The relationship of physicochemical properties and structure to the antioxidative activity of free amino acids in the aqueous Fenton system

Abstract

Iron is essential for plant growth, metabolism of mitochondria and chloroplasts, and a wide assortment of enzymes. However, it is also involved in Fenton reaction that generates hydroxyl radical (HO•), the most reactive species in plants. The aim of our study was to examine the effects of amino acids on HO• production. The rank order according to antioxidative activity (AA) of amino acids in Fenton system was: Trp > Phe, Leu > Ile > His > Arg > Val > Lys, Tyr, Pro > Gln, Thr, Ser > Glu, Ala, Gly, Asn, Asp. Sulfur-containing amino acids generated different secondary reactive products, which were discriminated by EPR spin-trapping spectroscopy. AA showed positive correlation with hydrophobicity and negative correlation with polarity. HO•provoked oxidation of amino acids was strongly positively affected by hydrophobic hydration. Group contribution method showed that the reactivity of amino acids with HO• is defined by the properties of side-chains (the contribution of NH3+CHCOO− group and antioxidative activity of Gly were next to zero). Our results might shed a new light on the role of iron in the regulation of free amino acids pool and on protein oxidation in plants. According to our findings, native proteins (with exposed polar residues) might not be particularly susceptible to oxidation by HO•. But once a protein is damaged and partially denatured it exposes hydrophobic side-chains and might become a likely target for HO•, thus potentially acquiring an antioxidative role. Further research on change of redox properties of proteins with denaturation is warranted.