| Abstract: |
Silicon is essential for some plants, diatoms, and sponges but, in
higher animals, its endogenous regulation has not been demonstrated.
Silicate ions may be natural ligands for aluminum and here
we show that, in the freshwater snail (Lymnaea stagnalis), intracellular
silicon seems specifically up-regulated in response to sublethal
aluminum exposure. X-ray microanalysis showed that exposure
of snails to low levels of aluminum led to its accumulation in
lysosomal granules, accompanied by marked up-regulation of
silicon. Increased lysosomal levels of silicon were a specific response
to aluminum because cadmium and zinc had no such effect.
Furthermore, intra-lysosomal sulfur from metallothionein and
other sulfur-containing ligands was increased after exposure to
cadmium and zinc but not aluminum. To ensure that these findings
indicated a specific in vivo response, and not ex vivo formation of
hydroxy-aluminosilicates (HAS) from added aluminum (555 g
liter) and water-borne silicon (43 gliter), two further studies
were undertaken. In a ligand competition assay the lability of
aluminum (527 gliter) was completely unaffected by the presence
of silicon (46 gliter), suggesting the absence of HAS. In
addition, exogenous silicon (6.5 mgliter), added to the water
column to promote formation of HAS, caused a decrease in lysosomal
aluminum accumulation, showing that uptake of HAS would
not explain the loading of aluminum into lysosomal granules.
These findings, and arguments on the stability, lability, and kinetics
of aluminum–silicate interactions, suggest that a silicon-specific
mechanism exists for the in vivo detoxification of aluminum, which
provides regulatory evidence of silicon in a multicellular organism.
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