Tissue silicon; a study of the ethanol-soluble fraction, using 31Si.

The view that silicon in animal tissues occurs in combination with organic groups has appeared repeatedly in the literature. As early as 1897 Drechsel & Winogradow described the extraction from goose feathers of an orthosilicate of the formula Si(OCuH5,90)4, stated to be an ester of a dihydric alcohol homologous with cholesterol. Isaacs (1924) inferred the presence of silicolipins in brain. Holzapfel (1943 a), investigating the composition of lung tissue in silico-tuberculosis, found Qrganosilicates, 'compounds in which the free valencies of the silica framework, instead of being satisfied with inorganic elements, such as calcium and magnesium, are satisfied with organic radicals'. She further reported both nitrogen-free and nitrogenous organic silicon compounds as derivatives of carbohydrates and proteins. Holzapfel (1942) reported that silicon compounds could be extracted from silicotic lungs by alcohol and ether, and that Debye-Scherrer photographs of these extracts and of substances similarly extracted from goose feathers showed similarities, but she gave no experimental evidence of this. Holzapfel (1943 b) found that ether removes only fats and cholesterol from blood, but that subsequent extraction with a mixture of ether and ethanol gives a fraction containing silicon, together with phosphatides, cholesteryl esters and soaps, and assumed that compounds similar to the phosphatides exist in which one or more phosphorus atoms are replaced by silicon. Assuming that the whole ofthe blood silicon is in organic combination, the insolubility of the greater portion was explained by the suggestion that compounds of polysilicic acids possess a very low solubility. Similar extraction of lung tissue showed that it behaved differently from blood (Holzapfel, 1947); the phosphatide fractions were free from silicon. As it was found in the ethereal extract containing fats and esters, Holzapfel suggested that silicon also occurred in combination with glycerol esters. Ohlmeyer & Olpp (1944) assumed that esters of silicic acid are present in the tissues and compared the ethanol-ester soluble silicon levels in several tissues. They found no significant variation in the 'organic silicon', although the urine gave a high value for non-extractable (inorganic) silicon. Although silicon has been reported as occurring in so many different types of organic compounds in biological material, there is no case recorded in which a compound has been isolated, and this work was undertaken in a further attempt to elucidate the problem. It soon became apparent that the usual chemical methods of analysis for the determination ofsilicon were inadequate to deal with the very small amounts present in the fractions. For this reason much of this investigation has been carried out with isotopically labelled silicon. Preliminary work (Holt, Yates & Tomlin, 1951) had shown that radioactive silicon, injected as silicic acid, is absorbed and is distributed over the tissues generally. A more extensive survey has confirmed that there is no large accumulation in any organ except the kidney, although the more vascular tissues give rather higher values. The liver, being the largest vascular organ, was chosen as the source of material for this investigation into the nature of the silicon complexes. Earlier workers have used ethanol or etherethanol mixtures to remove 'organic silicon' compounds from tissue. Ethanol also removes water and it is possible that a part of the silicon removed may be silicic acid which is associated with this water. For this reason tissues were usually dried by azeotropic distillation before extraction. As there is no evidence that ethanol is the most efficient solvent and particularly because it has been shown that ethyl silicate may be formed by some extraction procedures (Holt & Yates, 1950), other solvents have also been used. Fractionation of the ethanol-soluble extracts by the usual methods of fractional distillation and solvent partition have been attempted with little success; indirect methods have therefore been used. Attempts made to form the complexes in vitro have indicated their probable nature, and have explained why the usual methods of fractionation are unsuccessful.