Rat peptide methionine sulphoxide reductase: cloning of the cDNA, and down-regulation of gene expression and enzyme activity during aging

Mitochondrial targeting of the human peptide methionine sulfoxide reductase (MSRA), an enzyme involved in the repair of oxidized proteins

Peptide methionine sulfoxide reductase (MSRA) catalyzes the reduction of methionine sulfoxide to methionine. This widely expressed enzyme constitutes an important repair mechanism for oxidatively damaged proteins, which accumulate during the manifestation of certain degenerative diseases and aging processes. In addition, it is discussed to be involved in regulatory processes. Here we address the question of how the enzyme's diverse functions are reflected in its subcellular localization. Using fusions of the human version of MSRA with the enhanced green fluorescence protein expressed in various mammalian cell lines, we show a distinct localization at mitochondria. The N-terminal 23 amino acid residues contain the signal for this mitochondrial targeting. Activity tests showed that they are not required for enzyme function. Mitochondrial localization of native MSRA in mouse and rat liver slices was verified with an MSRA-specific antibody by using immunohistochemical methods. The protein was located in the mitochondrial matrix, as demonstrated by using pre-embedding immunostaining and electron microscopy. Mitochondria are the major source of reactive oxygen species (ROS). Therefore, MSRA has to be considered an important means for the general reduction of ROS release from mitochondria.

Cyclic oxidation and reduction of protein methionine residues is an important antioxidant mechanism

Almost all forms of reactive oxygen species (ROS) oxidize methionine residues of proteins to a mixture of the R- and S-isomers of methionine sulfoxide. Because organisms contain methionine sulfoxide reductases (Msr's) that can catalyze the thioredoxin-dependent reduction of the sulfoxides back to methionine, it was proposed that the cyclic oxidation/reduction of methionine residues might serve as antioxidants to scavenge ROS, and also to facilitate the regulation of critical enzyme activities. We summarize here results of studies showing that organisms possess two different forms of Msr--namely, MsrA that catalyzes reduction of the S-isomer and MsrB that catalyzes the reduction of the R-isomer. Deletion of the msrA gene in mice leads to increased sensitivity to oxidative stress and to a decrease (40%) in the maximum lifespan. This suggests that elimination of both Msr's would have more serious consequences.

High-quality life extension by the enzyme peptide methionine sulfoxide reductase

Cumulative oxidative damages to cell constituents are considered to contribute to aging and age-related diseases. The enzyme peptide methionine sulfoxide reductase A (MSRA) catalyzes the repair of oxidized methionine in proteins by reducing methionine sulfoxide back to methionine. However, whether MSRA plays a role in the aging process is poorly understood. Here we report that overexpression of the msrA gene predominantly in the nervous system markedly extends the lifespan of the fruit fly Drosophila. The MSRA transgenic animals are more resistant to paraquat-induced oxidative stress, and the onset of senescence-induced decline in the general activity level and reproductive capacity is delayed markedly. The results suggest that oxidative damage is an important determinant of lifespan, and MSRA may be important in increasing the lifespan in other organisms including humans.

Free radicals and disease

Free radicals and reactive oxygen species (ROS) have been associated with the etiology and/or progression of a number of diseases and in aging. Many of the proteins oxidatively modified by free radicals contain side-chain carbonyl derivatives, which can be used as markers for protein oxidation. The protein carbonyl content has been quantitated as a function of age for human cultured dermal fibroblasts, lens, and brain tissue. These data were analyzed using a simple autocatalytic model with the assumption that free radicals randomly oxidize proteins or peptides to form carbonyl derivatives and lead to their inactivation. The carbonylated proteins and peptides are highly susceptible to proteolytic degradation. Implication of free radicals in aging and in age-dependent susceptibility to neurodegenerative diseases will be discussed in light of this simplified kinetic model.

Peptide methionine sulfoxide reductase: structure, mechanism of action, and biological function

Reactive oxygen and nitrogen intermediates can cause damage to many cellular components and have been implicated in a number of diseases. Cells have developed a variety of mechanisms to destroy these reactive molecules or repair the damage once it occurs. In proteins one of the amino acids most easily oxidized is methionine, which is converted to methionine sulfoxide. An enzyme, peptide methionine sulfoxide reductase (MsrA), catalyzes the reduction of methionine sulfoxide in proteins back to methionine. There is growing evidence that MsrA plays an important role in protecting cells against oxidative damage. This paper reviews the biochemical properties and biological role of MsrA.

Purification and characterization of methionine sulfoxide reductases from mouse and Staphylococcus aureus and their substrate stereospecificity

Many organisms have been shown to possess a methionine sulfoxide reductase (MsrA), exhibiting high specificity for reduction the S form of free and protein-bound methionine sulfoxide to methionine. Recently, a different form of the reductase (referred to as MsrB) has been detected in several organisms. We show here that MsrB is a selenoprotein that exhibits high specificity for reduction of the R forms of free and protein-bound methionine sulfoxide. The enzyme was partially purified from mouse liver and a derivative of the mouse MsrB gene, in which the codon specifying selenocystein incorporation was replaced by the cystein codon, was prepared, cloned, and overexpressed in Escherichia coli. The properties of the modified MsrB protein were compared directly with those of MsrA. Also, we have shown that in Staphylococcus aureus there are two MsrA and one nonselenoprotein MsrB, which demonstrates the same substrate stereospecificity as the mouse MsrB.

Methionine sulfoxide reductase (MsrA) is a regulator of antioxidant defense and lifespan in mammals

Oxidation of proteins by reactive oxygen species is associated with aging, oxidative stress, and many diseases. Although free and protein-bound methionine residues are particularly sensitive to oxidation to methionine sulfoxide derivatives, these oxidations are readily repaired by the action of methionine sulfoxide reductase (MsrA). To gain a better understanding of the biological roles of MsrA in metabolism, we have created a strain of mouse that lacks the MsrA gene. Compared with the wild type, this mutant: (i) exhibits enhanced sensitivity to oxidative stress (exposure to 100% oxygen); (ii) has a shorter lifespan under both normal and hyperoxic conditions; (iii) develops an atypical (tip-toe) walking pattern after 6 months of age; (iv) accumulates higher tissue levels of oxidized protein (carbonyl derivatives) under oxidative stress; and (v) is less able to up-regulate expression of thioredoxin reductase under oxidative stress. It thus seems that MsrA may play an important role in aging and neurological disorders.