Exploring the binding of quercetin-3-O-metabolites with transthyretin

Transthyretin (TTR) is a homotetrameric transport protein and the primary carrier for thyroxine (T4) in cerebrospinal fluid, but only a secondary carrier in the blood. The main role of peripheral TTR is the transport of retinol complexed to retinol-binding protein. Wild-type (wt) TTR has a stable tetrameric structure, although some unknown conditions could decrease its stability, increasing dissociation and aggregation to form amyloid fibrils. Along with the already known capability of some chemical compounds to stabilize TTR tetramer and inhibit TTR amyloidosis1, the neuroprotective effects of polyphenols present in food and beverage are recently recognized. Dietary intake of nutraceuticals, such as polyphenols from fruits and vegetables, wine and tea, extra virgin olive oil and cocoa, has an adjuvant effect in fighting TTR amyloidosis. A recent investigation has been conducted on genistein, apigenin and their metabolic derivatives with the aim to understand their biological activities. Enzymatic activity within the liver and intestines act on these flavonoids, to yield glucuronides. When glucoronidated in position 7, genistein and apigenin showed a poorly defined electron density, in agreement with the very weak in vitro interactions with TTR, probably because of steric hindrance from the glucoronide2. Quercetin is another natural polyphenol with good binding affinity for TTR3. Its principal metabolic transformation occurs at position 34 (figure 1). In contrast with the attempts to highlight the binding of 7-susbstituted flavons with TTR, nothing is known about the position 3- of flavonols. Our study aims to elucidate the potential activity of such derivatives against TTR with a special focus on derivatives isolated from plants: O-glucuronide and O-glycosydes. We investigated the binding mode, which is predicted from molecular modeling studies to be different from that of known natural 7-O-substituted derivatives, by crystallization screening of 3-O-derivatives complexed with TTR (figure 1 A, B). Results will be discussed.