Repair of Protein Radicals by Antioxidants

Abstract

In vivo, proteins are the main targets for radicals and other reactive species. Their reactions result in formation of amino acid radicals on the protein surface that often yield tryptophan and tyrosyl radicals or, in the presence of O2, protein peroxyl radicals and hydroperoxides. All these species may propagate damage to biomolecules. Low molecular weight antioxidants, such as ascorbate, urate, and glutathione, are part of the defense system and function by repairing damaged proteins. We briefly review the existing knowledge about protein and amino acid radicals and their repair by antioxidants, including results of our investigations. The main question addressed is whether the antioxidants ascorbate, urate, and glutathione are able to repair amino acid radicals in model compounds and in proteins in vitro by pulse radiolysis. We show that ascorbate and urate repair tryptophan and tyrosyl radicals efficiently and inhibit proton‐coupled electron transfer from tyrosine residues to tryptophan radicals in a number of proteins. In contrast, repair by glutathione is much slower. Ascorbate also rapidly reduces the peroxyl radicals of the N‐acetylamide derivatives of glycine, alanine, and proline, whereas glutathione reduces peroxyl radicals in lysozyme. In vivo urate, ascorbate, and glutathione may prevent biological damage or, at least, reduce its rate, because they: (a) repair tryptophan and tyrosyl radicals in proteins and (b) reduce protein peroxyl radicals to the corresponding protein hydroperoxides. Most likely, in vivo, ascorbate and glutathione do not inhibit the reaction of C‐centered amino acid radicals with O2. Glutathione is less efficient that urate and ascorbate in repairing protein radicals; furthermore, the resulting glutathiyl radical is harmful. Ascorbate may be the more important repair agent in cells and tissues characterized by high ascorbate concentrations, such as the lens and brain; urate may be mainly responsible for repair in tissue compartments with higher urate concentrations, such as in plasma and saliva.

Kinetics of the reactions of catechins with hypochlorite, peroxynitrite, and amino acid–derived peroxyl‐ radicals

“…This confirms previous assumptions that peroxynitrite scavenging capacity of catechins is due to scavenging radical intermediates of peroxynitrite isomerization and/or repairing molecules damaged by peroxynitrite. Comparing the rate constants of the reactions of hypochlorite, peroxynitrite, and amino acid peroxyl radicals with the main low-molecular antioxidants in vivo, like ascorbate and glutathione, 47,48 with those obtained in this work for catechins, and having in mind that catechins concentration in plasma and tissue is several order of magnitude lower than that of ascorbate and glutathione, 46,49 it seems that catechins absorbed from food do not play a significant role in scavenging hypochlorite, peroxynitrite, and amino acid peroxyl radicals.…”

“…This value is comparable with the rate constants of the reactions of catechins with superoxide, 41,45 but two order of magnitude lower that the rate constants reported for the reaction of ascorbate and urate with glycine, alanine, and proline peroxyl radicals. 46 It seems that the values of the rate constants of the reactions of flavonoids with peroxyl radicals evaluated from the kinetics of flavonoid aroxyl radical formation 20 are overestimated, because part of flavonoid radicals may be formed in the fast reaction with C-centered radicals (see reactions A4 and A5 in the Appendix).…”

Fast Antioxidant Reaction of Polyphenols and Their Metabolites

“…This cannot be achieved under OS by the normal levels of the endogenous antioxidants. In fact, measurements of the rate constants of reactions of ascorbate, urate and GSH with C-centred radicals have shown their reactions to be too slow to ensure effective scavenging of the radicals in tissues with a low antioxidant content [ 21 , 22 , 23 ]. The obvious defensive tactic of increasing the levels of lthe endogenous antioxidants to levels sufficient to prevent the consequences of OS is not possible: in the case of enzymes and metal-chelating proteins, their activities are tightly regulated and are not amenable to manipulation in human populations.…”

Initiation and Prevention of Biological Damage by Radiation-Generated Protein Radicals

“…A similar mechanism of electron transfer to more electron-deficient sites in proteins exposed to ionizing radiations would explain the selective loss of specific residues and the concept of “radical sinks” [ 42 ]. LRET was virtually abolished in the presence of ascorbate with simultaneous formation of the ascorbyl radical [ 43 ].…”

“…According to this scheme, the most effective point of repair is reduction of the Pr ● , preventing its reaction with O 2 and the generation of a new range of reactive species. The obvious candidates for this role are the endogenous low molecular weight antioxidants, ascorbate (HAsc − ), urate (H 2 Ur − ), and glutathione (GSH) [ 34 , 43 ]. This defense system is adequate under normal conditions, but it is insufficient under the unfortunately common condition of oxidative stress, when the oxidative challenge exceeds the antioxidant potential of the organism [ 51 ].…”