## 1 Introduction  
High blood pressure or hypertension is considered to be 130/80 mmHg (systolic/diastolic blood pressure) in stage 1 and higher than 140/90 mmHg in stage 2 in a patient [1, 2]. It has been confirmed as one of the most common chronic health crises, not to mention that it also enhances the risk of developing other metabolic disorders, such as cardiovascular disease and diabetes [3, 4]. In addition, hypertensive patients are more vulnerable to developing serious COVID-19 complications, which can lead to an increase in mortality rates [5, 6].  
The angiotensin-I-converting enzyme (ACE; EC 3.4.15.11) is known to regulate blood pressure and inflammation in the human body through the renin-angiotensin system (RAS). A zinc dipeptide carboxypeptidase contributes to the conversion of decapeptide angiotensin I (Ang I) to octapeptide angiotensin II (Ang II) through the cleavage of the carboxyl-terminal His-Leu dipeptide. **Ang II's primary function is to induce systemic arteriolar contraction, and consequently, high blood pressure is** an inevitable result [1; 4]. Therefore, the inhibition or inactivation of ACE is one of the most efficient therapeutic strategies for hypertension treatment [4; 7]. The high-speed development of pharmaceutical drugs has played an important role in hypertension therapies with increased efficiency; however, their undesirable side effects have also been mentioned in parallel [3; 4]. Accordingly, the application of natural peptide therapies has become an attractive alternative to synthetic compounds. A large number of studies concerning the discovery of ACE inhibitory (ACEI) peptides have been carried out on different dietary-derived proteins, including milk proteins [5; 8; 9] and chicken egg white [10; 11].  
Previously published evidence of ACEI efficiency from soft-shelled turtles (SST) has been highlighted for egg, meat, and powder hydrolyses [12; 13; 14]. Consequently, potential ACEI peptides were identified from egg whites and egg yolks of SST [15; 16; 17]. Recently, dipeptidyl peptidase-IV (DPP-IV) inhibitory peptides were also identified in SST yolk (SST) hydrolyses using gastrointestinal enzymes [18]. This suggests that the SST egg is a powerful bioactive peptide source. Multiple-function peptides show promising results in the research of novel peptide therapy, as they could reduce costs and are more convenient for patients undergoing treatment of diseases. Interestingly, the activity of ACE could be inhibited by some DPP-IV inhibitors [19; 20]. The DPP-IV inhibitory effect is often associated with the N-terminal of a peptide and the ACE inhibitory activity is affected by the C-terminal of a peptide. Therefore, both terminal positions play significantly important roles in allowing some inhibitors to possess these dual functional properties, while others only have a single impact. The representation of proline (P) and/or hydrophobic amino acids has been commonly reported in the sequence of DPP-IV inhibitory peptides [19; 20]. Peptides with hydrophobic amino acid residues (W, F, Y, or P) at the C-terminal position are often associated with potential ACE inhibitory activity [20; 21; 22].  
Therefore, in this study, five DPP-IV inhibitory peptides (LPSW, LPLF, VPGLAL, WLQL, and LVGLPL) previously identified from SSTY hydrolyses were examined for their ACEI effects using in vitro and in silico tests [18]. The ACEI efficiency of these peptides was determined by calculating their IC\({}_{50}\) values using synthetic peptides. We used Lineweaver-Burk plots to investigate the peptides' inhibitory effect on ACE, as well as the interaction mechanism between the peptides and the ACE through docking simulation. Additionally, the overall structural stability of the peptides with ACE was evaluated through an ACE pre-incubation assay.