Abstract
Oxidative stress, an excess of reactive oxygen species (ROS), may lead to oxidative post-translational modifications of proteins resulting in the cleavage of the peptide backbone, known as α-amidation, and formation of fragments such as peptide amides and α-ketoacyl peptides (α-KaP). In this study, we first compared different approaches for the synthesis of different model α-KaP and then investigated their stability compared to the corresponding unmodified peptides. The stability of peptides was studied at room temperature or at temperatures relevant for food processing (100 °C for cooking and 150 °C as a simulation of roasting) in water, in 1% (m/v) acetic acid or as the dry substance (to simulate the thermal treatment of dehydration processes) by HPLC analysis. Oxidation of peptides by 2,5-di-tert-butyl-1,4-benzoquinone (DTBBQ) proved to be the most suited method for synthesis of α-KaPs. The acyl side chain of the carbonyl-terminal α-keto acid has a crucial impact on the stability of α-KaPs. This carbonyl group has a catalytic effect on the hydrolysis of the neighboring peptide bond, leading to the release of α-keto acids. Unmodified peptides were significantly more stable than the corresponding α-KaPs. The possibility of further degradation reactions was shown by the formation of Schiff bases from glyoxylic or pyruvic acids with glycine and proven through detection of transamination products and Strecker aldehydes of α-keto acids by HPLC–MS/MS. We propose here a mechanism for the decomposition of α-ketoacyl peptides.
Highlights
Aerobic cellular respiration is common for all known animals
The synthesis approaches were compared based on the overall complexity of the process and the yield of α-ketoacyl peptides (Table 2)
This reaction is only occurring in the absence of water, which is disadvantageous for synthesis of α-ketoacyl peptides
Summary
Aerobic cellular respiration is common for all known animals. Though highly energetically beneficial, it still bears some risks, since it makes the exposure of the organism to reactive oxygen species (ROS) inevitable. ROS have been shown to be formed in vivo in mitochondria as a byproduct of glucose metabolism due to leakage of electrons from the transport chain (Turrens and Boveris 1980) or in activated phagocytic cells as a response to microorganisms (Curnutte and Babior 1987). These reactive compounds have the potential to inflict severe damage on cells, so aerobic organisms use an abundance of protective endogenous antioxidant systems to maintain a precise balance between the formation and neutralization of ROS. This process is generally described as oxidative stress, and since proteins make up to half of dry cell mass, they are likely to become a major subject of an oxidative attack resulting in a post-translational modification (Esterbauer et al 1992; Davies 2005)
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