Effect Of Sodium Dodecyl Sulfate (SDS) On Protein Amyloid Fibrosis

Protein is one of the most important materials in maintaining life activities of human beings. Keeping a protein in a correct conformation is a prerequisite for its normal physiological functions. Changes in the three-dimensional structure of a protein may lead to loses of biological activity and even occurrence of diseases. More than 20 human diseases have been recognized to be associated with mis-folding and amyloid fibrillation of proteins or peptides, including Alzheimer’s and Parkinson’s diseases. One of the hallmarks of these diseases is amyloid fibrillation of related proteins under some environments. Numerous investigations suggest that three phases are involved in protein fibrillation, namely lag phase, growth phase, and eventually a maturity phase. There are nevertheless a few proteins on which the fibrillation process does not rely on the lag phase. During amyloid fibrillation, a protein undergoes reducing a-helical structure which gradually transforms into β-sheet-rich species and exposuring its inner hydrophobic domains to aqueous environment. One of the therapeutic strategies for the treatment of amyloidogenic disorders is to screen small organic molecules which are able to inhibit amyloid formation and disrupt the formed amyloid assemblies. An intensive investigation in the anti-amyloidogenic role of an inhibitor will be conducive to shed light into the molecular mechanism and provide effective lead structures in the drug design.In the present study, bovine serum albumin (BSA) has been utilized as an in vitro model to explore protein amyloid fibrillation under different conditions and the inhibitory effect of a surfactant sodium dodecyl sulfate (SDS). The main lines of this study include amyloid fibrillation of BSA in neutral and acidic mediums, and the anti-amyloid effects of SDS and its structural analogues. Intrinsic fluorescence and gel electrophoresis have also utilized to elucidate the molecular mechanism of interactions between BSA and small molecules.Experimentals and Results1. Fibril growth of BSA under different environmentsThioflavin T (ThT),8- anilino-1-naphthalenesulfonic acid (ANS) fluorescence probes and circular dichroism (CD) have been utilized monitoring the growth kinetics and changes in the secondary structures of BSA fibrillation. The resultant fibrillar morphology was observed under a transmission electron microscopy (TEM). The results indicated that BSA was able to form amyloid fibrils under both pH 7.4 and pH 3.0 through a pathway without a nucleation phase. Upon amyloid fibrillation, decreases in the a-helical structure and surficial hydrophobicity and an increase in the p-sheet structure of BSA molecules have been observed. The mature BSA fibrils showed a unique amyloid morphology, characterized by dense and net-like fibrils with branched structures, different from the ordinary amyloid morphology. At pH 3.0, BSA took a longer time to reach the maturity phase than a neutral medium. An analysis of intrinsic fluorescence suggested that BSA had a tight packing under an acidic medium.2. The influence of SDS no BSA fibrillationSDS is capable of inhibiting BSA fibrillation under both pH 7.4 and pH 3.0 in a dose-dependent manner. The inhibitory role of SDS was weaker at pH 3.0 than pH 7.4. The interactions between SDS and BSA were analyzed by intrinsic fluorescence. The resultant data showed that SDS had a higher binding constant to BSA at pH 7.4 with a binding ratio of 1:1.3. The inhibitory roles of SDS analogues on BSA fibrillationSDS is an anionic surfactant, consisting of a sulfonate end and a hydrophobic straight-chain alkane. The binding of SDS to BSA may involve hydrophobic and/or electrostatic interactions. To further explore the binding mechanism, the inhibitory roles of several SDS analogues on BSA fibrillation have been detected. The results indicated that at pH 7.0, hydrophobic interaction was predominated in the amyloid inhibition by SDS and its analogues. The inhibitory role was proportional to the length of alkane chain, although longer alkane chain did not increase amyloid inhibition if the carbon-number was greater than 10. at pH 3.0, sodium laurate demonstrated a promotive role on BSA fibrillation, originated possibly from the interaction between the carboxyl group and a proton-receptor, leading to a transformation of the protein into a conformation prone to amyloid fibrillation. Indeed, dodecanic acid can keep non-ionized under a weak acidic condition.Conclusion:BSA was able to form amyloid fibrils under both neutral (pH 7.4) and weak acidic (pH 3.0) mediums. The growth of BSA amyloid does not rely on a nucleation process. During amyloid fibrillation, changes in the properties of BSA occur, including decreases in the a-helical structure and surficial hydrophobicity and an increase in the β-sheet structure. The resultant mature BSA fibrils are characterized as dense, net-like and branched morphology, somewhat different from the ordinary amyloid structure. Comparatively, BSA fibrils form in a small growth rate at pH 3.0. The anionic surfactant SDS is capable of inhibiting BSA fibrillation, with a higher efficiency at pH 7.4 than at pH 3.0. Hydrophobic interaction is predominated in the inhibitory role of SDS on BSA amyloid formation at pH 7.4. However, at pH 3.0, the binding force of SDS on BSA decreases and therefore the capability of SDS on amyloid formation is reduced.