Table 1 summarizes many of the physiological substrates that compete for the pool of methyl groups transferred via SAM (Michal, 1999). In addition to identifying the methyla-tion substrate, the table also includes the name of the responsible enzyme and its methylated product. Each of the substrates listed in Table 1 is of critical metabolic importance. DNA and RNA are central to cellular function and propagation. The methylation status of these biopolymers is important to the regulation of their activity. Phosphatidyl choline is a phospholipid found in cell membranes and, thus, must be available to support cell division, and growth. Spermine and spermidine are polyamines that stabilize DNA in the chromosomes.
A second cluster of methyl group acceptors are involved with energy metabolism (Fig. 1). Trimethyllysine is used to make carnitine which transports fatty acids into mitochondria for the production of ATP and acetyl CoA via beta oxidation. Creatine can accept phosphate from ATP when it is present in excess and provides a reservoir of high energy phosphate in muscle cells to maintain activity when energy demands exceed the amount of ATP that can be provided by the glucose in muscle. This maintains muscle activity as cellular metabolism adjusts to draw on glycogen for additional ATP and mobilize glucose from the liver. Epinephrine is a hormone that enables the cell to respond to stress by increasing glycogen utilization. In addition, it modulates the flow of fatty acids to and from cells (Montgomery et al., 1990).
Two other products that are generated via SAM transmethylation are sarcosine and methylcobalamin (see Table 1). Both are noteworthy because they play a role in the methionine regeneration process. Methylcobalamin is a cofactor for the enzyme methionine synthase. Sarcosine can donate its methyl group to 5,10-methylene tetrahydrofolate via the activity of sarcosine dehydrogenase (Michal, 1999). It is also formed during the demethyl-ation of betaine. It is unique, and illustrative of the importance of SAM reactions, that among the acceptors of its methyl group are metabolites that insure that at least a portion of the methyl groups will be recaptured. This insures availability of methionine for protein synthesis.
Metabolites which accept the methyl group from SAM function to sustain energy
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