Protein Z-based Promising Carriers for Enhancing Solubility and Bioaccessibility of Xanthohumol

Xanthohumol (XN), as an abundant chalcone in hops, has received increasing attention due to its pharmacologic effects. However, poor solubility and bioaccessibility in aqueous solutions have limited its application. This work reported a novel carrier to improve the solubility and bioaccessibility of XN by using heat-stable, serpin-like protein Z (PZ) derived from barley malt. In this study, the solubility of XN was increased by 145-fold (vs. free XN in water) using 200 mu M PZ as the carrier. Fluorescence results demonstrated that XN interacted with PZ via static quenching process, with binding constant (K (a)) of 5.70 x 10 (4) L/mol at 298 K. PZ as a serine protease inhibitor exhibited excellent in vitro digestive stability that could inhibit trypsin with a low half inhibitory concentration (IC50) of 3.90 +/- 0.03 mg/L and showed only 26.54 +/- 0.70% degree of hydrolysis after gastrointestinal digestion. Furthermore, the retention ratio of XN was 50.28 +/- 1.80% after gastrointestinal digestion, which is 8-fold higher than XN alone, suggesting that PZ remarkably enhanced the bioaccessibility of XN. Molecular dynamics simulation analysis revealed that XN was bound within the hydrophobic pocket of PZ, near the Glu338 residue of PZ, whereas the external active loop of PZ was involved in binding with trypsin. This helped to elucidate the roles of PZ in improving the bioaccessibility of XN that PZ encapsulated XN in its hydrophobic cavity, without influencing its inhibition on trypsin. These findings suggest that PZ may be developed as carriers that protect XN or other hydrophobic bioactive molecules in the gastrointestinal tract.