The Reaction of Hydrogen Atoms with Methionine Residues: A Model of Reductive Radical Stress Causing Tandem Protein–Lipid Damage

Abstract

The occurrence of tandem damage, due to reductive radical stress involving proteins and lipids, is shown by using a biomimetic model. It is made of unsaturated lipid vesicle suspensions in phosphate buffer in the presence of methionine, either as a single amino acid or as part of a protein such as RNase A, which contains four methionine residues. The radical process starts with the formation of H(.) atoms by reaction of solvated electrons with dihydrogen phosphate anions, which selectively attack the thioether function of methionine. The modification of methionine to alpha-aminobutyric acid is accompanied by the formation of thiyl radicals, which in turn cause the isomerization of the cis fatty acid residues to the trans isomers. The relationship between methionine modification and lipid damage and some details of the reductive radical stress obtained by proteomic analysis of irradiated RNase A are presented.

Structural lesion to ribonuclease A caused by reductive stress: Assessment by Raman spectroscopy

“…On the contrary, H • atoms probably attack the enzyme at a more restricted number of sites, perhaps away from the surface and also the far-reaching Tyr can be damaged. A confirm of this behaviour is given by the absence of aggregate formation after H • atom attack [5] , whereas highly aggregated forms of the enzyme have been found under oxidative stress conditions [6,10], since the TyrO • radicals give rise easier to inter-molecular reactions than intra-molecular ones. Since three Tyr "buried" can be probably identified with Tyr-25, -92 and -97, the initial H • radical attack should take place on one or two of them.…”

“…In fact, the H • -induced desulfurisation involving these residues yield α-aminobutyrric acid (Abu) and Ala (reactions (5) and (6)). In addition, the reductive radical attack towards Met residues has been recently found to yield diffusible sulfur radicals able inducing damages in cellular membranes [4][5][6]. ,…”

“…Since recently the reductive stress has been placed in the context of a biological damage, constituting the basis of tandem-lipid damage the separate contribution of a reducing species such as H • to the changes produced in RNAse was investigated more in detail in the present work [5,6]. Neutral hydrogen atom is a simple one-electron equivalent reducing agent and its reactions are of great interest since they can shed light on basic problems of one-electron equivalent reaction kinetics.…”

Biomimetic Chemistry: Radical Reactions in Vesicle Suspensions

“…This first detection is then associated to proteomic analyses, and this approach is expected to open the way to molecular libraries useful for detecting damages in biological samples. We studied bovine pancreatic RNase A individuating both methionine and cystine reactive sites, with modification of Met and Cys to -amino butyric acid (Aba) and alanine (Ala) residues, respectively (Ferreri et al, 2006b;Ferreri et al, 2008). The sequence of amyloid -peptide of 40 amino acids, where only the Met residue is present as a S-containing amino acid, was also studied for its connections with the alteration reported in the Alzheimer disease.…”

Radiation chemical studies of Gly-Met-Gly in aqueous solution

“…We showed that the reaction of H atoms with peptides/proteins determines Met conversion into the non-genetically coded amino acid a-aminobutyric acid (Aba). We have identified this chemical mutation in Met-enkephalin [25], b-amyloid [26], bovine RNase A [27,28], human serum albumin [29], and various metalloproteins [30]. This reactivity has also been placed in a biomimetic context, since it has been suggested as molecular basis of the tandem protein-lipid damage [23,24].…”