Page 19 - Fall2012

A. During each methylation (reaction 3), methionine adenosyltransferase uses ATP to convert methionine to S-adenosylme-
thionine (SAM). Methyltransferases then use SAM to methylate a wide variety of molecules, generating S-adenosylhomo-
cysteine (SAH). S-adenosylhomocysteine hydrolase then allows a small amount of SAH to generate homocysteine, which
is potentially toxic. Vitamin B and folate assist in the recycling of homocysteine back to methionine. 5-methyltetrahy-
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drofolate (a form of folate, abbreviated as methyl-folate in the figure) methylates vitamin B (reaction 1), and methionine
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synthase then uses B to methylate homocysteine and thereby form methionine (reaction 2). Folate is then remethylated
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through several different pathways not shown in the figure, which depend on niacin, riboflavin, and vitamin B .
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B. When methionine concentrations are high, an alternative set of reactions predominates in the liver. Choline dehy-
drogenase and betaine aldehyde dehydrogenase convert choline to betaine with the assistance of niacin and oxygen
(reaction 1). Betaine-homocysteine methyltransferase uses betaine to convert homocysteine to methionine, generating
dimethylglycine (reaction 2), part of which is converted to glycine and part of which is lost in the urine. More SAM is
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generated than is needed for methylation reactions.
With the assistance of vitamin B , cystathionine b-synthase and cystathionine g-lyase use serine to convert homo-
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cysteine to cysteine in two successive steps (reaction 5). Serine is obtained directly in the diet or derived from glycine
with the assistance of vitamin B (reaction 4). Using glycine, glutamate, and ATP, glutamate cysteine ligase and glutathione
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synthase convert cysteine to glutathione in two successive steps (reaction 6). Excess cysteine is converted to taurine
and sulfate (reaction 7). As a result, glycine, obtained directly from the diet or synthesized from dietary serine with the
assistance of vitamin B (reaction 4), accepts methyl groups from SAM, generating dimethylglycine (reaction 3), part of
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which is lost in the urine. N-methyltransferases catalyze this reaction in two successive steps.
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Wise Traditions FALL 2012 FALL 2012 Wise Traditions 19

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