Protein denaturation induced by cyclic silicone.

While it has been demonstrated that such low-molecular-weight cyclic silicones as octamethylcyclotetrasiloxane (D4) exhibit adjuvant activity, the mechanism of immunological response to silicone is still not clear. This study therefore used fluorescence and circular dichroism (CD) spectroscopy to investigate the denaturation and conformational change of two proteins, fibronectin (Fn) and fibrinogen (Fbg), induced by D4 in vitro. Incubating D4 with Fbg or Fn at D4-to-protein ratios of > 100 or for > 10 h yielded white and mould-like precipitates of the proteins, indicating massive denaturation and aggregation. The fact that the decrease in fluorescence intensity of D4-treated Fn and Fbg was accompanied by a red shift in the maximum wavelength also indicated that denaturation of the proteins had taken place. These changes in fluorescence might result from exposure of tryptophan residuals in the proteins to polar water molecules inasmuch as the denaturation would lead to changes in the tertiary structures of the proteins and rearrangement of the tryptophan residues. The loss of the tertiary structure leads to protein denaturation and, consequently, precipitation. The difference in CD spectra between control Fbg (or Fn) and D4-treated Fbg (or Fn) indicated conformational changes of the proteins when incubated with D4. Thus it has been demonstrated that D4 can induce denaturation and conformational changes in Fbg and Fn and it can be expected that protein molecules that have undergone denaturation or conformational change induced by D4 may act as antigens and stimulate the immune system to generate antibodies, ultimately resulting in autoimmune disease.