proposed, using the physiological substrate, orthosilicic acid (Schr€ oder et al. 2010).
This mechanism which starts with a nucleophilic attack at the silicon of the silicic
acid substrate under formation of a covalently bound intermediate (Fig. 10.3b, c)
explains the increased rate of silica deposition in the presence of silicatein by the
enzyme-catalyzed formation of cyclic silicic acid species (Schr€ oder et al. 2010).
These reactive intermediates which, at a lower rate, are also generated at nonenzymatic conditions (see Fig. 10.1a) strongly enhance the silica polycondensation
reaction. The final product consisting of silica nanospheres may fuse via a
sintering-like mechanisms, (biosintering; M€ uller et al. 2009a, b) forming layered
(lamellar; Schr€ oder et al. 2007b) or other biosilica structures (Fig. 10.3d). Silicatein
displays not only silica polymerase but also silica esterase activity (M€ uller et al.
2008b). Besides inorganic substrates, e.g. orthosilicic acid, organic oxysilanes, e.g.
bis(p-aminophenoxy)-dimethylsilane (formation of silica; M€ uller et al. 2008b) and
dimethoxy dimethylsilane (formation of silicones; Wolf et al. 2010) can be used as
substrates.
Analyses of the composite structure of the biosilica material formed by silicatein
revealed that after silica formation, silicatein is present not only on the surface of
the silica lamellae which are formed as the result of the appositional growth of the
spicules (M€ uller et al. 2005; Woesz et al. 2006; Schr€ oder et al. 2007b), but also
entrapped within biosilica particles (M€ uller et al. 2010). Recent results demonstrate
that the interaction of silicatein with a scaffold protein, silintaphin-1, allows for the
generation of spicule-like three-dimensional structures (M€ uller et al. 2008b; Wiens
et al. 2009).
The expression of silicatein is induced by silicate (Krasko et al. 2000; M€ uller
et al. 2006). In addition, silicate induces the expression of myotrophin which
triggers collagen synthesis (Schr€ oder et al. 2000a). Moreover, in sponge tissue
cultures (primmorphs), silicate stimulates differentiation of stem-like cells into
the spicule-forming sclerocytes (M€ uller et al. 2006).
10.6 Silicon Metabolism
The silicon content of the human body is low and amounts to 1–2 g totally
(Jugdaohsingh 2007). The highest concentrations of silicon are found in bone,
connective tissue, and blood vessels (Carlisle 1972; Sripanyakorn et al. 2009).
In soft tissue, silicon may be complexed to glycosaminoglycans, polyuronides,
or silicic acid-binding polysaccharides and proteins (Schwarz 1973). Based on
these data, silicon has been proposed to be required for bone formation and the
formation of cartilage glycosaminoglycans during development and calcification
(Carlisle 1976, 1981). Dietary silicon is taken up through gastrointestinal absorption (Reffitt et al. 1999; Jugdaohsingh et al. 2002). It is excreted from the body via
the gastrointestinal tract and via the kidney by glomerulal filtration into the urine
(Berlyne et al. 1986; Adler and Berlyne 1986; reviewed in: Jugdaohsingh 2007).
Tracer experiments with
31 Si injected into rats showed an accumulation of silicon in
10 Biosilica-Based Strategies for Treatment of Osteoporosis and Other Bone Diseases
291
This mechanism which starts with a nucleophilic attack at the silicon of the silicic
acid substrate under formation of a covalently bound intermediate (Fig. 10.3b, c)
explains the increased rate of silica deposition in the presence of silicatein by the
enzyme-catalyzed formation of cyclic silicic acid species (Schr€ oder et al. 2010).
These reactive intermediates which, at a lower rate, are also generated at nonenzymatic conditions (see Fig. 10.1a) strongly enhance the silica polycondensation
reaction. The final product consisting of silica nanospheres may fuse via a
sintering-like mechanisms, (biosintering; M€ uller et al. 2009a, b) forming layered
(lamellar; Schr€ oder et al. 2007b) or other biosilica structures (Fig. 10.3d). Silicatein
displays not only silica polymerase but also silica esterase activity (M€ uller et al.
2008b). Besides inorganic substrates, e.g. orthosilicic acid, organic oxysilanes, e.g.
bis(p-aminophenoxy)-dimethylsilane (formation of silica; M€ uller et al. 2008b) and
dimethoxy dimethylsilane (formation of silicones; Wolf et al. 2010) can be used as
substrates.
Analyses of the composite structure of the biosilica material formed by silicatein
revealed that after silica formation, silicatein is present not only on the surface of
the silica lamellae which are formed as the result of the appositional growth of the
spicules (M€ uller et al. 2005; Woesz et al. 2006; Schr€ oder et al. 2007b), but also
entrapped within biosilica particles (M€ uller et al. 2010). Recent results demonstrate
that the interaction of silicatein with a scaffold protein, silintaphin-1, allows for the
generation of spicule-like three-dimensional structures (M€ uller et al. 2008b; Wiens
et al. 2009).
The expression of silicatein is induced by silicate (Krasko et al. 2000; M€ uller
et al. 2006). In addition, silicate induces the expression of myotrophin which
triggers collagen synthesis (Schr€ oder et al. 2000a). Moreover, in sponge tissue
cultures (primmorphs), silicate stimulates differentiation of stem-like cells into
the spicule-forming sclerocytes (M€ uller et al. 2006).
10.6 Silicon Metabolism
The silicon content of the human body is low and amounts to 1–2 g totally
(Jugdaohsingh 2007). The highest concentrations of silicon are found in bone,
connective tissue, and blood vessels (Carlisle 1972; Sripanyakorn et al. 2009).
In soft tissue, silicon may be complexed to glycosaminoglycans, polyuronides,
or silicic acid-binding polysaccharides and proteins (Schwarz 1973). Based on
these data, silicon has been proposed to be required for bone formation and the
formation of cartilage glycosaminoglycans during development and calcification
(Carlisle 1976, 1981). Dietary silicon is taken up through gastrointestinal absorption (Reffitt et al. 1999; Jugdaohsingh et al. 2002). It is excreted from the body via
the gastrointestinal tract and via the kidney by glomerulal filtration into the urine
(Berlyne et al. 1986; Adler and Berlyne 1986; reviewed in: Jugdaohsingh 2007).
Tracer experiments with
31 Si injected into rats showed an accumulation of silicon in
10 Biosilica-Based Strategies for Treatment of Osteoporosis and Other Bone Diseases
291