complex leads, through the specific opening of the acetal function at C-5O-6, to 1,4-dioxanes bearing a hydroxymethyl group. The stereo-chemistry of the dioxanes obtained is discussed with emphasis on the intramolecular hydrogen bond.     

Does aluminium affect root growth of maize through interaction with the cell wall – plasma membrane – cytoskeleton continuum?

The mechanism of aluminium-induced inhibition of root elongation is still not well understood. It is a matter of debate whether the primary lesions of Al toxicity are apoplastic or symplastic. The present paper summarises experimental evidence which offers new avenues in the understanding of Al toxicity and resistance in maize. Application of Al for 1 h to individual 1 mm sections of the root apex only inhibited root elongation if applied to the first 3 apical mm. The most Al-sensitive apical root zone appeared to be the 1–2 mm segment. Aluminium-induced prominent alterations in both the microtubular (disintegration) and the actin cytoskeleton (altered polymerisation patterns) were found especially in the apical 1–2 mm zone using monoclonal antibodies. Since accumulation of Al in the root apoplast is dependent on the properties of the pectic matrix, we investigated whether Al uptake and toxicity could be modulated by changing the pectin content of the cell walls through pre-treatment of intact maize plants with 150 mM NaCl for 5 days. NaCl-adapted plants with higher pectin content accumulated more Al in their root apices and they were more Al-sensitive as indicated by more severe inhibition of root elongation and enhanced callose induction by Al. This special role of the pectic matrix of the cell walls in the modulation of Al toxicity is also indicated by a close positive correlation between pectin, Al, and Al-induced callose contents of 1 mm root segments along the 5 mm root apex. On the basis of the presented data we suggest that the rapid disorganisation of the cytoskeleton leading to root growth inhibition may be mediated by interaction of Al with the apoplastic side of the cell wall – plasma membrane – cytoskeleton continuum. This revised version was published online in June 2006 with corrections to the Cover Date.     

Effects of aluminium and micro-nutrients on the sorption of phosphorus byTrifolium repens L. cv. ‘Grasslands Huia’ from nutrient solutions during plant induced pH changes

Summary The sorption of phosphorus from nutrient solution and the pH change in the nutrient solution were monitored over a 24 hour period forTrifolium repens L. cv. ‘Grasslands Huia’ plants. Two different concentration levels of micro-nutrients (B, Cu, Fe, Mn and Zn) and Al formed the factors of a fractional replicate of a 2⁶ factorial design. Measurements were made at four time intervals (30 minutes after the plants were placed on the pots, 3 hours, 6 hours and 24 hours later). In addition to phosphorus, fourteen other nutrients (including nitrate and ammonium) were monitored throughout the experiment. The sorption of phosphorus was significantly influenced by both aluminium and iron. The effect of aluminium and iron on phosphorus sorption is attributed to physico-chemical sorption processes involving the root surface. However the effect on the removal of phosphorus by boron, copper, manganese and zinc was observed only as first order interaction effects —B−Zn, Cu−Zn, Mn−Zn. Thus these three elements (B, Cu and Mn) only affect phosphorus removal in conjunction with zinc. Aluminium and iron together had a separate but very significant effect on the removal of phosphorus at most periods throughout the experiment. In contrast, pH was affected only by aluminium, iron (the pH drop was enhanced) and manganese (the pH drop was decreased) as main effects independent of the other treatment elements.     

Combined effects of air and soil pollution by fluoride emissions on Tibouchina pulchra Cogn., at Cubatão,SE Brazil,and their relations with aluminium

High deposition of gaseous/particulate fluorides and other air pollutants has resulted in an acidification and probable formation of soluble AlF_x complexes in the soil in the vicinity of the industrial complex of Cubatão, SE Brazil. With the present field study we aimed at determining the contribution of F and Al uptake from fluoride-contaminated soil, supposedly as AlF_x complexes, to the increase of foliar F and Al contents in saplings of an Al-accumulator tree species (Tibouchina pulchra) which were concomitantly exposed to fluoride-contaminated air and also the proportional contribution of both air and soil contamination to the mentioned foliar accumulation of these elements. The seasonal variations in F and Al accumulation and possible metabolic changes in the plants due to F and Al accumulation were also investigated. The saplings were exposed during three consecutive periods of 16 weeks to: (a) air and soil from a reference site (PV_noF); (b) air or soil from two polluted sites (CM-high air pollution, low F and MV-high air pollution, high F); and (c) both air and soil from these polluted sites. After exposure, the changes in the foliar concentrations of F and Al, the relations between both element contents and their relationships with oxidative stress indicators were determined. The data were grouped in three matrices: PV_noF–CM_lwF and PV_noF–MV_hgF, taking in account the possible air/soil exposure combinations in each, and soil/air from all sites. The slight F accumulation in plants of PV_noF–CM_lwF matrix was a result of higher uptake from soil than from air (54 and 46%, respectively). At PV_noF–MV_hgF matrix, the extremely high F accumulation in leaves of T. pulchra could be attributed to the combination of both air and soil contamination (83 and 17%, respectively). T. pulchra always showed higher foliar Al concentrations than 1000 g g^–1 dry mass, mainly after exposure to air and soil of both polluted sites (CM_lwF and MV_hgF). A highly significant linear regression was estimated between molar Al and F contents, taking in account the data obtained for saplings of T. pulchra cultivated in the different soils and exposed to ambient air of PV_noF, suggesting that both elements were taken as Al–F complexes from soil. The uptake of fluorides from air and/or soil of MV_hgF caused significant metabolic changes in T. pulchra, but visible injury supposedly induced by fluorides were observed only when the foliar F contents surpassed 700 g g^–1 dry mass. On the contrary, Al did not cause any metabolic stress to the plants.     

Callose in roots of Norway spruce (Picea abies (L.) Karst.) is a sensitive parameter for aluminium supply at a forest site (Höglwald)

Under controlled environmental conditions in nutrient solution experiments induction of non-constitutive callose in roots has been shown to be a symptom of aluminium (Al) toxicity. In the present study roots of Norway spruce were sampled from a forest site where soil conditions had been modified by acidic irrigation and liming (Höglwald Experiment in Bavaria, Germany). A significant positive relationship was found between the callose content in short roots and the Al concentration in the soil solution, particularly if free Al, rather than total concentrations of soluble Al, were used for prediction. At the same sites root growth of Norway spruce was not affected by free Al concentrations in the range of 2.5 to 199 µM Al. The results show that also under field conditions a positive relationship between Al supply and callose content can be established. In Norway spruce callose content in roots is a much more sensitive parameter for Al supply than root growth.     

Adenosine 5′-monophosphate decreases citrate exudation and aluminium resistance in Tamba black soybean by inhibiting the interaction between 14-3-3 proteins and plasma membrane H<Superscript>+</Superscript>-ATPase

Our previous study suggested that aluminium (Al) stress increased plasma membrane (PM) H⁺-ATPase activity and citrate secretion and simultaneously enhanced the interaction between 14-3-3 proteins and phosphorylated PM H⁺-ATPase in Al-resistant Tamba black soybean (RB). Adenosine 5′-monophosphate (AMP) is known as an inhibitor of the interaction between 14-3-3 proteins and PM H⁺-ATPases. To investigate the effects of AMP on Al resistance, PM H⁺-ATPase activity and citrate exudation, AMP was used to treat Al-stressed RB. The results showed that after treatment with either 100 μM AMP or 50 μM Al for 8 h, RB root growth was inhibited by approximately 50 and 30%, respectively. However, simultaneous treatment with 100 μM AMP and 50 μM Al for 8 h resulted in a 60% inhibition of RB root growth, indicating that the presence of AMP reduced Al tolerance in RB. The interaction of PM H⁺-ATPase and 14-3-3 proteins in the root tips of Al-treated RB was stronger than that in the untreated control. However, the interaction of the two proteins was greatly reduced (lower than that in the control) after co-treatment with Al and AMP, suggesting that the presence of AMP under Al stress reduced the Al-enhanced interaction between PM H⁺-ATPase and 14-3-3 proteins. Consequently, PM H⁺-ATPase activity decreased by approximately 50%, which led to a significant decrease in H⁺ efflux and citrate secretion in RB roots under Al stress. Collectively, these results indicate that AMP reduced citrate exudation and Al resistance in RB by inhibiting the interaction between 14-3-3 proteins and PM H⁺-ATPases under Al stress.