Does folate-dependent one-carbon metabolism drive methylation and glutathione production?

Folate-dependent one-carbon metabolism serves as a core metabolic hub that coordinates cell signaling, epigenetic regulation, and antioxidant defense.

Biochemical mechanisms of methylation

• Methyl group generation: The folate cycle reduces dietary folate to tetrahydrofolate (THF). Serine hydroxymethyltransferase transfers a one-carbon unit to generate 5,10-methylene-THF, which methylenetetrahydrofolate reductase (MTHFR) irreversibly reduces to 5-methyl-THF.
• SAM synthesis: Methionine synthase transfers the methyl group from 5-methyl-THF to homocysteine, producing methionine. This is subsequently converted to S-adenosylmethionine (SAM), the cell's universal methyl donor for DNA, histone, and lipid methylation.

Intersection with transsulfuration and glutathione synthesis

• The homocysteine junction: Homocysteine represents the metabolic branch point between remethylation and transsulfuration.
• Allosteric regulation: High levels of SAM allosterically activate and stabilize cystathionine $\beta$-synthase (CBS), committing homocysteine to the transsulfuration pathway rather than recycling.
• Glutathione production: CBS and cystathionine $\gamma$-lyase (CSE) convert homocysteine to cysteine, the rate-limiting substrate for glutathione synthesis. Hepatic models show that transsulfuration supplies approximately 50% of intracellular glutathione, a pathway selectively upregulated during oxidative stress to maintain cellular redox balance.

Bottom line

• Folate-dependent one-carbon metabolism is biochemically verified to drive cellular methylation via SAM synthesis and directly regulate transsulfuration flux, providing the rate-limiting cysteine necessary for glutathione production and cellular antioxidant defense.