Diastereoselective protein methionine oxidation by reactive oxygen speciesand diastereoselective repair by methionine sulfoxide reductase

Recent studies have shown that the "caldum-sensor" protein calmodulin (CaM) suffers an age-dependent oxidation of methionine (Met) to methionine sulfo xide (MetSO) in vivo. However, MetSO did not accumulate on the Met residues that show the highest solvent-exposure. Hence, the pattern of Met oxidatio n in vivo may give hints as to which reactive oxygen species and oxidation mechanisms participate in the oxidation of this important protein. Hen, we have exposed CaM under a series of different reaction conditions (pH, [Ca2], [KCl]) to various biologically relevant reactive oxygen species and oxid izing systems (peroxides, HOCl, peroxynitrite, singlet oxygen, metal-cataly zed oxidation, and peroxidase-catalyzed oxidation) to investigate whether o ne of these systems would lead to an oxidation pattern of CaM similar to th at observed in vivo. However, generally, these oxidizing conditions led to a preferred or exclusive oxidation of the C-terminal Met residues, in contr ast to the oxidation pattern of CaM observed in vivo. Hence, none of the em ployed oxidizing conditions was able to mimic the age-dependent oxidation o f CaM in vivo, indicating that other, yet unidentified oxidation mechanisms may be important in vivo. Some oxidizing species showed a quite-remarkable diastereoselectivity for the formation of either L-Met-D-SO or L-Met-LSO. Diastereoselectivity was dependent on the nature of the oxidizing species b ut was less a function of the location of the target Met residue in the pro tein. In contrast, diastereoselective reduction of L-Met-D-SO by protein me thionine sulfoxide reductase (pMSR) was efficient regardless of the positio n of the L-Met-D-SO residue in the protein and the presence or absence of c alcium. With only the L-Met-D-SO diastereomer being a substrate for pMSR, a ny preferred formation of L-Met-L-SO in vivo may cause the accumulation of MetSO unless the oxidized protein is substrate for (accelerated) protein tu rnover. (C) 2000 Elsevier Science Inc.