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The MTRR gene provides instructions for making an enzyme called 5-Methyltetrahydrofolate-Homocysteine Methyltransferase Reductase. This enzyme is required for the proper function of another enzyme called methionine synthase. Methionine synthase helps process amino acids, which are the building blocks of proteins.

Methionine is an essential amino acid and is required for numerous processes throughout the body. A major source of methionine is the enzyme methionine synthase which converts homocysteine into methionine, using 5-methyltetrahydrofolate (MTHF) produced by methylenetetrahydrofolate reductase (MTHFR) as a methyl donor. 

The G allele in the MTRR gene is associated with an accumulation of homocysteine, which is in turn associated with a variety of disorders including cancers, heart disease, stroke, raised blood pressure, and potential issues with birth defects. Vitamin B12 is the cofactor for methionine synthase. When bound methionine synthase can process homocysteine into the essential amino acid methionine. Over time the vitamin B12 molecule loses its effectiveness and must be removed, and replaced with a new molecule, which is achieved by MTRR. The risk G allele is associated with a reduction in its activity. Whilst supplementation with vitamin B12 will not benefit MTRR activity directly it may help ensure maximal methionine synthase function.

Methionine synthase converts homocysteine into the essential amino acid methionine. The risk G allele leads to reduced methionine synthase activity. This can promote homocysteine accumulation, but can also limit the availability of methionine. Supplementation with L-methionine can ensure that an appropriate supply of methionine is available for cells and tissues.

Knowing what your levels of potassium levels are important if you carry a medium or high impact genetic variation in this gene as this mineral is known to contribute to B12 deficiency.