Gallic acid, one of the components in many plant tissues, is a potential inhibitor for insulin amyloid fibril formation.

Proteins under stressful conditions can lead to the formation of an ordered self-assembled structure, referred to as amyloid fibrils, to which many neurodegenerative diseases such as Type II diabetes, Alzheimer's, Parkinson's, Huntington's, etc., are attributed. Inhibition of amyloid fibril formation using natural products is one of the main therapeutic strategies to prevent the progression of these diseases. Polyphenols are the mostly consumed as antioxidants in a human nutrition. Herein, we have studied the effect of a simple polyphenol, gallic acid (GA), one of the main components in plant tissues, especially in tea leaves, on the insulin amyloid fibril formation. Different biophysical characterizations such as turbidity, atomic force microscopy (AFM), Thioflavin T (ThT) assays, circular dichroism, and Fourier transform-infrared spectroscopy have been used to analyze the inhibition of amyloid fibril formation. The occurrence of fibrils in an AFM image and ThT fluorescence enhancement confirms the formation of insulin amyloid fibrils when incubated under acidic pH 2 at 65 °C. In the presence of GA, absence of fibrils in AFM image and no change in the intensity of ThT fluorescence confirms the inhibition of insulin amyloid fibrils by GA. Spectroscopic results reveal that GA inhibits the conformational transition of α-helix → β-sheet, which is generally induced during the insulin fibril formation. It was found that the inhibitory effect of GA is concentration dependent and non-linear. Based on the observed results, we propose that GA interacts with native insulin, preventing nuclei formation, which is essential for fibril growth, thereby inhibiting the amyloid fibril formation. The present results thus demonstrate that GA can effectively inhibit insulin amyloid fibril formation in vitro.
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