Different conformation of thiol protease inhibitor during amyloid formation: inhibition by curcumin and quercetin.

Cystatins are thiol proteinase inhibitors ubiquitously present in mammalian body and serve various important physiological functions. In the present study, we examined the effects of acid denaturation on newly identified thiol protease inhibitors from the lungs of Capra hircus (Goat) with a focus on protein conformational changes and amyloid fibril formation. Acid denaturation as studied by CD (Circular Dichroism) and fluorescence spectroscopy showed that purified inhibitor named GLC (Goat Lung Cystatin) populates three partly unfolded species, a native like state at pH 3.0, a partly unfolded intermediate at pH2.0, and unstructured unfolded state at pH 1.0, from each of which amyloid like fibrils grow as assessed by thioflavin T (ThT) spectroscopy. The result showed, native like structure formed at pH 3.0 is more responsive towards amyloid formation when compare to other conformation of proteins. Morphology of the protein species incubated for amyloid process was observed using transmission electron microscopy (TEM). Moreover, anti-fibrillogenic effects of curcumin and quercetin were analysed using ThT binding assay. Curcumin and quercetin produced a concentration dependent decline inThT fluorescence suggesting deaggregation of the fibrils. When added prior to amyloid fibril initiation 50 μM curcumin inhibited amyloid aggregation. However, more quercetin is needed to prevent the same extent of fibrillation. Implications for therapeutics in view of polyphenols as essential nutrients are suggested in lung diseases.