Methionine selenoxide

The excessive replacement of Met by SeMet lowers the protein stability in vitro, but not necessarily in vivo. At the low levels of SeMet normally present in culture media, the substitution of Met by the more oxygen-sensitive SeMet is unlikely to affect the properties of enzymes in an adverse manner. Moreover, since methionine selenoxide is readily converted back to SeMet on reaction with biogenic thiols such as GSH, a mechanism of repair of this type of oxidative damage is available. The reversibility of SeMet oxidation in the... 

Assmann, A., Briviba, K., and Sies, H. 1998. Reduction of methionine selenoxide to selenomethionine by glutathione. Arch. Biochem. Biophys. 349, 201-203. 

D,L-Selenomethionine is oxidised by peroxynitrite by two competing mechanisms, a one-electron oxidation that leads to ethylene, and a two-electron oxidation that gives methionine selenoxide (Pad-MAjA et al. 1997). Kinetic modelling of the experimental data suggests that both peroxynitrous acid and the peroxynitrite anion react with d,l-selenomethionine to form methionine selenoxide with rate constants of 20,4601440 M" s" and 2001170 M" s", respectively at 25 °C. In the presence of added bicarbonate, the yield of ethylene obtained from the reaction of 0.4 mM peroxynitrite with 1.0 mM selenomethionine, decreased by 35% with an increase in the concentration of bicarbonate from 0 to 25 mM. Kinetic simulations showed that the decrease in the yield of methionine selen-... 

Selenium compounds are more active electron donors compared to analogous sulphur compounds and play a wide role in biochemical systems. They can decompose peroxides (Caldwell and Tappel 1964,1965) and can act as antioxidants (Dillard et al. 1978) and free radical scavengers (Ursini and Bindoli 1987). Masumoto and Siess (1996) and Masumoto et al. (1996) have shown that peroxy-nitrite rapidly oxidises ebselen and its main metabolite to the corresponding selenoxides. The results of Padmaja et al. (1997) suggest that CO2 partially protects methionine selenoxide from peroxynitrite-mediated oxidation and that 0=N-00-C02 or its derivatives do not mediate the oxidation of d,l-selenomethionine or methionine selenoxide. 

The sulfur analog of the selenoxide pyrolysis is also known. In this sulfoxide pyrolysis the C-S bond is broken. The C-S bond is stronger than the C-Se bond and this explains why sulfoxides must typically be pyrolyzed at 200 °C to achieve elimination. Figure 4.13 shows the transformation of protected L-methionine into the corresponding sulfoxide, which then undergoes sulfoxide pyrolysis. This two-step sequence provides an elegant access to the nonnatural amino acid L-vinyl glycine. 

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