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. 2016 Jan 5;6(1):e704. doi: 10.1038/tp.2015.204

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Homocysteine may be recycled to methionine or eliminated in urine as sulfate. Under conditions in which excess methionine is present, homocysteine condenses with serine to form cystathionine in a reaction catalyzed by the vitamin B6–dependent rate-limiting enzyme cystathionine β synthase. B12, vitamin B12; B6, vitamin B6; CβS, cystathionine β-synthase; MTHFR, methylene-tetrahydrofolate reductase; THF, tetrahydrofolate. Adapted from ref. 66.