Biological and Therapeutic Effects of Ortho-silicic Acid and Some Ortho-silicic Acid-releasing Compounds

New Perspectives for Therapy

Lela Munjas Jurkić; Ivica Cepanec; Sandra Kraljević Pavelić; Krešimir Pavelić


Nutr Metab. 2013;10(2) 

In This Article

Zeolites as a Source of Ortho-silicic Acid

Zeolites are a class of aluminosilicates of general formula (Mn+)x/n[(AlO2)x(SiO2)y·mH2O, wherein M represents a positively charged metal ion such as sodium (Na+), potassium (K+), magnesium (Mg2+), or calcium (Ca2+). Zeolites are crystalline aluminosilicates with open 3D framework structures built of SiO4 and AlO4 tetrahedra linked to each other by sharing all the oxygen atoms to form regular intra-crystalline cavities and channels of molecular dimensions.[92] The positively charged metal ions (e.g. Na+, K+, Ca2+, Mg2+) are positioned in these cavities of aluminosilicate skeleton which are termed as micro- (2–20 Å), meso- (20–50 Å), and macro-(50–100 Å) -pores. These ions are readily exchangeable in contact with aqueous solution of other positively charged ions (e.g. heavy metal ions like Hg2+). This structural characteristic of zeolites is the base of their ion (cation)-exchange property.[93]

At present, 191 unique zeolite frameworks have been identified,[94] while over 40 naturally occurring zeolite frameworks have been described. Zeolites have been widely employed in chemical and food industries, agriculture, and environmental technologies as adsorbents, absorbents, adsorbent filter-aids, ion-exchangers, catalysts, active cosmetic and pharmaceutical ingredients, soil improvers, etc..[95–103] Besides, zeolites exhibit a number of interesting biological activities[5,104,105] (Figure 4). For example, nontoxic natural zeolite clinoptilolite affects tumour cells proliferation in vitro and might act as an adjuvant in cancer therapy.[105] Katic et al.[106] confirmed that clinoptilolite influences cell viability, cell division, and cellular stress response that results in antiproliferative effect and apoptosis induction in vitro. Obtained results demonstrated that clinoptilolite biological effect on tumour cells growth inhibition might be a consequence of adsorptive and ion-exchange characteristics that cause adsorption of some serum components by clinoptilolite.[106] Similarly, clinoptilolite showed antiviral effects in vitro and a potential in antiviral therapy either for local skin application against herpesvirus infections or oral treatment of adenovirus or enterovirus infections.[107] The antiviral mechanism is probably non-specific and is based on adsorption of viral particles on external cavities at the clinoptilolite surface rather than a consequence of ion-exchange properties.

Figure 4.

Biomedical application of zeolites (adopted from: K Pavelic, M Colic, B Subotic. In: Studies in Surface Science and Catalysis, Vol. 135. Amsterdam: Elsevier, 2001, p 170).

Each zeolite particle acts like a large inorganic molecule and acts as a molecular sieve with a potential in molecular medicine in molecular medicine. Their pores are indeed, rather small (less than 2 nm to 50 nm),[108] and these structural similarities between the cages of zeolites and binding sites of enzymes resulted in development of zeolite structures that mimic enzyme functions,[108] e.g. haemoglobin, cytochrome P450 or iron-sulphur proteins.[109]

Important data on biological zeolites fate (Figure 5) and effects in vivo have been widely reported so far in the scientific literature. For example, it was shown that zeolites bear detoxifying and decontaminant properties when added to animal diets, reducing levels of heavy metals (e.g. lead, mercury, and cadmium) and various organic pollutants, i.e. radionuclides (Figure 6) and antibiotics.[108] Furthermore, zeolites have been successfully utilized for haemodialysis, for cartridges in haemoperfusions, for wound healing, and surgical incisions.[108] For instance, QuikClot and Zeomic formulations are already being marketed for haemorrhage control[110] and dental treatment,[5] respectively.

Figure 5.

Structural and biochemical changes of zeolites in the digestive system (by courtesy from Application of natural zeolites in medicine and cosmetology – ZEOMEDCOS. SWB, Baku-London, 2010).

Figure 6.

The fate of isotope labelled activated clinoptilolite-zeolite in the gastro-intestinal tract (by courtesy from Application of natural zeolites in medicine and cosmetology – ZEOMEDCOS. SWB, Baku-London, 2010).

Several toxicological studies proved that certain natural zeolite, e.g. clinoptiolite are non-toxic and completely safe for use in human and veterinary medicine.[105] In vitro and in vivo controlled animal studies have shown that clinoptilolite is an inert substance that may cause, in some instances, only moderate but not progressive fibrosis or mesothelioma.[111] This effect might be attributed to side-substances present in natural zeolites, e.g. silica or clay aluminosilicates.[112] It should be also stated that some zeolites might be extremely dangerous for human health and exert negative biological effects. For example, erionite, a fibrous type of natural zeolite, causes a high incidence of mesotheliomas and fibrosis in humans and experimental animals.[113]

Animal studies have also shown the possibility of zeolite A (sodium aluminosilicate) as a viable source of silicon.[4,6,114] The latter is one of known zeolites that breaks down into bioavailable ortho-silicic acid (H4SiO4) in the digestive system. This property arises from the structure of zeolite A which is characterized by the same number of aluminium and silicon atoms in zeolite A.[115] Zeolite A is hydrolysed at low pH (stomach hydrochloric acid) into ortho-silicic acid (H4SiO4) and aluminium ions (Al3+). These are combined back to the amorphous aluminosilicate. Such process readily provides additional source of bioavailable silicon to the organism.[114,116] Indeed, randomized placebo-controlled studies on dogs[114] proved that silicon is absorbed upon oral administration of zeolite A. Comparable results have been obtained in a randomized placebo-controlled research on horses as well.[6] Addition of zeolite A to the diet of young racing quarter horses have resulted in decreased skeletal injury rates and better training performance.[117] However, increased bone formation was found in randomized controlled studies on broodmare horses,[118] but not in yearling horses.[119] Food supplementation with zeolite A in calves showed no changes in bone architecture or mechanical properties.[120] However, in a controlled study Turner et al.[120] showed increased aluminium content in the bone and cartilage of zeolite A-fed calves which is an important safety issue for the zeolite A therapeutic usage.