ABSTRACT
The Search for Mechanism . . . . . . . . . . . . . . . . . . . 758
A Test of the Concept: Prolylhydroxylase
Activity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 758
Aluminum in Biological Systems . . . . . . . . . . . . . . 759
A Test of the Al-Si Balance Hypothesis . . . . . . . . 760
The Exclusion Mechanism . . . . . . . . . . . . . . . . . . . . 760
The Significance of the
Environmental Si:Al Balance . . . . . . . . . . . . . . . . 761
Silicic Acid: Geochemistry
and Health . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 762
Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763
References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 763
Silicon is listed as an essential element. Its removal from the diet of experimental animals has been shown
to result in reduced growth rate (reversed on silicon supplementation) and changes to bone formation and
the synthesis of collagenous connective tissue. However, in spite of much effort, no organic binding (e.g.,
to proteins) of silicon has been convincingly demonstrated under physiological conditions in which
silicon exists as silicic acid, Si(OH)4, and no biochemical rationale has been proposed to account for
the effects of silicon deficiency. However, recent research indicates that a major role for silicon (as
silicic acid) is to reduce the bioavailability of aluminum, which is toxic when it gains entry into biological
systems, but which is normally largely excluded. The formation of subcolloidal hydroxyaluminosilicate
species is shown to prevent the absorption of aluminum in fish via gill epithelia. The generality of this
effect is discussed. The symptoms of silicon deficiency in experimental animals seem likely to result
from aluminum toxicity, so that the environmental balance for the two elements may be critical. This
chapter reviews the present position.
