S-Adenosyl-L-methionine; SAM-e; SAMe, AdoMet, ademethionine
|3D model (Jmol)||Interactive image|
|Molar mass||398.44 g·mol−1|
Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa).
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S-Adenosyl methionine[alternative names 1] is a common cosubstrate involved in methyl group transfers, transsulfuration, and aminopropylation. Although these anabolic reactions occur throughout the body, most SAM-e is produced and consumed in the liver. More than 40 methyl transfers from SAM-e are known, to various substrates such as nucleic acids, proteins, lipids and secondary metabolites. It is made from adenosine triphosphate (ATP) and methionine by methionine adenosyltransferase (EC 220.127.116.11). SAM was first discovered by Giulio Cantoni in 1952.
Biochemistry of S-adenosyl methionine
The reactions that produce, consume, and regenerate SAM-e are called the SAM-e cycle. In the first step of this cycle, the SAM-dependent methylases (EC 2.1.1) that use SAM-e as a substrate produce S-adenosyl homocysteine as a product. This is hydrolysed to homocysteine and adenosine by S-adenosylhomocysteine hydrolase EC 18.104.22.168 and the homocysteine recycled back to methionine through transfer of a methyl group from 5-methyltetrahydrofolate, by one of the two classes of methionine synthases (i.e. cobalamin-dependent (EC 22.214.171.124) or cobalamin-independent (EC 126.96.36.199)). This methionine can then be converted back to SAM-e, completing the cycle. In the rate-limiting step of the SAM cycle, MTHFR (methylenetetrahydrofolate reductase) irreversibly reduces 5,10-methylenetetrahydrofolate to 5-methyltetrahydrofolate.
Radical SAM-e enzymes
A large number of iron-sulfur cluster-containing enzymes cleave SAM-e reductively to produce a 5′-deoxyadenosyl 5′-radical as an intermediate, and are called radical SAM enzymes. Most enzymes with this capability share a region of sequence homology that includes the motif CxxxCxxC or a close variant. The radical intermediate allows enzymes to perform a wide variety of unusual chemical reactions. Examples of radical SAM enzymes include spore photoproduct lyase, activases of pyruvate formate lyase and anaerobic sulfatases, lysine 2,3-aminomutase, and various enzymes of cofactor biosynthesis, peptide modification, metalloprotein cluster formation, tRNA modification, lipid metabolism, etc. Some radical SAM-e enzymes use a second SAM-e as a methyl donor. Radical SAM enzymes are much more abundant in anaerobic bacteria than in aerobic organisms.
Another major role of SAM-e is in polyamine biosynthesis. Here, SAM-e is decarboxylated by adenosylmethionine decarboxylase (EC 188.8.131.52) to form S-adenosylmethioninamine. This compound then donates its n-propylamine group in the biosynthesis of polyamines such as spermidine and spermine from putrescine.
SAM-e is required for cellular growth and repair. It is also involved in the biosynthesis of several hormones and neurotransmitters that affect mood, such as epinephrine. Methyltransferases are also responsible for the addition of methyl groups to the 2' hydroxyls of the first and second nucleotides next to the 5' cap in messenger RNA.
Some research, including multiple clinical trials, has indicated taking SAM on a regular basis may help fight depression, liver disease, and the pain of osteoarthritis. All other indications are not yet well-evidenced.
At first, a line of evidence suggested abnormally low levels of endogenous SAM may play an important role in the development of Alzheimer's disease, and that SAM may therefore have therapeutic potential in the treatment of Alzheimer's disease. However, further research has indicated this effect is likely due to vitamin B12 deficiencies, which result in neurologic defects due to the inability to conduct one carbon transfers (with folate) in the absence of B12.[medical citation needed]
In the United States and Canada, SAM is sold as a nutritional supplement under the marketing name SAM-e (also spelled SAME or SAMe; pronounced "sam ee" or "Sammy"). Approved in Russia, Italy, and several countries of the European Union, SAM is also marketed as a prescription drug under the brand names Gumbaral, Samyr, Adomet, Heptral, Agotan, Donamet, Isimet and Admethionine. In India, SAM is also marketed as Nusam under dietary supplement category. In Serbia, the drug is marketed as "Tensilen". Therapeutic use of SAM has increased in the US as dietary supplements have gained in popularity, especially after the Dietary Supplement Health and Education Act was passed in 1994. This law allowed the distribution of SAM as a dietary supplement, and therefore allowed it to bypass the regulatory requirements of the Food and Drug Administration (FDA) for drugs.
Oral SAM achieves peak plasma concentrations three to five hours after ingestion of an enteric-coated tablet (400–1000 mg). The half-life is about 100 minutes.
Another reported side effect of SAM is insomnia; therefore, the supplement is often taken in the morning. Other reports of mild side effects include lack of appetite, constipation, nausea, dry mouth, sweating, and anxiety/nervousness, but in placebo-controlled studies, these side effects occur at about the same incidence in the placebo groups.[medical citation needed]
Interactions and contraindications
Taking SAM at the same time as some drugs may increase the risk of serotonin syndrome, a potentially dangerous condition caused by having too much serotonin. These drugs include dextromethorphan (Robitussin), meperidine (Demerol), pentazocine (Talwin), and tramadol (Ultram). SAM may also interact with antidepressant medications increasing the potential for their side effects and reduce the effectiveness of levodopa for Parkinson's disease.[medical citation needed]
- SAM-e, SAMe, SAM, S-Adenosyl-L-methionine, AdoMet, ademetionine
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