Does BHMT rs3733890 AG modestly affect betaine-dependent remethylation of homocysteine?

The BHMT rs3733890 (Arg239Gln) polymorphism influences one-carbon metabolism by altering the kinetic properties of the betaine-homocysteine S-methyltransferase enzyme.

Mechanistic explanations

• Kinetic alterations: Functional genomic and biochemical studies show that the variant A allele exhibits a significantly lower Michaelis constant ($K_m$)—approximately half that of the wild-type—for both betaine and homocysteine. This represents an increased substrate binding affinity that modestly modulates the rate of betaine-dependent remethylation.
• Substrate partitioning: Isotope-tracer investigations demonstrate that this kinetic shift alters metabolic flux, directing whether dietary choline is oxidized to generate betaine or partitioned toward phosphatidylcholine synthesis via the CDP-choline pathway.

Clinical and physiological evidence

• Modest metabolic impact: The net effect of the rs3733890 AG genotype on circulating plasma homocysteine is modest and highly dependent on overall nutritional status.
• Dietary reliance: Clinical depletion studies indicate that the variant does not elevate the risk of overt clinical organ dysfunction during acute choline restriction. However, because it dictates how the body partitions available methyl donors, there is a plausible biochemical reliance on adequate exogenous choline and betaine to support optimal one-carbon flux.

Bottom line

• The BHMT rs3733890 AG genotype modestly modulates betaine-dependent remethylation of homocysteine by increasing enzyme substrate affinity. This kinetic alteration shifts choline partitioning, making adequate dietary intake of choline or betaine highly beneficial for maintaining optimal metabolic pathways.