We examined the inhibitory activity of angiotensin I converting enzyme (ACE) in protein hydrolysates from dulse, Palmaria palmata. The proteins extracted from dulse were mainly composed of phycoerythrin (PE) followed by phycocyanin (PC) and allophycocyanin (APC). The dulse proteins showed slight ACE inhibitory activity, whereas the inhibitory activity was extremely enhanced by thermolysin hydrolysis. The ACE inhibitory activity of hydrolysates was hardly affected by additional pepsin, trypsin and chymotrypsin treatments. Nine ACE inhibitory peptides (YRD, AGGEY, VYRT, VDHY, IKGHY, LKNPG, LDY, LRY, FEQDWAS) were isolated from the hydrolysates by reversed-phase high-performance liquid chromatography (HPLC), and it was demonstrated that the synthetic peptide LRY (IC50: 0.044 μmol) has remarkably high ACE inhibitory activity. Then, we investigated the structural properties of dulse phycobiliproteins to discuss the origin of dulse ACE inhibitory peptides. Each dulse phycobiliprotein possesses α-subunit (Mw: 17,477–17,638) and β-subunit (Mw: 17,455–18,407). The sequences of YRD, AGGEY, VYRT, VDHY, LKNPG and LDY were detected in the primary structure of PE α-subunit, and the LDY also exists in the APC α- and β-subunits. In addition, the LRY sequence was found in the β-subunits of PE, PC and APC. From these results, it was suggested that the dulse ACE inhibitory peptides were derived from phycobiliproteins, especially PE. To make sure the deduction, we carried out additional experiment by using recombinant PE. We expressed the recombinant α- and β-subunits of PE (rPEα and rPEβ, respectively), and then prepared their peptides by thermolysin hydrolysis. As a result, these peptides showed high ACE inhibitory activities (rPEα: 94.4%; rPEβ: 87.0%). Therefore, we concluded that the original proteins of dulse ACE inhibitory peptides were phycobiliproteins.

We found that the red alga dulse (Palmaria palmata) contains a lot of proteins, which is mainly composed of phycoerythrin (PE) and the protein hydrolysates showed high angiotensin I converting enzyme (ACE) inhibitory activities. Therefore, we investigated the structure of dulse PE to discuss its structure‐function relationship. We prepared the chloroplast DNA and analyzed the nucleotide sequences encoding PE by cDNA cloning method. It was clarified that dulse PE has α‐ and β‐subunits and they are composed by 164 amino acids (MW: 17,638) and 177 amino acids (MW: 18,407), respectively. The dulse PE contained conserved cysteine residues for chromophore attachment site. On the alignment of amino acid sequences of dulse PE with those of other red algal PE, the sequence identities were very high (81–92%). In addition, we purified and crystallized the dulse PE, and its crystal structure was determined at 2.09 Å resolution by molecular replacement method. The revealed 3D structure of dulse PE which forms an (αβ)6 hexamer was similar to other red algal PEs. Conversely, it was clarified that the dulse PE proteins are rich in hydrophobic amino acid residues (51.0%), especially aromatic amino acid and proline residues. The data imply that the high ACE inhibitory activity of dulse protein hydrolysates would be caused by the specific amino acid composition and sequence of dulse PE. Practical Applications Dulse is an abundant and underused resource, which contains a lot of phycobiliproteins. Then, the dulse protein hydrolysates strongly inhibited the activity of ACE. Therefore, it has the potential to be an ingredient of functional food.

In this study, we investigated antioxidant activity of proteins from the red alga dulse (Palmaria sp.) harvested in Hokkaido, Japan. The dulse proteins that contain phycoerythrin (PE) as the main component showed a high radical scavenging activity. To clarify the key constituent of antioxidant activity in dulse proteins, we prepared recombinant dulse PE β‐subunit (rPEβ) (apoprotein) and chromophores from the dulse proteins. As a result, the rPEβ showed lower radical scavenging activity than that of dulse proteins. On the other hand, the dulse chromophores composed mainly of phycoerythrobilin (PEB) indicated extremely higher radical scavenging activity (90.4% ± 0.1%) than that of dulse proteins (17.9% ± 0.1%) on ABTS assay. In addition, on cell viability assay using human neuroblastoma SH‐SY5Y cells, the dulse chromophores showed extracellular and intracellular cytoprotective effects against H2O2‐induced cell damage. From these data, we concluded that the dulse proteins have antioxidant ability and the activity principally derives from the chromophores. Practical application Dulse is an abundant and underused resource, which contains a lot of proteins, especially phycoerythrin. We here demonstrated that the practically prepared dulse proteins possessed antioxidant activity and clarified that chromophores from the dulse proteins were the key components. Therefore, the dulse proteins have a potential for functional material.