Abstract: The relationship between rice glutelin structure and the antioxidant activity and angiotensin-converting enzyme (ACE) inhibitory activity of its hydrolysates at different times of in vitro simulated digestion was studied under different hydrolysis time. Endogenous fluorescence spectroscopy and Fourier transform infrared spectroscopy (FTIR) were used to determine the structural changes during glutelin hydrolysis. The biological activity of the hydrolysate was characterized by measuring its antioxidant capacity and ACE inhibitory activity. Quadrupole tandem time-of-flight mass spectrometry was used to identify the most antioxidant and ACE inhibitory components of the hydrolysate. The results showed that the degree of hydrolysis (DH) of glutelin increased rapidly in the first 2 h of digestion, and then tended to level off. After the hydrolysis of glutelin, the maximum absorption wavelength (λmax) was red-shifted, together with an increase in the fluorescence intensity. In addition, the proportion of α-helix structure significantly increased, resulting in decreased antioxidant capacity, while the proportion of β-sheet structure decreased significantly, resulting in increased ACE inhibitory activity (P < 0.05). The hydrolysate of glutelin had the strongest antioxidant capacity at 0.5 h, with half maximal inhibitory concentration (IC50) of (1.113 ± 0.015), (0.518 ± 0.053) and (0.678 ± 0.019) mg/mL for 1,1-diphenyl-2-picrylhydrazyl free radical scavenging, hydroxyl radical scavenging and Fe2+-chelating capacity, respectively. The ACE inhibitory activity first increased, reaching the maximum at 2 h, with an IC50 of (0.693 ± 0.011) mg/mL, and then decreased. The increase in the DH of glutelin reduced the antioxidant capacity, but enhanced the ACE inhibitory activity. The antioxidant capacity of the hydrolysate from 0.5 to 2 h was negatively correlated with the ACE inhibitory activity and then both decreased. The molecular masses of the antioxidant peptides and the ACE inhibitory peptides in the hydrolysate were both below 2 kDa, and VEGGFLFIV was selected as a representative antioxidant peptide. Most of the ACE inhibitory peptides were N-terminal hydrophobic amino acids or basic amino acids. Therefore, optimal functional peptide products with antioxidant or antihypertensive activity can be prepared by controlling the DH during the in vitro digestion of rice glutelin.

Key words: rice glutelin; trypsin; antioxidant activity; angiotensin-converting enzyme inhibitory activity