Methionine sulfoxide reduction and

Methionine sulfoxide chemistry and biochemistry TABLE 6. Requirements for the reduction of Met(0)-a-l-PI ... 

Reduction of Ozonized Lysozyme. Methionine sulfoxide can revert to methionine with the generation of the lytic activity for photo-oxidized lysozyme (14). We tested whether the ozonized lysozyme could be reactivated by chemical reduction. The ozonized lysozyme was treated with 2-mercaptoethanol, dialysed, purified by passage through a column of Sephadex G-25 and lyoph-ilized. The product showed no increase in its lytic activity. This is not surprising because residues other than methionine are oxidized, but it may be concluded that the oxidation of methionine alone cannot account for enzyme inactivation. 

The delay of about 30 min in the appearance of the peak of 14C02 and the high level of utilization of methionine sulfoxide as source of methionine indicate that this molecule readily and easily is reduced into methionine. The reduction process has been studied on perfused rat liver (68). 

Other tissues probably are involved in the reduction process of methionine sulfoxide. The metabolic studies performed on the living animals suggest that methionine sulfoxide is transported actively through the intestinal wall and reduced during its passage or reduced in the lumen and actively transported as free methionine. 

Reports of the presence of methionine sulfoxide are less frequent with respect to protein hydrolyzates than to free amino acid extracts. This is at least partially due to the fact that reduction and other changes occur upon hydrolysis with hydrochloric acid (16). The extent of decomposition depends on the conditions of hydrolysis and may approach 100%. One occasionally finds a published report of an amino acid analysis of protein in which data for methionine sulfoxide but not methio-... 

More important than these possible differences in the methionine sulfoxide content of the extractable fraction and the protein are the differences in bound sulfoxide between the control and the sulfate-injected insects. Regardless of the method of extraction or of hydrolysis, the amount of bound methionine sulfoxide-S35 m the sulfate-injected insects is higher than that formed as a result of oxidation during hydrolysis and chromatography of the protein (control). As in the soluble fractions, these differences may be partially a reflection of a reduction-oxidation shift actuated by the presence of exogenous methionine in the control. Inspection of the data shows, in addition, that the percentage... 

The function, if any, of methionine sulfoxide residues in peptides or proteins is a matter of conjecture. There is no evidence that they are of any structural significance. Indeed, since various enzymes such as ribonuclease and chymotrypsin are either partially or completely inactivated by oxidation of the methionine residues (15, 16), one hesitates to suggest any functional role for the sulfoxide. However, a role in the maintenance of oxidation-reduction potential of a biological system, as suggested by Dent (3), is conceivable. 

The repair of oxidative damage may be incomplete. Only reduction of the d-diastereomer of calmodulin-bound methionine sulfoxide (L-Met-D-SO) by methionine sulfoxide reductase was demonstrated, while in the cells both d- and l-steoreoisomers are formed. Such incomplete, diastereoselective repair by methionine sulfoxide reductase contributes to the accumulation of methionine sulfoxide residues during oxidative stress and aging in vivo (S24). 

Sharov, V. S., Ferrington, D. A., Squier, T. C., and Schoneich, C., Diastereoselective reduction of protein-bound methionine sulfoxide by methionine sulfoxide reductase. FEBS Lett. 455, 247-250(1999). 

Mammalia contain two methionine sulfoxide reductases, forms A and B. Methionine sulfoxide reductase B is a selenoprotein, which reduces the R form of free or protein-bound methionine sulfoxides to methionine with thioredoxin as the reductant. Together with isoform A (which does not contain selenium), the enzyme can repair oxidatively damaged protein and thus contributes to the antioxidant system of the cell. ... 

The thioredoxin system, consisting of thioredoxin and thioredoxin reductase, was originally discovered as the hydrogen carrier system, which provides, with NADPH, the reducing potential for the reduction of ribonucleotides (5, 35). Since then considerable evidence has been accumulated to indicate that this or a closely related system also participates in a variety of other enzymatic reductions. For instance thioredoxin can function as an electron carrier between NADPH and several disulfides, such as insulin, lipoate and oxidized glutathione. Furthermore Porque et al. (114) have shown that thioredoxin and thioredoxin reductase from yeast can function as hydrogen carriers in the reduction of methionine sulfoxide and sulfate. 

Etienne, F., Resnick, L., Sagher, D., Brot, N., and Weissbach, H. (2003) Reduction of sulindac to its active metabolite, sulindac sulfide assay and role of the methionine sulfoxide reductase system. Biochem. Biophys. Res. Commun. 312, 1005-1010. 

Thioredoxin (1,2) are small (M. 11-12,000) ubiquitous redox proteins with two half-cystine residues in the conserved active site structure Trp-Cys-Gly-Pro-Cys. The oxidized form Trx-S2 is reduced by NADPH and thioredoxin-reductase the reduced rorm Trx(SH)2 is a powerful protein disulfide oxido-reductase which regulatesthe activity of enzymes by thiol redox control it serves as hydrogen donor for various reductive enzymes such as ribonucleotide reductase or enzymes reducing sulfate or methionine sulfoxide. Also, Trx(SH)2 is essential for phage T7 DNA replication as a subunit of T7 DNA polymerase and assembly of filamentous phages (fl and M13), at least in E, ooti. 

The formation of methionine sulfoxide was observed in connection with hpid peroxidation, phenol oxidation and light exposure in the presence of oxygen and sensitizers such as riboflavin. After in vivo reduction to methionine, protein-bound methionine sulfoxide is apparently biologically available. 

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