SAMe (S-Adenosyl Methionine).

SAMe is formed when methionine adenosyltransferase joins methionine to ATP. Methionine supply is the direct upstream input to the whole methylation cycle.

5-MTHF hands its methyl group to homocysteine to rebuild methionine, which is then re-adenosylated to SAMe. Without folate the cycle stalls at homocysteine and SAMe regeneration slows.

Methionine synthase carries a cobalamin cofactor that shuttles the methyl group from folate to homocysteine. B12 status therefore sets how fast methionine, and with it SAMe, is regenerated.

Betaine-homocysteine methyltransferase uses TMG as an alternative methyl donor to rebuild methionine independently of folate. It gives the cycle a second lane back to SAMe.

Cystathionine beta-synthase and cystathionine gamma-lyase both need pyridoxal-5-phosphate to move homocysteine down the transsulfuration route. That clears the by-product of every SAMe methyl transfer.

MTHFR is a flavoprotein that needs FAD derived from riboflavin to generate 5-MTHF. Low riboflavin slows the folate arm that feeds methionine regeneration.

Choline is oxidised to betaine, the methyl donor for the alternative remethylation route, and its synthesis by PEMT consumes three SAMe molecules per phosphatidylcholine. Choline intake therefore both spares and supplies methyl groups.

The PEMT route methylates phosphatidylethanolamine three times using SAMe to make phosphatidylcholine. Supplying phosphatidylcholine directly lowers how much SAMe that pathway draws.

Guanidinoacetate methyltransferase uses SAMe to make creatine and accounts for a large share of daily methyl transfer. Dietary creatine reduces the pull on the SAMe pool.

Glycine N-methyltransferase methylates glycine to sarcosine specifically to soak up surplus SAMe. Glycine availability is how the body buffers a high methyl donor load.

Methionine adenosyltransferase is an ATP-dependent enzyme that requires magnesium at its active site. Magnesium status gates the step that actually forms SAMe.

Betaine-homocysteine methyltransferase is a zinc metalloenzyme, with the zinc holding the methyl-accepting thiol of homocysteine in place. Zinc supply supports that route back to methionine.

Homocysteine generated by SAMe methyl transfer can be routed through transsulfuration to cysteine. NAC supplies cysteine directly, easing demand on that route and supporting glutathione synthesis.

Transsulfuration of homocysteine is a main endogenous source of the cysteine used for glutathione. SAMe and glutathione therefore sit at two ends of the same sulfur pathway.

Folinic acid enters the folate pool upstream of 5-MTHF and can be reduced onward to the methyl donor form. It supports the same remethylation step that regenerates methionine.

After SAMe donates its methyl group it becomes S-adenosylhomocysteine and then homocysteine. One disposal route condenses that homocysteine with serine to form cystathionine, which is cleaved to cysteine. Cysteine availability therefore sits directly downstream of methyl donation. This is settled one-carbon and transsulfuration biochemistry.

Cysteine generated through transsulfuration is oxidised to cysteine sulfinate and decarboxylated toward taurine. SAMe is an allosteric activator of cystathionine beta-synthase, the entry enzyme of that branch, so methyl-donor status influences how much sulfur is routed this way. The link is enzymology, not a supplement trial.

Sulfur that leaves the methionine cycle through transsulfuration ends as sulfite and then sulfate. That last oxidation is carried out by sulfite oxidase, which requires a molybdenum cofactor. Adequate molybdenum is what lets the pathway terminate cleanly. The cofactor requirement is textbook; no combination study exists.

Nicotinamide is methylated by NNMT using SAMe as the methyl donor, producing N1-methylnicotinamide and S-adenosylhomocysteine. High nicotinamide intake therefore acts as a methyl sink and draws on the same SAMe pool other methyltransferases use. This is a well-characterised competition for methyl groups. It says nothing about a clinical outcome in either direction.

Three sequential methyl transfers from SAMe convert protein-bound lysine to trimethyllysine, the committed first step toward carnitine. Lysine supplies the carbon skeleton and SAMe supplies the methyl groups. Neither substitutes for the other. The stepwise pathway is established biochemistry.

Endogenous carnitine formation depends on SAMe for the trimethyllysine step. Supplying carnitine directly bypasses that methyl demand, which is why intake of preformed carnitine spares methyl groups for other acceptors. The relationship is a metabolic dependency, not a tested clinical pairing.

Acetylserotonin O-methyltransferase transfers a methyl group from SAMe to N-acetylserotonin to give melatonin. Methyl donor availability is therefore built into the last step of the pathway. Supplemental melatonin bypasses it entirely. The enzymology is settled; the practical significance of methyl supply for endogenous production has not been quantified in people.

Hepatic phosphatidylcholine can be made by three sequential SAMe methylations of phosphatidylethanolamine, and phosphatidylserine decarboxylation feeds that ethanolamine pool. The membrane phospholipid classes are therefore linked through methyl donor supply. This is a hepatic pathway relationship rather than a supplement combination with outcome data.

Phosphatidylethanolamine N-methyltransferase preferentially generates phosphatidylcholine species enriched in long-chain polyunsaturates, and each molecule costs three SAMe methyl groups. Methyl donor supply and long-chain fatty acid supply therefore meet at the same product. The link is biochemical, and no human combination trial anchors it.

SAMe influences cysteine supply through the transsulfuration branch, and cysteine is what limits glutathione synthesis. Alpha lipoic acid works on the reduced-to-oxidised thiol ratio once glutathione exists. The two act at different points of the same thiol economy. Thiol status is a marker, not a clinical outcome.

SAMe is synthesised largely in the liver and its methyl transfers dominate hepatic one-carbon flux. Silymarin is a flavonolignan complex studied against the same hepatic oxidative markers. The pairing is common in formulation and mechanistically plausible on shared hepatic redox ground. It has not been resolved as a combination in controlled human work.

Phosphatidylinositol and phosphatidylcholine draw on overlapping diacylglycerol precursors in the liver, and one of the two routes to phosphatidylcholine costs three SAMe methyl groups. The connection is indirect and sits at the level of lipid class balance. Regard it as mechanistic background rather than a supported pairing.

Methylenetetrahydrofolate reductase carries an FAD cofactor derived from riboflavin, and its product supplies the methyl group that converts homocysteine back to methionine, from which SAMe is remade. Low riboflavin status therefore constrains the recycling arm of the methionine cycle. This is a settled cofactor relationship.

Excess niacin is disposed of by N-methylation, which consumes SAMe and generates S-adenosylhomocysteine. High-dose niacin therefore competes for the same methyl pool as every other methyltransferase reaction. The competition is documented pharmacology. What it means for any endpoint depends on dose and on the rest of the one-carbon supply.