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Gene-nutrient interactions in homocysteine metabolism: Regulation of human methionine synthase by B12

Sebastian Oltean, University of Nebraska - Lincoln


Studies spanning the last decade have linked elevated plasma homocysteine with increased risk for cardiovascular and other diseases. Homocysteine is a sulfur containing amino acid and its metabolism is directly controlled by the activity of three enzymes: B6 dependent cystathionine beta-synthase, B12-dependent methionine synthase (MS), and betaine homocysteine methyl transferase. Homocysteine can either condense with serine in a reaction catalyzed by cystathionine beta-synthase or be methylated to methionine in a reaction catalyzed by betaine homocysteine methyl transferase (with limited tissue distribution) or by MS. Several studies indicate the benefit of multivitamin (B6, B12 and folate) treatment in lowering plasma homocysteine. Mutations in MS are correlated with hereditary hyperhomocysteinemia. ^ It was first reported over 30 years ago that the activity of MS in cultured cells was enhanced several fold upon supplementation with vitamin B12 . However, the mechanism of this activation was not elucidated. It is important to emphasize that the B12 effect was not incurred by a change in the culture conditions from B12 depleted to a B 12 replete medium but rather by B12 supplementation of medium already containing “normal” B12 levels. Previous work done in our laboratory revealed that this regulation occurs at a post-transcriptional level. In my studies I have demonstrated that B12 supplementation does not affect either MS mRNA stability or MS protein turnover, but increases translation of methionine synthase by unmasking mRNA molecules from the ribonucleoprotein pool and shifting them to the polysomal pool. Reporter gene constructs have revealed that an element in the 5 untranslated region (UTR) of the MS mRNA is involved in mediating the B12 response. ^ Furthermore, I have demonstrated that the MS 5UTR contains an internal ribosome entry site, that is modulated by B12. Structure probing analysis of the 5UTR has proved that B12 does not bind directly to the mRNA. Instead it probably exerts its action with the help of a protein. ^ The observed increase in the metabolic flux through methionine synthase in the presence of B12 suggests a mechanism for lowering of homocysteine levels by B12. ^

Subject Area

Biology, Molecular

Recommended Citation

Oltean, Sebastian, "Gene-nutrient interactions in homocysteine metabolism: Regulation of human methionine synthase by B12" (2004). ETD collection for University of Nebraska - Lincoln. AAI3131553.