Sarcosine.

Sarcosine is N-methylglycine and blocks the GlyT1 transporter that clears glycine from the synapse, so the two compete at the same carrier while raising glycine availability at the NMDA co-agonist site. Dosing one changes what the other does.

D-serine occupies the glycine co-agonist site on the NMDA receptor directly, while sarcosine raises the local glycine that competes for the same site. Both converge on occupancy of one binding pocket, so their effects are not independent.

Glycine N-methyltransferase makes sarcosine by moving a methyl group from SAM-e onto glycine, and this reaction is the main route the body uses to drain surplus SAM-e. Sarcosine sits directly downstream of SAM-e in that cycle.

Sarcosine dehydrogenase is a flavin-dependent enzyme that needs FAD built from riboflavin to strip the methyl group back off. Without adequate riboflavin the enzyme cannot turn sarcosine over.

When sarcosine is demethylated the methyl group is handed to tetrahydrofolate to form 5,10-methylene-tetrahydrofolate. Folate is the acceptor that carries sarcosine's methyl group back into the one-carbon pool.

Betaine gives up a methyl group to become dimethylglycine, which is then demethylated to sarcosine. Betaine intake feeds the pathway that generates sarcosine endogenously.

Choline is oxidised to betaine, which passes through dimethylglycine to sarcosine and finally to glycine. Choline intake sets the size of the pool that flows through sarcosine.

Methionine is converted to SAM-e, the methyl donor that glycine N-methyltransferase uses to make sarcosine. Higher methionine load increases flux through the sarcosine step, which acts as an overflow valve for surplus methyl groups.

B12 lets methionine synthase recycle homocysteine back to methionine using the methyl-folate that sarcosine demethylation helps generate. The two sit on opposite ends of the same one-carbon loop.

Serine hydroxymethyltransferase converts serine to glycine while loading a one-carbon unit onto tetrahydrofolate, and that reaction is the main source of both the glycine pool and the folate one-carbon units. Sarcosine is metabolised back to glycine by a folate-dependent dehydrogenase. The two nutrients meet at the folate cycle rather than at a receptor.

Serine hydroxymethyltransferase and the glycine cleavage system both require pyridoxal 5-phosphate. Those enzymes set how quickly glycine, the product of sarcosine demethylation, is generated and disposed of. Without adequate B6 the surrounding one-carbon traffic slows regardless of sarcosine intake.

Sarcosine dehydrogenase transfers sarcosine's methyl group onto tetrahydrofolate to form 5,10-methylene-THF. That acceptor has to be available for the reaction to run. Folate status therefore governs the clearance of sarcosine back to glycine, and this is settled biochemistry rather than a trial finding.

Betaine-homocysteine methyltransferase is a zinc metalloenzyme, and it is the step that converts betaine to dimethylglycine, one reaction upstream of sarcosine. Zinc status therefore shapes how much substrate arrives at the sarcosine node. The link is upstream and enzymatic, not a direct interaction with sarcosine itself.

Creatine synthesis consumes a large share of the body's S-adenosylmethionine through guanidinoacetate N-methyltransferase, and glycine N-methyltransferase competes for the same methyl donor when it makes sarcosine from glycine. Supplemental creatine lowers endogenous synthesis demand and frees SAM for other methylation reactions. The two therefore pull on one shared currency, which is why they are worth considering together.

Nicotinamide N-methyltransferase methylates nicotinamide using S-adenosylmethionine, the same donor glycine N-methyltransferase uses to make sarcosine. High nicotinamide intake measurably increases SAM consumption and raises homocysteine in reported work. Anyone stacking high-dose nicotinamide with methylation-active ingredients should account for that draw.

Glycine is one of the three amino acids in glutathione and is often the limiting one in adults. Sarcosine demethylation returns carbon to the glycine pool. That connection is indirect and depends on folate-dependent clearance running normally.

Cysteine is produced from homocysteine through the transsulfuration route, which is the disposal arm of the same methylation cycle that generates and clears sarcosine. Supplying cysteine reduces demand on that arm. The two connect through cycle economics rather than through a shared enzyme.

The NMDA receptor channel is blocked by magnesium at resting membrane potential, and that block has to lift before glycine-site occupancy can influence current. Sarcosine acts at the glycine transporter, upstream of that co-agonist site. The magnesium block is a gate on the same channel and belongs in any discussion of glycine-site modulation.

Taurine and sarcosine are both small neutral molecules handled by sodium- and chloride-coupled transporters of the SLC6 family, the family that also contains the glycine transporters. Substrate overlap within that family is well described. Whether co-dosing shifts either one's transport in people has not been measured.

Molybdenum is the cofactor metal for sulfite oxidase, the terminal enzyme of sulfur amino acid disposal downstream of the methylation cycle. It has no direct role in sarcosine handling. It is listed here as a background cofactor for the wider sulfur pathway rather than as a partner with a measured effect.