Presence of Modified Peptides with High Bioavailability and Angiotensin-Converting Enzyme Inhibitory Activity in Japanese Fermented Soybean Paste (<i>Miso</i>)

Not only free amino acids and normal short-chain peptides but also modified amino acids, such as <i>N</i>-acetyl- and <i>N</i>-formyl amino acids, monoamines, polyamines, and modified peptides, such as isomerized aspartyl peptides, pyroglutamyl peptides, and diketopiperazines, were identified in Japanese fermented soy paste (<i>miso</i>) prepared using different fungal starters, rice, barley, and soybean-<i>koji.</i> One hour after oral administration of water extract of soybean-<i>koji miso</i> to rats, the modified peptides increased significantly in the lumen upon the ingestion, while the normal peptides did not. In the blood from the portal vein and abdominal vena cava, 17 and 15 diketopiperazines, 16 and 12 isomerized aspartyl peptides, and 2 and 1 pyroglutamyl peptides significantly increased to approximately 10-400 nM, respectively. The modified peptides, which increased in rat blood, showed angiotensin-converting enzyme (ACE) inhibitory activity in a dose-dependent manner, indicating multiple ACE inhibitory peptides with high bioavailability in <i>miso.</i> Among them, l-β-Asp-Pro showed the highest ACE inhibitory activity (IC₅₀ 4.8 μM).