Presence of γ-glutamyl and β-aspartyl isopeptides, diketopiperazines, pyroglutamyl peptides, in addition to normal peptides in fish and soy sauces: Structures, contents and their bioavailability

This study identified peptides in fish and soy sauces and elucidated their bioavailability in rats. Over 96 peptides including γ-glutamyl, pyroglutamyl, β-aspartyl peptides, and diketopiperazines were detected. The content of these peptides varied greatly between the products tested. After the administration of Vietnamese fish sauce which had the highest peptide content among samples tested, most normal peptides did not significantly increase in the blood; whereas γ-glutamyl and pyroglutamyl peptides significantly increased in the small intestine and some hydrophobic γ-glutamyl isopeptides and pyroglutamyl-proline significantly increased in the blood. Diketopiperazines and β-aspartyl isopeptides significantly increased in the small intestine and the blood. These findings highlight the presence of modified peptides in fish and soy sauces, which are commonly consumed in daily dishes in East Asia. Only modified peptides such as diketopiperazines, β-aspartyl isopeptides and hydrophobic γ-glutamyl isopeptides survived gastrointestinal digestion, entering blood circulation, suggesting their potential biological activities.