S-Adenosyl-L-Methionine (Molecular Biology)

S-Adenosyl-L-methionine (AdoMet), one of nature’s most interesting biological objects, is a sulfonium compound, first described by Cantoni (1), that is synthesized in every cell from L-methionine and adenosine triphosphate (ATP) by methionine adenosyl transferase (MAT).

MAT catalyzes a most unusual reaction where ATP serves as the donor of its adenosine moiety, while its triphosphate chain is cleaved nonrandomly by the intrinsic triphosphatase activity of MAT itself, with the a and the b phosphates yielding inorganic pyrophosphate (PPi) and the g phosphate yielding orthophosphate, Pj (2). It is noteworthy that the triphosphatase activity of MAT is stimulated by AdoMet (3). MAT is found in every cell, and the amino acid sequences of enzymes isolated from species that are separated by more than a billion years in evolutionary divergence (see Divergent Evolution), such as Escherichia coli (4) and humans (5, 6), exhibit an extraordinary degree of homology.

Elucidation of the structure of AdoMet as a sulfonium compound and clarification of the mechanism of its biosynthesis revealed that, in the reaction catalyzed by MAT, the chemical energy of the pyrophosphate bonds in ATP is utilized for the biosynthesis of a new class of energy-rich compounds. At physiological pH, the free energy of hydrolysis of a sulfonium compound is equivalent to that of the pyrophosphate bond in ATP (see Adenylate Charge; Free Energy Relationships), and the reaction is essentially irreversible, because it is accompanied by the release of a proton (7).

AdoMet was first identified as the source of methyl groups for biological transmethylation reactions (8), but it was anticipated that the energetic equivalence of the bonds linking the three ligands to the sulfur atom would enable AdoMet to function biologically as a source of either alkyl or adenosyl groups, thereby endowing AdoMet with a unique biochemical versatility (9). Experimentally, Tabor et al. (10, 11) made the important discovery that £-adenosylpropylamine, a sulfonium compound resulting from the enzymatic decarboxylation of AdoMet (12), can serve as a donor of propylamine in the synthesis of spermidine from putrescine, and of spermine from spermidine (11). AdoMet decarboxylase is particularly interesting from the mechanistic point of view, since it has a covalently bound pyruvate that is required for activity (13). AdoMet decarboxylase and the propylaminetransferases are widely distributed in nature having been found in bacteria, yeast (14), and vertebrates. The synthesis of polyamines is of great biological importance although, quantitatively, it is less significant than biological transmethylation.

The biochemical availability of the groups linked to the sulfonium center of AdoMet was demonstrated by Nishimura et al. (15), who showed that AdoMet can serve as a donor of the aminobutyryl group to S-RNA, and more recently by Knappe (16) and by Reichardt and collaborators, who demonstrated that AdoMet can serve as an adenosine donor (17, 18). Finally Adams and Yang (19, 20) discovered in fruit ripening that AdoMet is cleaved to thiomethyl-adenosine and 1-aminocyclopropane-1-carboxylic acid. This reaction is of great biological significance, in that 1-aminocyclopropane-carboxylic acid is further metabolized to ethylene, a plant hormone that initiates fruit ripening and regulates many aspects of plant growth and development. Thiomethyl-adenosine is cleaved phosphorolytically to adenine and thiomethyl ribose 1-phosphate, in the first step of a salvage pathway that results in the regeneration of methionine (21).

In these reactions, AdoMet functions as a donor of the three ligands attached to the sulfonium atom. In addition, it has been shown that its 6 amino group is utilized in the synthesis of 7,8-diaminopelargonic acid, an intermediate in the synthesis of biotin (22). The mechanism of this reaction is entirely unknown, but it demonstrates the biochemical versatility of AdoMet.

In addition, AdoMet plays a role as a positive or as a negative allosteric effector (see Allostery) in the regulation of the key reactions that affect the metabolism of homocysteine . This amino acid can either be remethylated to methionine by 5-methyl-tetrahydrofolate (see Aminopterin, Methotrexate, Trimethoprim, and Folic Acid) or conjugated with serine to yield cystathionine. As an inhibitor of methylene-tetrahydrofolate reductase (23, 24), AdoMet regulates the availability of methyl-tetrahydrofolate, the key intermediate needed for the methylation of homocysteine in the de novo synthesis of methionine, whereas as an allosteric effector of cystathionine synthase (25) AdoMet favors commitment of homocysteine to the transulfuration pathway.

Quantitatively, AdoMet’s role of serving as a methyl donor predominates. With the exception of the de novo synthesis of methionine, AdoMet is the sole source of the methyl groups utilized by a myriad of substrate-specific methyltransferases for the synthesis of a great variety of compounds, such as creatine, phospatidylcholine, yeast sterols, plant alkaloids, fungal and bacterial antibiotics, ubiquinone, triterpenes, lignins, and methyl chloride. AdoMet is also the source of the methyl groups required for the post-translational modification of proteins and nucleic acids.

Inversion of configuration at the stereospecific sulfonium center is a constant feature of the enzymatic methyl group transfer reactions (26-29). This indicates that methyltranferases, and by extrapolation alkyltransferases in general, operate by a mechanism involving direct transfer of the methyl, or alkyl, group from the sulfonium atom to the acceptor substrate, thus precluding a methylated intermediate or a ping-pong mechanism (see Enzymes).

S-Adenosyl-L-homocysteine (AdoHcy) is a product (30) and consequently a competitive inhibitor (31, 32), of all the reactions in which AdoMet participates as a methyl donor. AdoHcy also regulates fruit ripening by inhibiting the formation of ethylene (33) from AdoMet. In eukaryotes and most prokaryotes, the only pathway for the further metabolism of AdoHcy is catalyzed by adenosylhomocysteine hydrolase (AdoHcyase), an enzyme that, as first shown by de la Haba and Cantoni (34), catalyzes the reversible hydrolysis of AdoHcy to homocysteine and adenosine. The equilibrium of this reaction lies far in the direction of synthesis; physiologically, however, the reaction proceeds in the hydrolytic direction because homocysteine and adenosine are efficiently removed by methionine synthase and by adenosine deaminase, respectively. It was shown by Eloranta (35) that in most tissues the activity of AdoHcyase is 100 to 1000 times greater than the activity of MAT and that consequently the intracellular concentration of AdoHcy is 20 to 100 times smaller than that of AdoMet.

The mechanisms regulating the utilization of AdoMet by many different competing reactions are not well understood. Many methyltransferases have been purified and studied in detail. The kinetics of most are relatively simple, and the physiological activity of these enzymes appears to be related directly to the availability of methyl acceptor substrates, to their affinity (^m) for AdoMet, and to their inhibition constant for AdoHcy. Comparison of these parameters in a number of enzymes revealed that the kinetic features of different methyltransferases can be very different, and they appear to vary independently. This led to the suggestion that the intracellular ratio of AdoMet/AdoHcy, also known as the "methylation ratio" ," might play a key role in the regulation of AdoMet utilization (36).

The validity of this hypothesis can be explored by modulating experimentally the intracellular AdoMet/AdoHcy ratio. This ratio will decrease whenever inhibition of the enzymes responsible for the removal of adenosine and/or homocysteine shifts the equilibrium of AdoHcyase towards AdoHcy synthesis, or when removal of AdoHcy generated by methyltransfer reactions is prevented by inhibition of AdoHcyase. It is also possible to take advantage of the broad specificity of AdoHcyase for adenosine by supplying adenosine analogs, like 3-deazaadenosine (DZA), that, being adenosine deaminase-resistant, can be converted to 3DZAHcy (37). Modulation of the AdoMet/AdoHcy ratio or the generation of analogues containing groups other than adenosine in response to the administration of various AdoHcyase inhibitors and/or substrates, indicates that in different tissues the physiological responses may be ascribed to the inhibition of specific methyltransferases (38). These results therefore appear to support the hypothesis that the utilization of AdoMet by various competing reactions is regulated by the intracellular AdoMet/AdoHcy ratio. It should be noted, however, that interpretation of these experiments is complex because the accumulation of AdoHcy and/or of other analogues may also result in an increase in the intracellular level of AdoMet, due to feedback inhibition of AdoMet-dependent methyl or alkyl transferases.

Unexpectedly and most importantly, AdoMet has been found to be a safe and effective agent in the treatment of certain forms of clinical depression (39, 40). While it has been clearly established that administration of AdoMet results in a significant increase in the concentration of AdoMet in the cerebrospinal fluid (CSF) (41), it has not been determined whether the therapeutic effects of AdoMet are related to its ability to function as a methyl donor. In the absence of an animal model for depressive disorders, it has unfortunately been possible only to formulate hypotheses to account for the mode of action of AdoMet in such illnesses.