Not only free amino acids and normal short-chain peptides
but also
modified amino acids, such as N-acetyl- and N-formyl amino acids, monoamines, polyamines, and modified
peptides, such as isomerized aspartyl peptides, pyroglutamyl peptides,
and diketopiperazines, were identified in Japanese fermented soy paste
(miso) prepared using different fungal starters,
rice, barley, and soybean-koji. One hour after oral
administration of water extract of soybean-koji miso 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 miso. Among them, l-β-Asp-Pro showed the
highest ACE inhibitory activity (IC50 4.8 μM)