Spirulina and One-Carbon Metabolism: MTHFR, Folate Cycle, and Nucleotide Synthesis

Tetrahydrofolate: Reduction and One-Carbon Unit Entry

Dietary folate (polyglutamate) is hydrolysed to monoglutamate by FOLH1 (folate hydrolase/ GCPII) in the small intestine, absorbed, and reduced to tetrahydrofolate (THF) by dihydrofolate reductase (DHFR, NADPH-dependent). THF acquires one-carbon units from serine (via serine hydroxymethyltransferase SHMT1/2, generating 5,10-methylene-THF and glycine), histidine catabolism (via FIGLU, generating 5-formiminino-THF), and formate (from mitochondria, via MTHFD2 producing 10-formyl-THF). 5,10-methylene-THF is the central intermediate for: thymidylate synthesis (TYMS: dUMP → dTMP, consuming 5,10-methylene-THF → DHF), purine synthesis (GART/ATIC using 10-formyl-THF), and methionine synthesis (MTHFR: 5,10-methylene-THF → 5-methyl-THF; MTR: 5-methyl-THF + Hcy → THF + Met).

MTHFR: The C677T Polymorphism and Hyperhomocysteinemia

MTHFR (methylenetetrahydrofolate reductase) catalyses the irreversible reduction of 5,10-methylene-THF to 5-methyl-THF (donating to MTR for methionine re-methylation). The common MTHFR C677T polymorphism (rs1801133, ~10% TT homozygosity in European populations) encodes a thermolabile variant with 30-70% reduced activity, leading to elevated plasma homocysteine (especially when folate is marginal). Spirulina's folate content (~95 microg/100g DW, primarily 5-methyl-THF-like forms in the natural state) provides the substrate that the reduced-activity MTHFR TT variant still processes, reducing Hcy in folate-deficient MTHFR C677T individuals.

SHMT1/2 and the Serine-Glycine Axis

Serine hydroxymethyltransferase 1 (cytosolic) and SHMT2 (mitochondrial) interconvert serine and glycine, donating the methylene to THF. This reaction is the primary entry of one-carbon units into the folate cycle in proliferating cells. SHMT2 is highly upregulated in cancer cells (driven by HIF-1alpha and c-Myc) to support nucleotide synthesis. AMPK-mTORC1 suppression attenuates c-Myc/HIF-1alpha-driven SHMT2 upregulation, reducing nucleotide synthesis in tumour cells. Spirulina's AMPK and PCB-HIF-1alpha effects thus reduce SHMT2/one-carbon flux in tumour cells while supporting physiological nucleotide synthesis in non-cancerous proliferating cells (immune cells, enterocytes) via adequate folate supply.

Mitochondrial One-Carbon Metabolism: MTHFD2 and Serine Catabolism

In mitochondria, SHMT2 generates glycine and 5,10-methylene-THF, which is further processed by MTHFD2 (NAD+-dependent) to 10-formyl-THF, then MTHFD1L exports formate to the cytosol. The mitochondria-cytosol one-carbon unit transfer (formate shuttle) supports cytosolic 10-formyl-THF for purine synthesis. Cancer cells upregulate MTHFD2 (c-Myc/MTHFD2 essential in multiple cancer types). Spirulina's AMPK- c-Myc suppression (AMPK-mediated GSK-3beta activation phosphorylates and destabilises c-Myc) attenuates MTHFD2-driven mitochondrial one-carbon flux selectively in proliferating cancer cells.

Purine Synthesis and Anti-Folate Relevance

De novo purine synthesis requires 10-formyl-THF at two steps: GART (5-phosphoribosyl- glycinamide formyltransferase) and ATIC (bifunctional 5-aminoimidazole-4-carboxamide ribonucleotide formyltransferase/IMP cyclohydrolase). Anti-folates methotrexate and pemetrexed inhibit DHFR (blocking THF regeneration from DHF after TYMS) and TYMS directly, depleting thymidylate and purines in rapidly proliferating cancer cells and immune cells. Spirulina's folate supports normal purine synthesis in physiological cell turnover while AMPK effects selectively suppress cancer cell purine demand.

Betaine and the Alternative Methylation Route

Betaine (trimethylglycine, from choline oxidation) serves as an alternative methyl donor for Hcy remethylation via BHMT (betaine-homocysteine methyltransferase), using betaine → dimethylglycine + Hcy → Met. This BHMT route supplements MTR when folate is limiting, particularly in liver (where BHMT is highly expressed). Spirulina contains betaine (~0.5-2 g/100g DW), providing both a direct methyl donor and competing with MTHFR/MTR-driven remethylation, complementing the folate-mediated Hcy lowering effect.