| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
|
|
|
|||||||
|
|||||||||
Original Research Communications |
1 From the Gastrointestinal Laboratory, The Rayne Institute, St Thomas' Hospital, London; the Department of Chemistry, University of Manchester, United Kingdom; and the Department of Nuclear Physics, Australian National University, Canberra.
2 Supported by The Special Trustees of St Thomas' Hospital (RJ and DMR) and The Medical Research Council (JJP); CO was supported by an Engineering and Physical Sciences Research Council studentship. The Royal Society supported the purchase of the aluminum-26 and The Wellcome Trust supported the use of the inductively coupled plasma optical emission spectrometer.
3 Address reprint requests to R Jugdaohsingh, Gastrointestinal Laboratory, The Rayne Institute, St Thomas' Hospital, London, SE1 7EH, United Kingdom. E-mail: ravin.jugdaohsingh{at}kcl.ac.uk.
 |
ABSTRACT |
|---|
 TOP ABSTRACT INTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Objective: By using 26Al as a tracer, we investigated the effects of oligomeric and monomeric silica on the bioavailability of aluminum (study 1) and compared the availability of silicon from oligomeric and monomeric silica in the human gastrointestinal tract (study 2).
Design: In study 1, three healthy volunteers each ingested aluminum alone (control), aluminum with oligomeric silica (17 mg), and aluminum with monomeric silica (17 mg). In study 2, five healthy volunteers ingested both the oligomeric and monomeric forms of silica (34 mg). Serum and urine samples were analyzed for aluminum and silicon.
Results: Oligomeric silica reduced the availability of aluminum by 67% (P = 0.01) compared with the control, whereas monomeric silica had no effect (P = 0.40). Monomeric silica was readily taken up from the gastrointestinal tract and then excreted in urine (53%), whereas oligomeric silica was not detectably absorbed or excreted.
Conclusions: The oligomeric, high-aluminum-affinity form of soluble silica reduces aluminum availability from the human gastrointestinal tract. Its potential role in the amelioration of aluminum toxicity in other biological systems requires attention.
Key Words: Oligomeric silica • monomeric silica • orthosilicic acid • silicon • silicic acid • silica • aluminum • absorption • bioavailability • gastrointestinal uptake • accelerator mass spectrometry
|
INTRODUCTION |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
However, studies have focused on monomeric silica [Si(OH)4], also termed orthosilicic acid, but this form has a low affinity for Al3+ (log Keff = 4.7 ± 0.05, where Keff is the effective stability constant, at pH 7.2) (19) and recent data suggest that for efficient binding of aluminum and silica, at least one needs to be polymeric. Duan and Gregory (20) showed an interaction between monomeric silica and polyhydroxy aluminum, whereas we showed the marked interaction between oligomeric silica, a polymer of Si(OH)4, and monomeric aluminum (log Keff =11.70 ± 0.30 at pH 7.2) (21).
Oligomeric silica is formed as a metastable intermediate in the progressive polymerization of silicic acid in neutral solutions at concentrations >2 mmol/L and, when diluted (<2 mmol/L), is transiently stable (<1–50 d) depending on the aluminum concentration (21). Thus, the weathering and dissolution of solid-phase silica and certain pore waters are probable environmental sources of oligomeric silica (2, 22). Oligomeric silica has yet to be fully characterized, but studies so far suggest that it is colloidal with a mean particle diameter of <20 nm (R Jugdaohsingh, RPH Thompson, and JJ Powell, unpublished observations, 1998) and that it forms a high-affinity complex with aluminum with a molar ratio of 35 silicon:1 aluminum (21).
In this article we report the effects of monomeric and oligomeric silica, given in amounts typical of human diets, on the bioavailability of aluminum in healthy subjects (study 1). Absorption of aluminum into serum and its excretion in urine were monitored by using the 26Al tracer isotope, allowing for investigation of typical dietary amounts (5 mg/d) of aluminum (23). We also looked separately at the bioavailability of the 2 different forms of silica when ingested in the absence of aluminum (study 2). This work aimed to clarify the conflicting results in the literature on the amelioration of aluminum availability by soluble silica (11–18).
|
SUBJECTS AND METHODS |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
Approval of the study was obtained from St Thomas' Hospital Local Research Ethics Committee. The dose of 26Al tracer isotope was approved by the Administration of Radioactive Substances Advisory Committee support unit of the Department of Health, United Kingdom. Details of the study and potential risks associated with the 26Al radioisotope were explained to the volunteers, all of whom signed consent forms before the study began.
Dietary restrictions
Throughout the study periods, subjects ingested an unrestricted diet except that they avoided foods high in silicon and aluminum and foods that might affect aluminum uptake and excretion. The latter included foods containing aluminum chelators, such as polyphenols and citric acid, and those that cause diuresis, such as tea, coffee, and alcohol. Subjects consumed 2–2.5 L fluid daily and recorded their diet during the study periods. Before ingestion of the solution, the subjects fasted overnight from 2200 and continued to fast the next day until 5 h after ingesting the test solution at 
1400.
Aluminum radioisotope
The 26Al radioisotope has a half-life of 730000 y and decays with emission of positrons and gamma radiation. The 3 men in study 1 each ingested 453 ± 12 ng (323.6 ± 8.6 Bq) 26Al on 3 occasions. This was a small amount of radiation with an effective dose equivalent to the whole body of 1.94 ± 0.05 µSv per dose (equivalent to one-fiftieth of a single, modern chest X-ray).
Materials
The water was ultrapure (18 M
/cm) from an Elga water purifier (Elga, High Wycombe, United Kingdom). Silica was prepared in water from a stock basic (4.87 mol NaOH/L) concentrated sodium silicate solution (7 mol Si/L; Aldrich Chemical Co, Gillingham, United Kingdom). 26Al (10 MBq/L in 1 mol HCl/L) was supplied by the Los Alamos National Laboratory, Isotope and Nuclear Chemistry Division, University of California. We prepared a stock aluminum tracer solution containing 4.47 µg 26Al and 1.35 mg 27Al in 1 mol HCl/L. We used citric acid monohydrate (crystalline, extra pure; Merck Ltd, Lutterworth, United Kingdom) prepared in ultrapure water. Polypropylene plastic ware (Merck Ltd, Aldrich Chemical Co, and Elkay Laboratory Products Ltd, Basingstoke, United Kingdom), acid-washed (10% nitric acid by vol) for 24 h and rinsed with ultrapure water, was used throughout. Clean-air facilities (class J clean-air room and class C laminar-air-flow workstation) were used when appropriate to avoid contamination of samples with silicon and aluminum.
Silica preparation
We prepared oligomeric silica, fresh on each occasion, at 42 mmol Si/L by dilution of the stock sodium silicate solution and neutralization to pH 7.2 with HCl. This 42-mmol/L solution was incubated at room temperature for 24 h before dilution (1–2 mmol Si/L) to prepare the doses. Monomeric silica was prepared either by incubation (>7 d) at room temperature (23°C) of the 2-mmol/L oligomeric solution to allow its depolymerization (study 2) or by direct dilution of the stock sodium silicate solution (to 1 mmol/L) followed by incubation (>7 d) at room temperature (study 1).
Study design
Study 1: ingestion of aluminum in the presence and absence of silica
This study was conducted in 3 male volunteers over 8 wk. Each subject ingested 3 different aluminum solutions: aluminum citrate with oligomeric silica (week 1), aluminum citrate with monomeric silica (week 5), and aluminum citrate alone (week 8). On day 1 of each study week, each subject collected his urine in a single container for 24 h; this was the predose urine. On day 2 at 0930, after fasting overnight, subjects ingested 621 mL of the dose solution (pH 7.2) containing aluminum citrate [137 µg 27Al (8.1 µmol/L) plus 453 ± 12 ng 26Al with 80.8 µmol citrate/L] that had been incubated, with or without silica (1 mmol Si/L), for 48 h. All postdose urine was collected for a total of 72 h (in 3 x 4 h, then 1 x 12 h, and then 2 x 24 h collections). Blood samples (10 mL) were also collected via an all-plastic intravenous cannula (Medicut, 1.3 mm outer diameter x 45 mm; Sherwood Medical, Tullamore, Ireland) inserted into a forearm vein; we collected 1 predose sample and additional samples at intervals during the 72-h postdose period. Isotopic abundance ratios of 26Al to 27Al were measured in urine, serum, and dose samples by using accelerator mass spectrometry (Australian National University, Canberra) as described previously (18, 24). Total silicon was measured in solutions by inductively coupled plasma optical emission spectrometry (ICPOES) as reported previously (25, 26). The percentage of monomeric silica in aliquots of the solutions was determined at the time of each ingestion, as described previously (21), by reaction with molybdic acid (27). In addition, amounts of ultrafilterable silicon and aluminum were determined in aliquots by using filter units (5000 nominal molecular weight, Biomax Ultrafree-15; Millipore, Watford, United Kingdom). Filtrates were analyzed for silicon and aluminum by ICPOES. 26Al that was adherent to emptied dose containers was measured in thorough acid rinses (1% nitric acid by vol) by accelerator mass spectrometry.
Study 2: ingestion of silica in the absence of aluminum
This study was conducted in 5 volunteers (3 men and 2 women) over 3 wk. Silica solutions (2 mmol Si/L) were ingested by subjects to facilitate the detection of urinary silica excretion above baseline. In week 1, each subject ingested 600 mL of 1.5 L of their oligomeric silica solution (2 mmol Si/L, pH 7.2). In week 3, after natural depolymerization of the silica solutions over time, an additional 600-mL dose was ingested by each volunteer. In both study weeks, day 1 was used as a run-in period when dietary restrictions (see above) were observed. On day 2, two predose urine collections, each of which lasted 8 h (0–8 h and 16–24 h), were performed. After ingestion of silica on day 3, postdose urine was collected for four 8-h periods (total of 32 h). In 2 volunteers, 1 predose and postdose blood samples (5 mL) were also collected via an intravenous all-plastic cannula at various intervals over 24 h. Urine, serum, and aliquots of the silica solutions were analyzed for total silicon by using ICPOES as described previously. Monomeric silica in the dose solutions was determined at the time of ingestion, as described above, by ultrafiltration and reaction with molybdic acid.
Statistical analysis
The results are expressed as means ± SDs. In study 1, we tested the hypothesis that soluble silica reduces the gastrointestinal availability of aluminum; comparisons were made by using paired, one-tailed Student's t tests. Because 2 forms of silica were investigated, a Bonferroni correction for multiplicity of testing was applied to the P values and significance was defined as P < 0.025 (0.05/2). Correlations between urinary and serum concentrations of aluminum were assessed by using Pearson's product-moment correlation coefficients, with significance defined as P < 0.05. In study 2, we investigated which of the 2 forms of silica was bioavailable from the gastrointestinal tract; comparison was by paired, one-tailed Student's t test and significance was defined as P < 0.05.
|
RESULTS |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
In all cases, ingestion of the solutions increased the concentrations of aluminum in serum over the 72-h study period (Figure 1A
), although oligomeric silica significantly reduced the appearance of aluminum in serum by 67 ± 13% (n = 3, P = 0.01) compared with the control solution of aluminum without silica (Figure 1B). In contrast, aluminum with monomeric silica did not differ significantly from the control. Results in urine (Figure 1C) mirrored those in serum (Figure 1A); urinary aluminum excretion was reduced by 72 ± 12% compared with the control (n = 3, P = 0.02) when aluminum was ingested with oligomeric silica but did not differ significantly from the control when aluminum was ingested with monomeric silica (Figure 1D). The area under the curve of aluminum in serum (0–72 h) was significantly correlated with aluminum excreted in urine over the same period (r = 0.86, P < 0.01; n = 9), which suggests that the absorption-excretion profiles of aluminum were similar for the 3 solutions.
|
). On each occasion (day 0 and day 14), 600 mL of solution (1.2 mmol Si) was ingested by each healthy volunteer (n = 5). In marked contrast with the bioavailability of monomeric silica on day 14, the bioavailability of oligomeric silica on day 0 was poor (Figure 3). Analysis of urine showed significantly increased excretion of silicon between 0 and 8 h postdose (P < 0.001) after ingestion of monomeric silica (53.2 ± 8.5% of dose solution, n = 5), but only a marginal increase after ingestion of oligomeric silica (Figure 3B). This marginal increase in urinary excretion of silicon after ingestion of oligomeric silica may represent contamination of the solution with monomeric silica (Figure 2). In 2 of the volunteers, sequential blood samples were also obtained and they confirmed that the availability of silicon from the monomeric silica solution (day 14) was >20% of the ingested dose, greatly exceeding its availability from oligomeric silica (day 0), which was not detectable (Figure 3A).
|
|
) and had a mean diameter of <20 nm by particle light scattering (R Jugdaohsingh, RPH Thompson, and JJ Powell, unpublished observations, 1998), probably precludes significant passive absorption (28). However, the low availability of silicon from oligomeric silica also suggests that even during transit through the gastrointestinal tract, the species was not greatly broken down or absorbed. |
DISCUSSION |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
8 µmol Al/L was used in this study with a concentration of silicon (1000 µmol/L) that ensured excess aluminum-binding capacity. Before ingestion, all solutions were incubated for 48 h to ensure that binding was complete between silica and aluminum and also to allow silica not bound to aluminum to depolymerize fully (21). Citrate was used as the aluminum counter ion because it maintains aluminum in a monomeric form in aqueous solution, it prevents adhesion of Al3+ to the dosing containers, and silica has been shown to affect aluminum availability even in the presence of citrate (12, 14). The concentrations of aluminum and silicon used in this study fall within the broad dietary ranges for these elements. Nonetheless, the results have wide application because oligomeric silica binds to aluminum rapidly in many different forms and under a range of experimental conditions (21; R Jugdaohsingh, RPH Thompson, and JJ Powell, unpublished observations, 1999). Oligomeric silica reduced both the serum concentrations and urinary excretion of aluminum. Indeed, urinary and serum aluminum concentrations were correlated, suggesting similar absorption-excretion profiles. Because there are no other routes of aluminum excretion (29), this is consistent with reduced aluminum absorption. Thus, our data suggest that the strong interaction observed in vitro (21) between oligomeric silica and aluminum is maintained in the human gastrointestinal tract, leading to a marked reduction in the absorption of aluminum. The large colloidal size of the oligomeric silica species virtually precludes passive diffusion of the silica-aluminum complex (28). This is unlike the situation with monomeric silica, which is easily absorbed and has a low affinity for aluminum (19, 21) and no effect on the gastrointestinal absorption of aluminum.
Clearly, research on environmental or biological systems that aims to show interactions between soluble silica and aluminum now must distinguish carefully between these different forms of silica. Indeed, traces of oligomeric silica in solution will profoundly affect the availability of aluminum and would readily account for discrepancies in results published regarding the amelioration of aluminum absorption and toxicity by silica (11–18). In studies in which concentrated silica is added to aluminum-containing solutions before dilution (13, 14), the formation of oligomeric silica–aluminum complexes with associated aluminum-ameliorating effects is likely. In contrast, in studies in which concentrated silica solutions are diluted before addition to aluminum (17, 18), the formation of monomeric silica without effects on aluminum binding or amelioration is likely. However, in any study in which the soluble silica concentration of freshly prepared solutions approaches or is >2 mmol/L at or near neutral pH (11, 15, 16), the formation of oligomeric silica is likely and amelioration of aluminum effects should be observed.
This work also raises questions about biologically relevant forms of dietary silica. Previous investigations (17, 18, 30) focused on absorbable monomeric silica, whereas the more common, poorly absorbed fraction of polymeric and oligomeric dietary silica may be of greater importance in preventing the uptake of aluminum. To determine whether oligomeric silica is absorbed at all will require further studies with silicon-based radioisotopes (31). Indeed, colloidal species of this size are normally absorbed passively at 1–5% of the ingested dose (32). Alternatively, if after absorption silica concentrates at its biological site of action, then the in situ formation of oligomeric silica could occur, leading to local sequestration of Al3+ ions. For example, in one study it was suggested that silica concentrates in the renal tubule allowing interaction with, and facilitated excretion of, aluminum (30). In the current study, ingestion of monomeric silica increased urinary aluminum excretion compared with ingestion of aluminum alone in 2 of the 3 subjects, but this effect was not significant and will need to be addressed in a separate study.
Finally, the current findings do not exclude a role for monomeric silica in other aspects of cellular or structural biology. However, monomeric silica itself is unlikely to contribute to the natural sequestration of toxic and ubiquitous Al3+ ions.
|
REFERENCES |
|---|
|
TOPABSTRACTINTRODUCTIONSUBJECTS AND METHODSRESULTSDISCUSSIONREFERENCES |
|---|
This article has been cited by other articles:
|
|
 |
|
  F. J. DOS SANTOS, O. S. SIQUEIRA, A. A. S. T. DELBEN, J. R. J. DELBEN, and P. MELNIKOV Spongolite: a hollow fibrous mineral from Mato Grosso do Sul State, Brazil Clay Minerals, June 1, 2002; 37(2): 249 - 253. [Abstract] [Full Text] [PDF]
|
|
|
|
 |
|
 M. Desouky, R. Jugdaohsingh, C. R. McCrohan, K. N. White, and J. J. Powell Aluminum-dependent regulation of intracellular silicon in the aquatic invertebrate Lymnaea stagnalis PNAS, March 19, 2002; 99(6): 3394 - 3399. [Abstract] [Full Text] [PDF]
|
|
| ||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||||
| HOME | HELP | FEEDBACK | SUBSCRIPTIONS | ARCHIVE | SEARCH | TABLE OF CONTENTS |
), 26Al-labeled aluminum citrate with monomeric silica (
), and 26Al-labeled aluminum citrate alone (control; 
); n = 3 (A and C). B: Mean (±SD) total uptake of 26Al into serum over 72 h derived from the area under the serum curve. *Significantly different from control, P = 0.01 (paired, one-tailed t test). D: Mean (±SD) total excretion of 26Al in urine over 72 h. *Significantly different from control, P = 0.02 (paired, one-tailed t test).
