As shown in Figure 1A, Captopril- and PPLLFAAL-treated SHRs exhibited a significant decrease in SBP. Captopril significantly reduced the SBP (from 190 to 151 mmHg at 4h, p < 0.05), which then increased to 161 mmHg at 24 h. The SBP reduction curve that was obtained for PPLLFAAL was similar to that obtained with captopril. It was notable that the PPLLFAAL could maintain lower SBP levels for a longer period compared with the captopril group after intravenous administration. The results indicated that PPLLFAAL substantially reduced the SBP between 2 and 4 h (p < 0.05), with the largest decrease in SBP from 193 to 145 mmHg occurring at 4 h. The SBP then began to recover and maineained a level of 154 mmHg at 24 h. In addition, PPLLFAAL could also affect the DBP (Figure 1B). PPLLFAAL could significantly reduce the DBP of SHRs from 135 to 107 mmHg at 4 h (p < 0.05), which was then restored to a level of 113 mmHg at 24 h.

Figure 1. Changes in spontaneously hypertensive rat's blood pressure after the intravenous administration of PPLLFAAL; (A) SBP changes and (B) DBP changes. Different letters indicate statistically significant differences, as demonstrated using multiple one-way analysis of variance tests (p < 0.05).

Enzymatic hydrolysis

The skins of T. flavidus were hydrolyzed using different enzymes under the corresponding optimal temperature and pH conditions (alcalase, pH 8.0, 55 ℃; neutral protease, pH 7.0, 55 ℃; pepsin, pH 2.0, 37 ℃), with the same enzyme/substrate ratio of 2000 U/g and hydrolysis time of 5 h. Alcalase was indicated that the low-MW peptides were generally more active than high-MW peptides, selected for the generation of ACE-inhibitory peptides from T. flavidus.

The results suggested that PPLLFAAL may have potential applications in functional foods or pharmaceuticals as an antihypertensive agent.