Methionine oxidation products as biomarkers of oxidative damage to proteins and modulators of cellular metabolism and toxicity

Abstract

Proteins are highly abundant and readily oxidized targets of reactive species formed in biological systems, with these often accounting for greater than 50 % of the dry mass of biological samples. Of the amino acids present in proteins, the sulfur-containing amino acids cysteine (Cys), cystine and methionine (Met) are some of the most reactive species with a range of biologically-relevant modifying agents including radicals, two-electron species and also many electrophiles. Reaction with Cys gives a wide range of both reversible and irreversible species. Lesser numbers of products are well-characterized for cystine and Met. For the latter, the sulfoxide is often the most abundant product, but other species including the cyclic species dehydromethionine, and methionine sulfone have been characterized and shown to be major species under some circumstances. Whilst the sulfone has been widely reported to arise from the sulfoxide as a result of further oxidation, increasing evidence suggests that it can also be formed directly, without the intermediacy of the sulfoxide, and particularly with singlet oxygen (¹O₂). Whilst the sulfoxide is subject to reduction (e.g. via methionine sulfoxide reductases) and further metabolism in vivo, the sulfone appears to be a stable product and may therefore under certain circumstances be a biomarker of Met oxidation. This article briefly reviews the oxidation chemistry of Cys and cystine, and a more detailed discussion of the mechanisms of Met oxidation, formation of the sulfoxide, dehydromethionine and sulfone, and the biological fates and activities of these species.