Demethylation of methionine

Homocysteine. A sulfur-containing amino acid, a homologue of cysteine, produced by the demethylation of methionine, and an intermediate in the biosynthesis from methionine via cystathionine. 

C HjNOjS mol wt 135.19. C 35.53%, H 6.71%, N 10,36%. O 23.67%, S 23.72%. HSCH,CHjCH(NH,)COOH. A sulfur containing amino acid, produced by the demethylation of methionine and an intermediate in the biosynthesis of cysteine from methionine. Originally obtained from the liver of mammals. Has also been obtained from cystathionine Binkley, Methods EnzymoL 2, 314 (1955). D-Homocysteine may be prepd from 5-benzyl -D-homocysteine, while L-homocysteine is best obtained from L-homocystrre du... 

One synthesis of SAM involves the initial conversion of adenosine (the synthesis of which has been described earlier, vide supra) to the corresponding primary chloride hydrochloride (Scheme 12.117) with thionyl chloride (SOCI2) in pyridine, demethylation of methionine (Met, M) with sodium metal (Na°) in Uquid ammonia (NHj) to produce homocysteine (the synthesis of which has been described earlier. 

The conversion of serine into cysteine is brought about by condensation of the serine with homocysteine, formed by the demethylation of methionine, or more exactly of S-adenosylmethionine, in the course of transmethylation reactions. 

Nonstandard amino acids are usually formed through modifications to standard amino acids. For example, homocysteine is formed through the transsulfuration pathway or by the demethylation of methionine via the intermediate metabolite S-adenosyl methionine, while hydroxyproline is made by a posttranslational modification of proline. 

Conversion of Methionine to Homocysteine. The conversion of methionine to the corresponding keto acid does not bring the thiomethyl group directly into action. On the contrary, the fact that this group plays the principal role in the demethylation of methionine has been demonstrated in the work of du Vigneaud and collaborators. Numerous observations have been carried out with living animals which have shown... 

This observation was explained by the assumption that a portion of the protoberberine was formed via norreticuline (96) present in the same incubation mixture and derived from enzymic demethylation of reticuline. Reaction of 96 with an unlabeled one-carbon fragment and subsequent ring closure would then lead to C-8 unlabeled protoberberines. The authors suggest that this one-carbon fragment may be derived from S-adenosyl-methionine, and that the product of its combination with 96 may be converted directly to 91 or 94 without the intermediacy of free reticuline (99). If their assumption is correct, the conversion of norreticuline to the protoberberine alkaloids may not involve the formation of reticuline itself, a suggestion that is at variance with the known intermediacy of reticuline in the biosynthesis of alkaloids of this group. 

Strong protic acids cleave phenolic methyl ethers. Thus hydrogen bromide accomplished the same double demethylation as that depicted in Scheme 4.104.189 The final step in a Merck synthesis of the potent dopamine agonist (A,R)-4-pro-pyl-9-hydroxy naphthoxazine (106 2), a phenolic ether deprotection was accomplished on a large scale using methanesulfonic acid in the presence of methionine as the nucleophile [Scheme 4.106]. 190 191... 

The principal substrate for glutamylation is free tetrahydrofolate one-carbon substituted folates are poor substrates. Because the main circulating folate, and the main form that is taken up into tissues, is methyl-tetrahydrofolate, demethylation by the action of methionine synthetase (Section 10.3.3) is essential for effective metabolic trapping of folate. In vitamin B12 deficiency, when methionine synthetase activity is impaired, there wUl be impairment of the retention of folate in tissues. 

Unlike most enzymes utilizing or metabolizing tetrahydrofolate, methionine synthetase has equal activity toward methyl-tetrahydrofolate mono- and polyglutamates. As discussed in Section 10.2.2, demethylation of methyl-tetrahydrofolate is essendal for the polyglutamylation and intracellular accumulation of folate. 

Impairment of methionine synthetase activity, for example, in vitamin B12 deficiency or after prolonged exposure to nitrous oxide (Section 10.9.7), will result in the accumulation of methyl-tetrahydrofolate. This can neither be utilized for any other one-carbon transfer reactions nor demethylated to provide free tetrahydrofolate. 

The cobalt atom is in the Co + oxidation state in hydroxo-, aquo-, methyl-, and cyanocobalamins in the Co+ oxidation state in adenosylcobalamin and, transiently, in the demethylated prosthetic group of methionine synthetase (Section 10.8.1). 

The cause of megaloblastosis is depressed DNA synthesis, as a result of impaired methylation of dCDP to TDP, catalyzed by thymidylate synthetase, but more or less normal synthesis of RNA. As discussed in Section 10.3.3, thymidylate synthetase uses methylene tetrahydrofolate as the methyl donor it is obvious that folic acid deficiency will result in unpaired thymidylate synthesis. It is less easy to see how vitamin B12 deficiency results in impaired thymidylate synthesis without invoking the methyl folate trap hypothesis (Section 10.3.4.1). The main circulating form of folic acid is methyl-tetrahydrofolate before this can be used for other reactions in tissues, it must be demethylated to yield free folic acid. The only reaction that achieves this is the reaction of methionine synthetase (Section 10.8.1). Thus, vitamin B12 deficiency results in a functional deficiency of folate. 

The key intermediate in the catalytic pathway is the supemucleophile cob(l)alamin, which attacks A -methyl-tetrahydrofolate, generating tetrahydrofolate and MeCbl. Then homocysteine (probably as its thiolate) attacks MeCbl, which yields methionine and regenerates cob(l)alamin (Scheme 2). The demethylation of A -methyltetrahydrofolate is not trivial, even for the supemucleophilic cob(l)alamin, and considerable efforts have been invested into understanding this reaction, dubbed improbable by Duilio Arigoni. The obvious mode of activation is by proton transfer to N-5 of A -methyl tetrahydrofolate, but as this is weakly basic (pAa 5.1) the nature of the proton source and mode of transfer has been difficult to pin down. Recent research from the Matthews group has shown how the reactivities of cob(I)alamin and methylcobalamin are modulated by the ligand trans to the lone pair of cob(l)alamin and methyl group of methylcobalamin (21). 

Methylcobalamia is iavolved ia a critically important physiological transformation, namely the methylation of homocysteine (8) to methionine (9) (eq. 2) catalyzed by A/ -methyltetrahydrofolate homocysteine methjitransferase. The reaction sequence involves transfer of a methji group first from A/5 -methjltetrahydrofolate to cobalamin (yielding methjicobalamin) and thence to homocysteine. Once again, the intimate details of the reaction are not weU known (31). Demethylation of tetrahydrofolate to tetrahydrofohc acid is a step in the formation of thymidine phosphate, in turn requited for DNA synthesis. In the absence of the enzyme, excess RNA builds up in ted blood cells. 

On the other hand, it was clarified by the incorporation of labeled methionine that the methyl groups incorporated during the biosynthesis of N-methyltyramine and hordenine were derived from methionine.The yield of N-methyltyramine in these experiments is less than half that of horde-nine however, the incorporation of into N-methyltyramine is 1.5 times that of hordenine. This indicates that N-methyltyramine is not formed by the demethylation of hordenine, but that N-methyltyramine was formed first, and hordenine is then formed by the methylation of the former alkaloid. If N-methyltyramine was formed by the demethylation of hordenine, the incorporation rate of of N-methyltyramine, which possesses only one N-methyl moiety, and is present in lower yield than hordenine, should not be superior to that of hordenine [4]. 

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