*4.2. Preparation of OPs*

OPs were prepared by enzymatic hydrolysis from the oyster meat according to the methods of Li et al., Peng et al., and Zhang et al. [17,41,47]. Briefly, three kilograms of oyster meat were ground into mince and then mixed with distilled water (1:3 *w*/*v*). The mixture was homogenized at 8000 rpm for 5 min by using a homogenizer. Homogenates were hydrolyzed at pH 7.0 with compound protease (enzyme concentration 1000 U g−<sup>1</sup> of raw material). The hydrolysis reaction lasted for 5 h in a 53 ◦C water bath. Subsequently, the protease was inactivated at 100 ◦C for 10 min and the enzymatic hydrolysate was centrifuged at 15,000g at 4 ◦C for 20 min to obtain the supernatant. The supernatant was fractionated by an ultrafiltration device (XX42PMINI, Millipore, USA) and 10 kDa, 5 kDa, and 3 kDa ultrafiltration membranes (Mili Pellicon, Millipore, USA) to obtain the components used in this study (<3 kDa hydrolysate fraction). The samples were freezedried into powder for subsequent experiments (FD-551, EYELA, Tokyo, Japan).
