Aluminum ion in biological systems

Effects of diabetes mellitus and aluminum toxicity on myocardial calcium transport

Diabetics have an increased risk of developing renal insufficiency, as well as congestive heart failure independent of coronary atherosclerotic or hypertensive heart disease. Aluminum toxicity is being recognized with increased frequency in patients with reduced renal function and aluminum accumulates to a greater degree in tissues of patients with diabetes. Studies in patients with end stage renal disease have implicated aluminum overload as a potential cause of reduced cardiac function. Since both diabetes and aluminum decrease the activity of (Ca + Mg)-ATPase, a key enzyme involved in myocardial calcium transport, the interaction of experimental diabetes mellitus and aluminum toxicity on myocardial sarcoplasmic reticulum calcium transport was investigated in rats. Aluminum alone had no effect on (Ca + Mg)-ATPase activity, while activities in both the diabetic ([DM]) and diabetic plus aluminum loaded ([DM + Al]) groups were significantly lower than controls ([C]). Oxalate-dependent calcium uptake in the [DM] rats was slightly, but not significantly lower than controls, however, uptake was markedly reduced in rats which were both diabetic and aluminum loaded. The calcium regulatory protein calmodulin was measured by a functional assay in the soluble fraction of myocardial tissue prepared from each of the four groups. Compared to [C], calmodulin activity was significantly reduced in both the [DM] and [DM + Al] groups but not affected by aluminum alone. These data indicate that diabetes mellitus is associated with decreased myocardial calmodulin activity that may contribute to reduced sarcoplasmic reticulum (Ca + Mg)-ATPase and calcium transport activities and that aluminium toxicity potentiates the adverse effects of diabetes on decreasing sarcoplasmic reticulum calcium uptake.

Aluminum inhibits calpain-mediated proteolysis and induces human neurofilament proteins to form protease-resistant high molecular weight complexes

We studied the effects of aluminum salts on the degradation of human neurofilament subunits (NF-H, NF-M, and NF-L, the high, middle, and low molecular weight subunits, respectively) and other cytoskeletal proteins using calcium-activated neutral proteinase (calpain) purified from human brain. Calpain-mediated proteolysis of NF-L, tubulin, and glial fibrillary acidic protein (GFAP), three substrates that displayed constant digestion rates in vitro, was inhibited by AlCl3 (IC50 = 200 microM) and by aluminum lactate (IC50 = 400 microM). Aluminum salts inhibited proteolysis principally by affecting the substrates directly. After exposure to 400 microM aluminum lactate and removal of unbound aluminum, human cytoskeletal proteins were degraded two- to threefold more slowly by calpain. When cytoskeleton preparations were exposed to aluminum salt concentrations of 100 microM or higher, proportions of NF-M and NF-H formed urea-insoluble complexes of high apparent molecular mass, which were also resistant to proteolysis by calpain. Complexes of tubulin and of GFAP were not observed under the same conditions. Aluminum salts irreversibly inactivated calpain but only at high aluminum concentrations (IC50 = 1.2 and 2.1 mM for aluminum lactate and AlCl3, respectively), although longer exposure to the ion reduced by twofold the levels required for protease inhibition. These interactions of aluminum with neurofilament proteins and the effects on proteolysis suggest possible mechanisms for the impaired axoplasmic transport of neurofilaments and their accumulation in neuronal perikarya after aluminum administration in vivo.

Fluoroaluminates do not affect the guanine-nucleotide binding centre of the peptide chain elongation factor EF-Tu

EF-Tu is often referred to as a model for guanine-nucleotide-binding regulatory proteins (G-proteins), since X-ray diffraction analysis of its GTP-binding domain shows a detailed location of the 'consensus' amino acid sequences involved in nucleotide binding. Fluoroaluminates are thought to mimick the gamma-phosphate in the GTPase centre on account of their activating effect on a variety of GTP binding proteins. In the case of EF-Tu, we could find no such effects on the basis of at least three independent functional assays. We did notice, however, complicating interactions between free nucleotides, fluoroaluminates and other ligands. By consequence, if indeed AlF4- behaves as a gamma-phosphate analogue in G-proteins, then EF-Tu must have a different GDP/GTP binding site, despite of the conserved consensus sequences.

Aluminum uptake by neuroblastoma cells

Aluminum uptake studies in viable neuroblastoma cells were performed. Aluminum uptake was largely dependent on the pH of the suspension medium. At physiological pH values, cells were apparently unable to incorporate detectable amounts of aluminum in the absence of proper mediators. Aluminum uptake was enhanced as the pH decreased, attaining a plateau at about pH 6.0. In experiments with 2 x 10(6) cells/ml, pH 6.0, and 25 microM aluminum in the medium, aluminum incorporation reached saturation at 5 nmol of aluminum/mg of cellular protein, accounting for 60-70% of aluminum added. At pH 6.0, cells showed a large capacity for accumulating aluminum; about 70% of intracellular aluminum was associated with the postmitochondrial fraction. At neutral pH, application of apotransferrin seemed to facilitate aluminum translocation into cells via membrane receptors. Fatty acids were also capable of mediating aluminum uptake at neutral pH, probably by forming aluminum-fatty acid complexes. Low molecular weight aluminum chelators, e.g., citrate, inhibited aluminum uptake. Treatment of cells with energy metabolism blockers had virtually no influence on aluminum uptake, indicative of passive mechanisms. The results suggest that aluminum uptake occurs via different modes dependent on growth conditions, such as medium pH.

Aluminium perturbs oscillatory phosphoinositide-mediated calcium signalling in hormone-stimulated hepatocytes

Aluminium is known to be toxic to cells from bone, brain and bone marrow but the molecular target(s) affected by Al3+ are not known. We show here that Al3+ disrupts the oscillatory free Ca2+ responses of hepatocytes exposed to the Ca2(+)-mobilizing agonist phenylephrine. Al3+ initially increases the frequency of the oscillations and later induces broad Ca2+ spikes lasting several minutes. These broad spikes persist after removal of both agonist and Al3+ from the medium. In the absence of agonist, Al3+ has no effect on free Ca2+. The data suggest that some component(s) of the receptor-phosphoinositide-Ca2+ signalling pathway might be the site at which Al3+ exerts toxic effects.

Aluminum, altered transcription, and the pathogenesis of Alzheimer's disease

The etiology of some, if not all, cases of Alzheimer's disease is linked to a mutation in the proximal portion of the long arm of chromosome 21∶21q11.2 → 21q22.2. While the functional consequences of the mutation are unknown, we speculate that one consequence of the mutation is loss of the natural barriers and intracellular ligands for aluminum. As a result, aluminum gains access to several brain sites including the nuclear compartment in certain neurons of the central nervous system.Both sporadic and familial Alzheimer's disease are associated with an increased compaction of DNA within chromatin as measured by physical shearing and resistance to digestion by micrococcal nuclease and DNase I. There is also an increase in linker histone Hl(o) content on dinucleosomes released by light (3-5% ASN) micrococcal nuclease digestion, and an increase in the affinity of histone Hl(o) for DNA as measured by a salt elution technique. The change in enzyme accessibility to chromatin also involves the 5' promoter region of at least one physiologically important gene: the gene which codes for the low molecular weight moiety of neurofilament (NF-L). The conformation change involving the 5' regulator region probably reduces transcription because the pool size of the mRNA coding for NF-L is reduced to 14% of age matched control in cerebral grey matter. Reduced transcription may account for many disorders in cellular metabolic processes including the regulation of phosphorylation, calcium homeostasis, free radical metabolism, proteolysis and neurotransmitter metabolism.The experimental evidence indicates that one important toxic action of aluminum in Alzheimer's disease neocortex is to increase the binding of histones, particularly Hl(o), to DNA which results in increased compaction of chromatin and reduced transcription. The supporting evidence includes: (1) A statistically reliable correlation between the aluminum to DNA ratio on intermediate euchromatin and the amount of highly condensed heterochromatin found in a given preparation from Alzheimer affected neocortex (Crapperet al., 1980). (2) A nine-fold increase in aluminum content in Alzheimer's disease in the di- and tri- nucleosome fraction released by light micrococcal nuclease digestion of nuclei from cerebral grey matter compared to age matched controls. Compared to age matched control dinucleosomes, the Alzheimer affected dinucleosomes contain an increased abundance of the linker histone Hl(o) and an increased proportion of DNA containing the promoter region of the gene coding for NF-L. (3) A reduction in abundance to 14% of control mRNA coding for NF-L in Alzheimer affected neocortex (Crapper McLachlanet al., 1988). (4) In vitro evidence that Alzheimer linker histones bind more tightly to DNA than control and that aluminum added to nuclei,in vitro, extracted from normal control brain, enhances DNA-protein binding of Hl and Hl(o) at concentrations found in the Alzheimer affected chromatin (Lukiwet al., 1987). (5) Application of a band retardation assay indicates that aluminum,in vitro, selectively binds human Hl(o) to a 300 bp human ALU DNA fragment from a crude extract of 5% per chloric acid soluble proteins. (6) Aluminum experimentally applied to rabbit CNS induces a marked reduction in NF-L mRNA in anterior horn cells (Mumaet al., 1988). We therefore conclude that aluminum plays a major role in the pathogenesis of Alzheimer's disease. Further understanding of the role of aluminum in Alzheimer's disease requires a detailed investigation of the precise sites of co-ordination of this trivalent metal within chromatin.

Aluminofluoride and beryllofluoride complexes: a new phosphate analogs in enzymology

The action of fluoride ions on G proteins as well as on various ATPases and phosphatases, is related to their complexation with traces of aluminium or beryllium. These fluorometallic complexes act as analogs of phosphate: they bind with high affinity, but reversibly, in phosphate sites or, concomitantly with nucleoside-diphosphate, in nucleoside-triphosphate sites. The beryllofluoride complexes are strictly tetrahedral; they cannot take on the pentavalent conformation adopted by phosphate in transition states hence they interfere with phospho-transfer reaction mechanisms.

Effects of aluminum on the parathyroid hormone receptors of bone and kidney

Aluminum intoxication is associated with low osseous remodeling rate and peripheral resistance to parathyroid hormone (PTH). The pathophysiological mechanism of these aluminum induced changes was investigated using cultured clonal osteoblastic UMR-106 cells as well as dog renal cortical membrane. Both systems possess high-affinity PTH receptors that are coupled to adenylate cyclase. The UMR-106 cells have typical osteoblastic features, including receptors for the tissue-specific hormones, formation and mineralization of a bone-like ground substance and exclusive synthesis of type 1 collagen. The results show that aluminum at a concentration of 4 microM and 40 microM significantly inhibits the cyclic AMP responses to PTH challenge in UMR-106 cells, and this is associated with significant decrease in the binding to the PTH receptor. At 200 microM, no PTH-responsive adenylate cyclase or binding to receptor can be demonstrated. The effect of aluminum on UMR-106 rat osteosarcoma cells is not due to changes in cell number, cell viability or rate of mitogenesis. Similar results are obtained with dog kidney membrane. At a concentration of 10 microM and 400 microM, there is significant inhibition of the binding of PTH to kidney membrane and proportional decrease in PTH-stimulated adenylate cyclase. With higher concentration of aluminum, no response or binding can be demonstrated. In conclusion, aluminum at concentrations of 4 to 400 microM is associated with a decrease in affinity of PTH receptor and concomitant suppression of PTH-stimulated adenylate cyclase.(ABSTRACT TRUNCATED AT 250 WORDS)

Inactivation of glucose-6-phosphate dehydrogenase isozymes from human and pig brain by aluminum

Prolonged intake of low levels of aluminum from the drinking water has been found to increase the aluminum content in rat brain homogenates and to reduce the activity of hexokinase and glucose-6-phosphate dehydrogenase (G6PD). To determine the interaction of G6PD with aluminum in the brain, we have recently purified two isozymes of G6PD (isozymes I and II) from human and pig brain. Unlike isozyme I, isozyme II also had 6-phosphogluconate dehydrogenase (6-PGD) activity. We report here that G6PD isozymes I and II from human and pig brain purified to apparent homogeneity are inactivated by aluminum. Aluminum did not affect the 6-PGD activity of isozyme II. The aluminum-inactivated enzyme contained 1 mol of aluminum/mol of enzyme subunit. The protein-bound metal ion was not dissociated by exhaustive dialysis at 4 degrees C against 10 mM Tris-HCl (pH 7.0) containing 0.2 mM EDTA. Preincubation of aluminum with citrate, NADP+, EDTA, NaF, ATP, and apotransferrin protected the G6PD isozymes against aluminum inactivation. However, when the G6PD isozymes were completely inactivated by aluminum, only citrate, NaF, and apotransferrin restored the enzyme activity. The dissociation constants for the enzyme-aluminum complex of the isozymes varied from 2 to 4 microM, as measured by using NaF, a known chelator for aluminum. Inhibition of G6PD by low levels of aluminum further strengthens the suggested role of aluminum toxicity in the energy metabolism of the brain.

Aluminum increases agonist-stimulated cyclic AMP production in rat cerebral cortical slices

The effects of AlCl3 on basal and stimulated cyclic AMP production in rat cerebral cortical slices were studied. AlCl3 (10-250 microM) had no effect on the cyclic AMP concentration in the absence of drugs that stimulate the synthesis of cyclic AMP. 2-Chloroadenosine (25-200 microM) significantly stimulated the synthesis of cyclic AMP in a concentration-dependent manner, and AlCl3 significantly potentiated this response at 50 and 100 microM 2-chloroadenosine. This effect of AlCl3 was dependent on preexposure of the slices to AlCl3 before addition of the agonist. The potentiation by AlCl3 of the 2-chloroadenosine-induced increase in cyclic AMP level was concentration dependent, with significant enhancement by 100 (142% of the control) and 250 (150% of the control) microM AlCl3. Lower concentrations of AlCl3 had no significant effect on the production of cyclic AMP stimulated by 2-chloroadenosine. AlCl3 also potentiated the isoproterenol-induced increase in cyclic AMP production. Forskolin-induced production of cyclic AMP was unaltered by the presence of AlCl3. These results demonstrate that AlCl3 can potentiate agonist-stimulated cyclic AMP production in a whole-cell brain preparation without the addition of fluoride. This may account for the previously reported aluminum-induced increase in cyclic AMP concentrations in rat brain in vivo.

Time-dependent inactivation of glucose-6-phosphate dehydrogenase from yeast by aluminum

Aluminum inhibited yeast glucose-6-phosphate dehydrogenase (EC 1.1.1.49) by a pseudo-first-order reaction. The inhibition was proportional to the incubation time and the concentration of aluminum. Double reciprocal plots gave a straight line with a kinact of 8.3 min-1 and indicated the presence of a binding step prior to inhibition. The kinetic study showed that 1 mol of aluminum was bound per mol of enzyme subunit.

Aluminium and calcium in soil and food from Guam, Palau and Jamaica: Implications for amyotrophic lateral sclerosis and parkinsonism-dementia syndromes of Guam

Low calcium and high aluminium concentrations in the soils, waters and native foods have been hypothesised as environmental factors contributing to the unusually high incidence of amyotrophic lateral sclerosis and parkinsonism with dementia (ALS-PD) found on the island of Guam. The amounts of elemental aluminium and calcium were measured in foods of the native diet of the Chamorro people of Guam. The amount of aluminium eluted from topsoil by water at pH 7 at 22 °C was also measured. For comparison, food, water and soil samples were collected from two islands which have not reported a high incidence of ALS-PD syndromes: Palau and Jamaica.Compared with agricultural soils of Jamaica or Palau, the agricultural soils of Guam averaged 42-fold higher yield of elutable aluminium. The food data, however, do not indicate a differentially high exposure to elemental aluminium or low calcium intake in the diet of any one population. While this study did not detect an unusually high dietary aluminium or low dietary calcium content, the soils and possibly the dusts of Guam may be a major source of aluminium entering the body of the native people, particularly through the respiratory epithelium. Since iipid soluble organic ligands of aluminium more readily penetrate epithelial membranes, further study of soil aluminium ligands is required.

Modulatory effects of aluminum, calcium, lithium, magnesium, and zinc ions on [3H]MK-801 binding in human cerebral cortex

The independent and combined effects of Ca2+, Mg2+, Zn2+, Al3+ and Li+ on [3H]MK-801 binding in human cerebral cortical membranes were studied to further characterize the modulatory effects of metal ions on the N-methyl-D-aspartate (NMDA) receptor-ionophore. Glycine, in the presence of glutamate, significantly intensified the Mg2+ inhibition of [3H]MK-801 binding whereas it masked the Ca2+ enhancement and slightly diminished the Zn2+ inhibition. Both Ca2+ and Mg2+ reduced the Zn2+ inhibitory potency. Aluminum demonstrated a potent, relatively glycine-insensitive inhibition of [3H]MK-801 binding as an amorphous Al(OH)3 polymer rather than as the free ion. Cationic modulation of the NMDA receptor-ionophore appears to be regulated at multiple sites which have significant allosteric interactions.

Inhibition by aluminum hydroxide of the voltage-dependent closure of the mitochondrial channel, VDAC

Micromolar quantities of aluminum have been found (Dill et al. (1987) J. Membrane Biol. 99, 187-196) to reduce the voltage dependence of the mitochondrial outer membrane channel, VDAC, from Neurospora crassa. In the present study, various metallic and organic ions were tested for possible aluminum-like effect, and only the trivalent metals exhibited a similar ability to reduce the channels voltage dependence. However, trivalency alone was not sufficient because lanthanum (III) had no effect. Quantitative analyses with three group IIIA metals (A1, Ga, and In) showed that, of the structural characteristics examined, the ability to form sufficient M(OH)3 at experimental pH was the primary property shared by all the effective metals. While providing new insight into the nature of VDAC's sensor, these results also indicate that aluminum-cell interaction may result from the presence of AI(OH)3 in solution in addition to the widely accepted AI3+-mediated interactions. While the [AI3+] is vanishingly low at neutral pH, the trihydroxide is the major form and should be considered as an important candidate for aluminum-induced cellular effects.

Inactivation of bakers' yeast glucose-6-phosphate dehydrogenase by aluminum

Preincubation of yeast glucose-6-phosphate dehydrogenase (G6PD) with Al(III) produced an inactive enzyme containing 1 mol of Al(III)/mol of enzyme subunit. None of the enzyme-bound Al(III) was dissociated by dialysis against 10 mM Tris-HCl, pH 7.0, containing 0.2 mM EDTA at 4 degrees C for 24 h. Citrate, NADP+, EDTA, or NaF protected the enzyme against the Al(III) inactivation. The Al-(III)-inactivated enzyme, however, was completely reactivated only by citrate and NaF. The dissociation constant for the enzyme-aluminum complex was calculated to be 4 x 10(-6)M with NaF, a known reversible chelator for aluminum. Modification of histidine and lysine residues of the enzyme with diethyl pyrocarbonate and acetylsalicylic acid, respectively, inactivated the enzyme. However, the modified enzyme still bound 1 mol of Al(III)/mol of enzyme subunit. Circular dichroism studies showed that the binding of Al(III) to the enzyme induced a decrease in alpha-helix and beta-sheet and an increase in random coil. Therefore, it is suggested that inactivation of G6PD by Al(III) is due to the conformational change induced by Al(III) binding.

Mechanism of GTP hydrolysis in tubulin polymerization: characterization of the kinetic intermediate microtubule-GDP-Pi using phosphate analogues

Beryllium fluoride (BeF3-) has previously been shown to bind tightly to microtubules as a structural analogue of Pi and to mimic the GDP-Pi transient state in tubulin polymerization [Carlier, M.-F., Didry, D., Melki, R., Chabre, M., & Pantaloni, D. (1988) Biochemistry 27, 3555-3559]. The interaction of BeF3- with tubulin is analyzed here in greater detail. BeF3- binds to and dissociates from microtubule GDP subunits at very slow rates (k+ congruent to 100 M-1 s-1; k- congruent to 6 x 10(-4) s-1), suggesting that a slow conformation change of tubulin, linked to the stabilization of the microtubule structure, follows BeF3- binding. The possibility is evoked that BeF3- acts as a transition-state analogue in the GTPase reaction of tubulin. BeF3- does not bind to dimeric nor to oligomeric GDP-tubulin with high affinity. Substoichiometric binding of BeF3- to microtubules provides extensive stabilization of the structure. An original mechanistic model that accounts for the data is proposed. The kinetic parameters for microtubule elongation in the presence of GTP- and GDP-tubulin with and without BeF3- have been determined. Data support the following views: (i) Microtubules at steady state and in a regime of slow growth in the presence of GTP are stabilized by a cap of GDP-Pi subunits functionally similar to GDP-BeF3 subunits. (ii) In the presence of BeF3-, microtubules elongate from GDP-tubulin within the following sequence of reactions: initial nonproductive binding of GDP-tubulin to microtubule ends is followed by the binding of BeF3- and the associated conformation change allowing sustained elongation.

Aluminum and chronic renal failure: sources, absorption, transport, and toxicity

In normal subjects the gastrointestinal tract is a relatively impermeable barrier to aluminum with a low fractional absorption rate for this metal ion. Aluminum absorbed from the gastrointestinal tract is normally excreted by the kidneys; in the presence of impaired renal function aluminum is retained and accumulates in body tissues. Aluminum-containing medications are given, by mouth, to patients with chronic renal failure as phosphate-binding agents for the therapeutic control of hyperphosphatemia. Patients with chronic renal failure are also exposed to aluminum in domestic tap-water supplies used either for drinking or, in those on dialysis treatment, in the preparation of their dialysate. In patients with end-stage chronic renal failure, particularly in those on treatment by hemodialysis, the accumulation of aluminum in bone, brain, and other tissues is associated with toxic sequelae. An increased brain content of aluminum appears to be the major etiological factor in the development of a neurological syndrome called either "dialysis encephalopathy" or "dialysis dementia"; an increased bone content causes a specific form of osteomalacia. An excess of aluminum also appears to be an etiological factor in a microcytic, hypochromic anemia that occurs in some patients with chronic renal failure on long-term treatment with hemodialysis. The various mechanisms involved in the toxic phenomena associated with the accumulation of aluminum in body tissues have not been clearly defined but are the subject of extensive investigations.