Inhibitory Effects of Imidazole Dipeptides and 2-Oxo-Imidazole Dipeptides on Intracellular ROS Generation and Degradation of Protein and DNA

Imidazole dipeptides (IDPs), including carnosine, anserine, and balenine, are functional food ingredients found in meats. They have been reported to exhibit high antioxidant activity. 2-Oxo-imidazole dipeptides (2-oxo-IDPs) are present in trace amounts in various tissues and show notably higher antioxidant activity compared with IDPs. Trace amounts of 2-oxo-IDPs are also present in commercial IDP reagents, suggesting that they affect the antioxidant activity of IDPs. Trace amounts of 2-oxo-IDPs were detected in IDP reagents from various manufacturers by HPLC. Some reagents with trace amounts of 2-oxo-IDPs exhibited higher antioxidant activity in a DPPH radical-scavenging assay compared with high-purity IDP reagents devoid of 2-oxo-IDPs. Therefore, it is important to use highly purified IDP reagents to measure antioxidant activity accurately. The antioxidant activity of highly purified IDPs and 2-oxocarnosine (2-oxo-Car) was evaluated through their ability to protect protein and DNA from ROS. 2-Oxo-Car markedly inhibited the protein and DNA degradation by ClO− and ONOO− compared with IDPs. Moreover, 2-oxo-Car was not cytotoxic, even at high concentrations, and suppressed pyocyanin-induced ROS generation in C2C12 cells compared with IDPs and glutathione. Overall, 2-oxo-IDPs are effective antioxidants and are equivalent or superior to known water-soluble antioxidants, such as glutathione and vitamin C.