Methionine reductase

Other Names

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Synopsis

Methionine Sulfoxide Reductase (MSR): A Comprehensive Reference

1. Identity and Chemical Classification

Methionine sulfoxide reductase (MSR; EC 1.8.4.11 and EC 1.8.4.12) refers to a family of evolutionarily conserved, thiol- or selenol-dependent oxidoreductase enzymes that catalyze the reduction of oxidized methionine residues — in both free amino acid and protein-bound forms — back to methionine. In the context of dietary supplementation and nutraceutical science, the term is used in two interrelated senses: (1) the endogenous enzyme system itself, and (2) natural compounds capable of enhancing or acting as substrates within the MSR catalytic antioxidant cycle.

The methionine sulfoxide reductase (Msr) family is composed of two structurally unrelated classes of monomeric enzymes named MsrA and MsrB, which display opposite stereo-selectivities towards the sulfoxide function. MsrA enzymes reduce the S-epimer of methionine sulfoxide (S-MetO) and MsrB enzymes reduce the R-epimer (R-MetO), respectively. The two classes are thus complementary — together, they are capable of repairing all diastereomeric forms of oxidized methionine within proteins.

Methionine sulfoxide reductases include one MSRA and three MSRBs (MSRB1, MSRB2, and MSRB3) proteins in mammals. A notable distinction among the MsrB subclass is that MSRB1 is the only member of the MsrB family that is a selenoprotein, containing a selenocysteine (Sec) residue at its active site, and it has the highest methionine-R-sulfoxide reductase activity compared to other members containing cysteine in place of Sec.

The human MSRA gene (GeneID: 4482) is located on Chromosome 8, locus 8p23.1, and encodes a 235 amino acid protein with a molecular mass of 26 kDa called Methionine Sulfoxide Reductase A (MsrA) or peptide-methionine-(S)-S-oxide reductase (EC 1.8.4.11).

1.1 Nomenclature and Synonyms

MSRA / MsrA: Peptide methionine sulfoxide reductase A; peptide-methionine-(S)-S-oxide reductase; EC 1.8.4.11
MSRB / MsrB: Peptide methionine sulfoxide reductase B; peptide-methionine-(R)-S-oxide reductase; EC 1.8.4.12
MSRB1: Also known as selenoprotein R (SelR) and selenoprotein X (SelX)
MSRB2: CBS-1 (CBS domain protein), primarily mitochondria-localized
MSRB3: Located in the endoplasmic reticulum and mitochondria
Related dietary substrate: S-methyl-L-cysteine (SMLC; CAS 1187-84-4), a natural food-derived compound that serves as a substrate for the MSR catalytic cycle

1.2 Natural Sources

Reduction of the sulfoxide back to methionine is catalyzed by methionine sulfoxide reductases, enzymes found in almost all organisms from microbes to humans. The endogenous enzyme is therefore not externally supplemented in isolated form; rather, the interest in dietary supplementation centers on compounds that activate or provide substrate for the MSR system.

The most studied dietary MSR-activating compound is S-methyl-L-cysteine (SMLC). S-methyl-L-cysteine is found in onions, garlic, cabbage, and legumes, and exhibits antioxidative, neuroprotective, and anti-obesity activities. One way to enhance the MSRA antioxidant system is dietary supplementation with S-methyl-L-cysteine, found abundantly in garlic, cabbage, and turnips.

Another substrate compound is dimethyl sulfide (DMS). Dimethyl sulfide, a main metabolite produced by marine algae, has emerged as a substrate for MsrA-catalytic antioxidation.

2. Structural Biology and Active Site

There are two major transcript variants of MsrA called the long and short forms. The long form of MsrA encodes a 235 amino acid peptide containing an N-terminal mitochondrial targeting sequence, a catalytic cysteine containing Gly-Cys-Phe-Trp-Gly sequence required for its methionine sulfoxide reductase activity and a C-terminal thioredoxin binding domain. The short form of MsrA lacks the mitochondrial targeting sequence while retaining the catalytic cysteine containing sequence and the thioredoxin binding domain.

The long form of MsrA has been localized to the mitochondria, nucleus, and cytosol, while the short form of MsrA has been localized to the nucleus and the cytosol.

The structures support a unique, thiol–disulfide exchange mechanism that relies upon an essential cysteine as a nucleophile and additional conserved residues that interact with the oxygen atom of the sulfoxide moiety.

3. Traditional and Historical Use

The MSR enzyme family was not known historically as a supplement ingredient in any traditional healing system. It is a molecular biology discovery of the twentieth century. The first bacterial form was characterized in the late 1970s and early 1980s, and eukaryotic forms were described progressively through the 1990s and 2000s. The gene for this enzyme has been cloned and sequenced from a variety of prokaryotic and eukaryotic cells, and the deduced amino acid sequence is very highly conserved.

However, certain traditional food plants that are now understood to be rich MSR substrates have long histories of medicinal use. Garlic (Allium sativum), one of the richest dietary sources of SMLC, has been documented in use across Egyptian, Greek, Roman, Chinese, and Indian (Ayurvedic) traditions for thousands of years, employed variously for cardiovascular, antimicrobial, and metabolic purposes. Garlic (Allium sativum, Liliaceae) is a rich source of bioactive compounds and is used in folk medicine for the treatment of various diseases. The recognition that specific sulfur-amino acid components such as SMLC may mediate some of these benefits via the MSR catalytic pathway is a modern scientific interpretation, not a traditional one.

4. Key Constituents and Active Compounds

4.1 The Methionine Oxidation/Reduction Cycle

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.

Conversion of methionine to its sulfoxide is mediated by a host of reactive species over a broad pH range. For example, hydrogen peroxide readily reacts with certain methionine residues in proteins even at pH 5 or lower. Oxidation by hydrogen peroxide, hypochlorous acid, and other reactive species creates a chiral center at the sulfur so that the methionine sulfoxide produced is a mixture of the S and R epimers.

4.2 SMLC as a Dietary MSR Substrate

S-methyl-L-cysteine, a natural analogue of methionine that is abundantly found in garlic and cabbage, can activate the Met oxidase activity of MsrA to scavenge free radicals. It operates by entering and amplifying the methionine-centered redox cycle (MCRC) in which MsrA catalyzes a repeated oxidation-reduction scavenging loop.

4.3 Dimethyl Sulfide (DMS)

Dimethyl sulfide, a main metabolite produced by marine algae, emerged as a substrate for MsrA-catalytic antioxidation. MsrA binds to DMS and promotes its antioxidant capacity via facilitating the reaction of DMS with reactive oxygen species through a sulfonium intermediate at residues Cys72, Tyr103, and Glu115, followed by the release of dimethyl sulfoxide.

5. Mechanisms of Action

5.1 Catalytic Mechanism

MsrAs and MsrBs share the same chemical mechanism implying sulfenic acid chemistry. The mechanism includes three steps: (1) formation of a sulfenic acid intermediate with a concomitant release of 1 mol of methionine per mol of enzyme; (2) formation of an intramonomeric disulfide Msr bond; followed by (3) reduction of the oxidized Msr by thioredoxin. This scheme is in accordance with the kinetic mechanism of both Msrs, which is of ping-pong type.

Methionine sulfoxide reductases are thiol (or selenol)-dependent oxidoreductases that reduce protein-based and/or free forms of Met-S-SO or Met-R-SO to Met. This function makes these enzymes important antioxidant proteins that can protect against oxidative stress through reversible oxidation and reduction of Met.

5.2 Catalytic Antioxidant Amplification

The amino acid methionine is readily oxidized to methionine sulfoxide, and its reduction is catalyzed by the MSR family. The reversible oxidation-reduction cycle of methionine involving MSRs has been postulated to act as a catalytic antioxidant system protecting cells from oxidative damage. Because each molecule of MsrA can repeatedly cycle methionine residues between their oxidized and reduced forms, a single enzyme molecule can neutralize many molecules of reactive oxygen species (ROS), giving the system a catalytic — rather than merely stoichiometric — antioxidant character.

Many proteins (e.g., GroEL, catalase, and recombinase) are significantly more methionine-rich than other bacterial proteins and, in all likelihood, are oxidized under oxidative stress and salvaged by the activity of MSR enzymes. Thus, MSR proteins not only repair oxidative damage to methionine residues through the oxidation/reduction cycle but also serve as scavengers of ROS and protect cells from more widespread oxidative damage.

5.3 Protein Function Regulation via Methionine Redox

Beyond pure antioxidant defense, MSR enzymes participate in regulatory signal transduction. Msrs may regulate protein function by controlling the redox state of critical Met residues. One case of such regulation involves CaMKII (calcium/calmodulin-dependent protein kinase II): this protein is activated by oxidation of two Met residues in the regulatory domain in the absence of Ca²⁺/CaM, and this activation is reversed by MsrA.

In addition to cytochrome c and α-crystallin, at least 8 other proteins have been identified to be affected by methionine oxidation and are repaired by MsrA. These include the shaker potassium channel, HIV-2 protease, the Ffh prokaryotic signal recognition particle component, calmodulin, alpha-1-proteinase inhibitor, ribosomal protein L12, Hsp21, and α-synuclein.

The methionine residues of Helix-3, Ca²⁺/calmodulin-dependent protein kinase II (CamKII), shaker voltage-dependent K⁺ channel, and Slo1 K⁺ channels can be oxidized and hereby regulate their function. It has been shown that oxidation of a methionine residue in the shaker voltage-dependent K⁺ channel disrupts its inactivation, and this effect can be reversed by co-expression with MsrA.

5.4 Thioredoxin-Dependent Regeneration

MetSO residues can be reduced by methionine sulfoxide reductases, evolutionarily highly conserved enzymes able to reduce MetSO back to Met using electrons derived from thioredoxin, thioredoxin reductase, and NADPH. The dependence of MSRB1 on selenium is notably direct: the expression and activity of MSRB1 are dependent on selenium availability in the diet.

5.5 Role in Macrophage Innate Immunity

MSRB1 plays a role in innate immunity by reducing oxidized actin, leading to actin repolymerization in macrophages. Recent findings of MSRB1 regulating the innate immunity response through reversible stereospecific Met-R-oxidation of cytoskeletal actin opened up new avenues for biological importance of MSRB1 and its role in disease.

6. Scientific Evidence by Area of Use

6.1 Oxidative Stress and Aging

Methionine is among the amino acids most susceptible to oxidation by almost all forms of reactive oxygen species, resulting in both S and R diastereomeric forms of methionine sulfoxide. These modifications can be repaired specifically by peptide methionine sulfoxide reductase A and B enzymes.

MsrA is tightly implicated in protection against oxidative stress and in protein maintenance, which is critical in the aging process. Several studies have shown that overexpression of MsrA led to an increased resistance against oxidative stress, while MsrA null mutants are more sensitive toward oxidative stress.

An influential set of experiments in Drosophila melanogaster showed that overexpression of MsrA extended the lifespan of flies. Overexpression of MsrA in transgenic flies renders them more resistant to oxidative stress and dramatically increases their lifespan. However, this longevity benefit did not translate uniformly to mammals. Despite their sensitivity to oxidative stress, mutant mice lacking MsrA did not differ from control mice in their longevity. A subsequent study in transgenic mice confirmed a more nuanced picture: previous studies in Drosophila had shown that increasing expression of MsrA can extend longevity, but in two transgenic mouse models, elevated expression of MsrA targeted specifically to the cytosol reduced the rate of age-related death in female mice when assessed by Gompertz analysis.

Evidence strength: Preclinical (invertebrate and rodent model) evidence is consistent that MSR expression modulates resistance to oxidative stress. Longevity effects are species- and compartment-dependent; no human clinical trials have directly tested longevity outcomes of MSR upregulation.

MSR activity declines at old age in mammals, and this decline in activity is associated with age-associated diseases.

6.2 Neurodegenerative Disease — Alzheimer's Disease

The sulfoxide form has been found to comprise 10–50% of Aβ in amyloid plaques of AD brain, although it is difficult to determine whether its existence in the plaques contributes to AD etiology or results from the highly oxidative environment around the amyloid plaques.

In patients with Alzheimer's disease, the decreased MSRA activity coincides with the oxidation of critical proteins. At the animal model level, the absence of MsrA modifies Aβ solubility properties and causes mitochondrial dysfunction in a mouse model of Alzheimer's disease. Furthermore, MSRB overexpression in APP/PS1 mice has been shown to reduce Aβ production by reducing APP and BACE1 expression.

The MSR system of enzymes, consisting of MSRA and MSRB, directly or indirectly mitigates AD pathogenesis by reversing the oxidation of methionine residues. While levels of MSRA and MSRB proteins are reduced in the aging brain, they may be reduced by varying degrees in different regions of the brain, for example, hippocampus versus cortex, and the reduction may occur at different times in the different regions of the aging brain.

Evidence strength: Primarily cellular and mouse model data. Human studies show correlative reduction of MSR activity in AD brains, but no clinical interventional trials in human AD patients have been completed using MSR-targeting strategies.

6.3 Neurodegenerative Disease — Parkinson's Disease

MSRA inhibits development of the locomotor and circadian rhythm defects caused by ectopic expression of human α-synuclein in the Drosophila nervous system. Furthermore, dietary supplementation with S-methyl-L-cysteine, found abundantly in garlic, cabbage, and turnips, can enhance the MSRA antioxidant system.

Supplementation of food with SMLC alleviates the locomotor defect in the animals expressing human α-synuclein in the nervous system. This Drosophila-model study (Wassef et al., 2007, Journal of Neuroscience) is the primary experimental basis for SMLC's reputation as an MSR-enhancing dietary compound in the context of Parkinson's-like pathology.

SMLC protected against antimycin A-induced mitochondrial membrane depolarization and alleviated 1-methyl-4-phenylpyridinium (MPP+)-induced neurotoxicity, highlighting the possibility for SMLC supplementation in the detoxification of mitochondrial damage.

Studies on several model systems as well as genome-wide association studies have been compiled to highlight the role of MSRA in schizophrenia, Alzheimer's disease, and Parkinson's disease.

Evidence strength: Evidence for neuroprotection by MSR activation (including via SMLC) is derived from invertebrate models, cell culture, and rodent studies. Genome-wide association data provide circumstantial human genetic evidence. No controlled human clinical trials have been conducted.

6.4 Eye Lens and Cataract Formation

The eye lens represents one of the most thoroughly investigated tissues for MSR function. Age-related cataract is an opacity of the eye lens, and its etiology is related to oxidative stress damage. Oxidation of methionine to methionine sulfoxide is a major oxidative stress product that reaches levels as high as 60% in cataract, while being essentially absent from clear lenses.

Accumulation of methionine sulfoxide is a significant feature of human cataract, and methionine sulfoxide reductase A (MsrA), which acts to repair Met(O), can defend human lens cells against oxidative stress-induced cell death. Decreased MsrA levels were found to be associated with loss of cell viability, decreased mitochondrial membrane potential, and increased ROS levels in the absence of oxidative stress.

PMSO levels increase in the eye lens upon aging and in age-related human cataract as much as 70% of total lens protein is converted to PMSO. MsrA is required for lens cell maintenance, defense against oxidative stress damage, mitochondrial function, and prevention of lens cataract formation.

Evidence strength: Robust correlative human tissue data (human cataract specimens show elevated Met oxidation), with strong mechanistic cell and mouse model data. No interventional supplementation trial in cataract patients.

6.5 Cardiovascular and Vascular Biology

Methionine oxidation has been observed to contribute to the redox regulation of several vascular proteins involved in thrombosis and atherosclerosis, suggesting that Msr may control specific signaling pathways via the regulation of protein methionine oxidation and reduction.

In a murine vascular injury model, deletion of the MsrA gene did not affect atherosclerotic lesion area in apolipoprotein E-deficient mice, and had no significant effect on susceptibility to experimental thrombosis after photochemical injury. In contrast, the neointimal area after vascular injury due to complete ligation of the common carotid artery was significantly greater in MsrA-deficient compared to control mice. In aortic vascular smooth muscle cells lacking MsrA, cell proliferation was significantly increased due to accelerated G1/S transition.

Methionine sulfoxide reductase A catalytically scavenges reactive oxygen species and also repairs oxidized methionines in proteins. Increasing MsrA protects cells and organs from a variety of oxidative stresses while decreasing MsrA enhances damage. The myristoylated form of cytosolic MsrA was specifically studied in a mouse ischemia-reperfusion model: approximately 25% of MsrA is targeted to the mitochondria while the other ~75% is targeted to the cytosol and is posttranslationally modified by myristoylation. Transgenic mice overexpressing MsrA targeted to either compartment were used in a Langendorff model of ischemia-reperfusion to assay cardiac protection.

Evidence strength: Preclinical animal models only. Results indicate MsrA modulates vascular smooth muscle proliferation and contributes to ischemia-reperfusion outcomes in rodent hearts, but no human cardiovascular trial data exist.

6.6 Antimicrobial and Host Defense

Deletion of msrA in the facultative intracellular pathogen Salmonella enterica serovar Typhimurium increased susceptibility to exogenous H₂O₂ and reduced bacterial replication inside activated macrophages, and in mice. Comparable findings apply across a range of pathogens, including Staphylococcus aureus, Helicobacter pylori, Fusobacterium nucleatum, and others.

From the host-immune perspective, MSRB1 plays a role in innate immunity by reducing oxidized actin, leading to actin repolymerization in macrophages. The implication is that adequate selenium-dependent MSRB1 activity supports macrophage motility and phagocytic function.

Evidence strength: Microbiology evidence is robust for MSR's role in bacterial pathogen survival. Host MSR involvement in immunity is established at the cellular/molecular level, with limited clinical data.

6.7 Signal Transduction and Ion Channel Regulation

Reversible methionine oxidation and reduction play a dynamic role in a variety of cellular signaling pathways. For example, the methionine residues of Helix-3, CaMKII, the shaker voltage-dependent K⁺ channel, and Slo1 K⁺ channels can be oxidized and hereby regulate their function.

MsrB1 interacts with the TRPM6 α-kinase domain but does not affect the channel activity under normoxic conditions. TRPM6 is the primary transepithelial magnesium channel in kidney and intestine, making MSRB1 a potential regulator of systemic magnesium homeostasis under oxidative stress conditions.

7. Body Systems Associated with MSR Function

Central Nervous System: MsrA is highly expressed in the brain; linked to Alzheimer's, Parkinson's, and schizophrenia via oxidative protein damage pathways. The MSRA gene encodes an enzyme that reduces oxidised methionine residues, contributing significantly to the defence against oxidative stress, and this gene is highly expressed in several areas of the brain.
Ocular (Lens): MsrA and all three MsrBs are expressed in human lens cells and protect against cataract formation through repair of oxidized crystallin proteins.
Cardiovascular: MsrA is present throughout the vascular wall and regulates vascular smooth muscle cell proliferation and ischemia-reperfusion injury.
Immune System: MSRB1 (selenoprotein) controls macrophage actin dynamics and innate immune responses.
Hepatic and Renal: MSRB1 is highly expressed in liver and kidney, and is localized to the nucleus and cytosol.
Mitochondria: MSRB2 is primarily located in the mitochondrion, where it plays roles in mitochondrial quality control and oxidative phosphorylation.
Retina: MsrA has been localized to the mammalian retina and has been shown to protect retinal pigmented epithelial cells against exogenously added oxidative stress.

8. Dosage Forms and Reported Dosages

Methionine sulfoxide reductase enzymes themselves are not currently available as purified isolated dietary supplement products. The supplementation strategy studied in peer-reviewed literature involves consumption of natural MSR substrates or activators, primarily SMLC.

S-methyl-L-cysteine, found abundantly in garlic, cabbage, and turnips, was used as a dietary supplement in Drosophila models of Parkinson's disease, where supplementation of food with SMLC alleviated the locomotor defect in animals expressing human α-synuclein in the nervous system. In this foundational invertebrate study (Wassef et al., 2007), SMLC was provided as a dietary admixture, with dose quantities derived from standard feeding protocols in the Drosophila experimental system; specific mg/kg dosage values applicable to humans were not reported.

In a cell-culture model of mitochondrial dysfunction, SMLC, abundantly found in garlic and cabbage, could activate the Met oxidase activity of MsrA to scavenge free radicals, and protected against antimycin A-induced mitochondrial membrane depolarization and alleviated MPP⁺-induced neurotoxicity in PC12 neuronal cells. Concentrations used in this in vitro work are not directly translatable to human oral dosages.

No human clinical trial establishing a defined therapeutic dose of SMLC or any MSR-upregulating supplement has been identified in the peer-reviewed literature at the time of writing. Quantitative dosage recommendations therefore cannot be stated with source-backed accuracy.

9. Safety Considerations and Interactions

9.1 Selenium Dependency of MSRB1

The expression and activity of MSRB1 are dependent on selenium availability in the diet. Conditions of dietary selenium deficiency may therefore directly impair the R-epimer repair arm of the MSR system. Conversely, excess selenium intake is associated with known toxicity in humans (selenosis), creating a narrow therapeutic window.

9.2 Genetic Variation and Phenotypic Complexity

In contrast to previous reports, MsrA⁻/⁻ mice showed no evidence for neuromuscular dysfunction in either young adult or older animals, and no difference between MsrA⁻/⁻ and control mice in either their median or maximum life span. Thus, MsrA regulates sensitivity to oxidative stress in mice but has no effect on aging as determined by life span. This finding illustrates the risk of over-extrapolating disease relevance from single-gene knockout phenotypes.

9.3 Dual Role of MsrA as an Oxidase

The antioxidant capability of MsrA may involve a Met oxidase activity that facilitates the reaction of Met with reactive oxygen species. This bidirectional catalytic role means that under certain conditions — such as mitochondrial MsrA overexpression — the enzyme can paradoxically increase ROS production rather than reduce it, a complexity that has been documented in skeletal muscle models.

9.4 SMLC: Known Safety Profile

SMLC is a naturally occurring amino acid found in common food plants (garlic, cabbage, onions, legumes) and has been consumed in these forms without reported harm across human populations worldwide. No formal human toxicology studies specifically on isolated SMLC supplementation at pharmacological doses have been identified in the peer-reviewed literature. The available evidence base is preclinical.

9.5 Interaction with Thioredoxin System

The full catalytic function of MSR enzymes is dependent on the thioredoxin/thioredoxin reductase/NADPH system for regeneration. Methionine sulfoxide reductases reduce MetSO back to Met using electrons derived from thioredoxin, thioredoxin reductase, and NADPH. Compounds or drugs that interfere with thioredoxin reductase activity (including certain gold compounds and some chemotherapy agents) could theoretically impair MSR regeneration and thus its antioxidant function.

9.6 Vascular Smooth Muscle Proliferation

In mice, MsrA deficiency resulted in a significantly greater neointimal area after vascular injury, and in aortic vascular smooth muscle cells lacking MsrA, cell proliferation was significantly increased due to accelerated G1/S transition. These observations, derived from murine models, suggest that MSR activity may constrain smooth muscle cell growth; the clinical implications in humans remain unexplored.

10. Summary of Evidence Quality

The bulk of the research base for methionine sulfoxide reductase in the context of aging, neurodegeneration, and ocular disease is preclinical. Strong biochemical and molecular evidence establishes the enzyme's mechanistic role in repairing oxidized proteins across virtually all aerobic organisms. Modifications to the MSR antioxidant system have been shown to impact the lifespan of several model system organisms, and in humans, methionine oxidation of critical proteins and deficiencies in the methionine sulfoxide reductase system have been linked to age-related diseases, including cancer and neurodegenerative disease. These linkages are predominantly associative or demonstrated in animal/cell models. The translation of MSR biology to human dietary supplementation protocols — via compounds such as SMLC — remains at an early, largely preclinical stage, with no established clinical trial evidence for efficacy or dosing in humans.

References

Health Conditions

Health conditions that Methionine reductase may help support.

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Body Systems

Body systems that Methionine reductase may help support.

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