Angiotensin I-converting enzyme (ACE) inhibitory peptides derived from seaweed represent a potential source of new antihypertensive. The aim of this study was to isolate and purify ACE inhibitory peptides (ACEIPs) from the protein hydrolysate of the marine macroalga Ulva intestinalis. U. intestinalis protein was hydrolyzed by five different proteases (trypsin, pepsin, papain, α-chymotrypsin, alcalase) to prepare peptides; compared with other hydrolysates, the trypsin hydrolysates exhibited the highest ACE inhibitory activity. The hydrolysis conditions were further optimized by response surface methodology (RSM), and the optimum conditions were as follows: pH 8.4, temperature 28.5 °C, enzyme/protein ratio (E/S) 4.0%, substrate concentration 15 mg/mL, and enzymolysis time 5.0 h. After fractionation and purification by ultrafiltration, gel exclusion chromatography and reverse-phase high-performance liquid chromatography, two novel purified ACE inhibitors with IC₅₀ values of 219.35 μM (0.183 mg/mL) and 236.85 μM (0.179 mg/mL) were obtained. The molecular mass and amino acid sequence of the ACE inhibitory peptides were identified as Phe-Gly-Met-Pro-Leu-Asp-Arg (FGMPLDR; MW 834.41 Da) and Met-Glu-Leu-Val-Leu-Arg (MELVLR; MW 759.43 Da) by ultra-performance liquid chromatography-tandem mass spectrometry. A molecular docking study revealed that the ACE inhibitory activities of the peptides were mainly attributable to the hydrogen bond and Zn(II) interactions between the peptides and ACE. The results of this study provide a theoretical basis for the high-valued application of U. intestinalis and the development of food-derived ACE inhibitory peptides.

In order to utilize seaweeds as a natural therapeutic agent in aquatic cultures, it is important to evaluate their antimicrobial activities. We analyzed that of the typical seaweeds of the Zhejiang Coast in order to improve their potential utilization. Six species of seaweeds were collected from the Zhejiang coast—Ulva pertusa, Ulva prolifera, Gloiopeltis furcata, Gracilariopsis lemaneiformis, Sargassum fusiforme and Ishige okamurae—and their ethanolic extracts were tested for antibacterial effects as well as antiviral activity against the white spot syndrome virus (WSSV). The seaweed extracts inhibited bacterial growth in vitro, and increased the immune responsiveness and survival of the crab Scylla paramamosain infected with WSSV in a concentration-dependent manner. U. prolifera, G. lemaneiformis, and S. fusiforme showed the most potent antibacterial activities and most significant enhancement of the innate immunity in healthy crabs. In conclusion, our study showed that the seaweed extracts have therapeutic effects and are a potential natural medicine for aquatic animals.