Methionine (Met) is an essential amino acid. In the Methionine Cycle, methionine's methyl group becomes activated by ATP with the addition of adenosine to the sulfur of methionine, adjacent to the methyl group to form S−Adenosyl Methionine (SAMe). Removal of the methyl group from SAMe results in the formation of S−Adenosyl Homocysteine (SAH), which is immediately converted to the amino acid homocysteine by removal of the adenosine molecule. Homocysteine can then be modified in three different ways. In liver cells (and only in liver cells) homocysteine can irreversibly enter the transsulfuration pathway (catalyzed by Vitamin B6) to produce the amino acid cysteine. It has been estimated that 60% of homocysteine is metabolized by transsulfuration in the liver, with glucocorticoids increasing that percentage. Cysteine can be incorporated into proteins, can be used in the formation of the anti-oxidant molecule glutathione (GSH), or can be oxidized to form the amino acid taurine. If homocysteine does not enter the transsulfuration pathway, it can be converted back to methionine by the addition of a methyl group by one of two pathways. For one pathway of methyl group addition, methionine synthase enzyme catalyzes the transfer of a methyl group from methylated folic acid (MTHF) to homocysteine assisted by vitamin B12, which takes the methyl group from MTHF and adds it to the homocysteine. In another pathway (only in the liver), betaine (TMG) is the source of the methyl group transferred to homocysteine.
One of the metabolites in the methionine cycle, S-adenosylmethionine (SAM), is the universal methyl donor and is the substrate for a host of methyltransferases among which are the DNA methyltransferases and histone methyltransferases that regulate gene silencing and epigenetic inheritance. The level of SAM varies with methionine input and folate status, and, together with its product S-adenosylhomocysteine (SAH), is used as an indicator of methylation capacity. Another metabolite in the methionine cycle is homocysteine, and elevated levels of homocysteine are generally accepted as a major biomarker for cardiovascular disease. In addition, via the cystathionine-β-synthase (CBS) reaction, the methionine cycle provides the first step in the synthesis of reduced glutathione (GSH), a key antioxidant.
The methionine cycle has three important functions in cellular metabolism. First, it regulates the balance between methionine and cysteine for protein synthesis; second, it provides the substrate for polyamine synthesis, and third, it provides the mechanism by which methyl groups are transferred from 5-methyltetrahydrofolate to a broad variety of substrates and constitutes the primary mechanism for transmethylation reactions in mammals. Normal functioning of the methionine cycle is essential for growth and development, and defects in methionine metabolism are associated with a variety of diseases ranging from cardiovascular disease to psychiatric disorders, DNA methylation status and cancer.
Because of its central role in cell metabolism, the operation of the methionine cycle has been the subject of numerous experimental studies. Studies have revealed complex responses to experimental variation in its various components. Some of this complexity arises from the fact that enzymes of the methionine cycle are activated and inhibited by several of the intermediates of the cycle. A significant part of the complexity arises from nonlinearities in the interactions among the components of the cycle that make the response to perturbation context-dependent, and therefore non-intuitive and unpredictable. Much of what is known about the properties and behavior of the pathway comes from a broad body of empirical experience, both in vivo and in vitro. By providing the exact molecular weights and retention time, LC–MS/MS techniques serves as a powerful analytical tool for identification and quantification of small molecules (metabolites). Creative Proteomics has established sensitive, reliable, and accurate HPLC-MS/MS method for quantification of metabolites in methionine cycle metabolites.
Methionine Cycle Metabolites Quantified in This Service
|Methionine Cycle Metabolites Quantified in This Service|
|2||S Adenosyl Methionine|
|8||Aethylated folic acid|
With integrated set of separation, characterization, identification and quantification systems featured with excellent robustness & reproducibility, high and ultra-sensitivity, Creative Proteomics provides reliable, rapid and cost-effective Methionine Cycle Metabolites targeted metabolomics services.
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