Betaine, as methyl donor

Figure 9.5. Methionine load test for vitamin Be status. Methionine synthetase, EC 2.1.1.13 (vitamin Bi2-dependent) 2.1.1.5 (betaine as methyl donor) cystathionine synthetase, EC 4.2.1.22 and cystathionase, EC 4.4.1.1. Relative molecular masses (Mr) methionine, 149.2 homocysteine, 135.2 cystathionine, 222.3 and cysteine, 121.2.

In other methyl transfers, there is no net proton production. For example, with betaine as methyl donor to homocysteine, the general reaction according to Eq. (5) [in this case Eq. (15a)] is balanced at pH 7.0 in aqueous solution by proton uptake by the product dimethylglydne ... 

Reaction 4 is catalysed by cystathionine synthase (EC 4.2.1.13), an enzyme widely distributed in the tissues. In homocystinuria, cystathionine synthase is virtually completely absent or inactive in all tissues examined liver, brain and fibroblasts grown in tissue culture [33]. In some cases 1 to 2% of the normal enzymic activity can be demonstrated, in others no enzymic activity has been found [34]. As a result of the metabolic block, homocysteine accumulates and is partly converted to homocystine, partly to homocysteine-cysteine mixed disulphide and partly S-methylated to methionine by reactions 6 and 7 with, respectively, N -methyltetrahydrofolic acid and betaine as methyl donors. In infancy methionine and homocysteine are present in high concentrations in the plasma while homocystine and homocysteine-cysteine mixed disulphide are excreted in the urine later the concentration of methionine in the plasma drops. Cystathionine is normally present in highest concentration in the cells of the brain, though traces are found elsewhere and in the urine in homocystinuria no cystathionine can usually be demonstrated in the brain or urine [35]. The body s cysteine and cystine are also largely biosynthesized from methionine, though some is obtained from cysteine and cystine in dietary proteins in homocystinuria, cysteine/cystine becomes an essential amino acid. 

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See also in sourсe #XX -- [ , , , ]

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