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. 2006 Feb 15;5:6. doi: 10.1186/1475-2891-5-6

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Copyright © 2006 McCaddon; licensee BioMed Central Ltd.

This is an Open Access article distributed under the terms of the Creative Commons Attribution License (http://creativecommons.org/licenses/by/2.0), which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

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Details of the methionine synthase reaction: Homocysteine reacts with the methyl group of methionine synthase-bound methyl-B₁₂to produce methionine and an unstable intermediate form of B₁₂, cob(I)alamin. Cob(I)alamin then reacts with methyl-folate (CH₃-THF) to generate free folate (THF) and regenerate methyl-B₁₂. The vitamin shuttles between methyl-B₁₂and cob(I)alamin states. Cob(I)alamin is occasionally deactivated by oxidation to cob(II)alamin. Reductive remethylation of Cob(II)alamin requires a methyl group donated by S-adenosylmethionine. Deactivation usually occurs every few hundred cycles, but AD and age-related oxidative stress might augment this process [23].