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BRS2 — Methylation & One-Carbon Metabolism

BRS2-2-5 - Transsulfuration Pathway​

(Converting Homocysteine and Serine into Cysteine)

1. Mission & Overview​

Mission​

Divert homocysteine toward cysteine so methylation demand is balanced against sulfur amino-acid and redox needs.

Intervention Dominance: Diet-Dominant

Vitamin B6 supplies an essential enzyme cofactor. Comparative intervention dominance and benefit from additional intake are not established; moderate restriction altered cystathionine handling without lowering total pathway flux. [7]

Overview​

Cystathionine beta-synthase joins homocysteine and serine to form cystathionine. [1] [2] Cystathionine gamma-lyase then cleaves cystathionine and releases cysteine. [3] [4] Both enzymes require pyridoxal 5′-phosphate, the active cofactor made from vitamin B6. [1] [5] This irreversible branch disposes of homocysteine and produces cysteine. [3] Studies of the purified enzymes establish these reactions and the cofactor requirement. [1] [2]

  • Benefits: Transsulfuration directs sulfur from homocysteine toward cysteine. [3] Cells can later use that cysteine to make glutathione, which links this branch to antioxidant defence.
  • Implementation Notes: Vitamin B6 is the dietary source of pyridoxal 5′-phosphate. [1] [5] Homocysteine, serine, and cystathionine are substrates already in the pathway, not dietary targets. These enzyme studies do not show that eating more vitamin B6 speeds this conversion in people. [5]
  • Biological Relevance: Glutathione synthesis is a later, separate use of the cysteine made here. [3]

2. Primary Biological Effects​

↑ cystathionine formation from homocysteine and serine
↑ cysteine release from cystathionine
↑ irreversible disposal of homocysteine through this branch

3. Intervention Levers​

4. Mechanistic Basis​

Summary​

Homocysteine can leave the methyl cycle when it is joined to serine and the product is cleaved to cysteine. Those two enzyme steps set the chemistry of this conversion. They do not, by themselves, show how a change in vitamin B6 intake changes the rate in people.

4.1 Scientific Findings (6)​

Summary​

Purified and recombinant human enzyme studies establish the two transsulfuration reaction boundaries and their PLP dependence: CBS forms cystathionine, and CGL uses it to release cysteine. [1] [2] [4] [3] The findings converge on biochemical pathway identity, including cysteine’s position as a product of the second step. They do not measure whole-body dietary responsiveness or glutathione synthesis, so neither increased flux from extra vitamin B6 nor downstream benefit is established by these assays. Human tracer evidence adds a physiological boundary: moderate vitamin B6 restriction changed cystathionine handling without significantly changing total transsulfuration. [7]

5. BRS Pathways and Connections​

5.1 BRS Pathways​

BRS2-FM2-PM5 — Transsulfuration Pathway ↓ BRS2-FM2-PM6 — Glutathione Synthesis

5.2 Cross-BRS Mechanism Relationships​

  • None listed

5.3 Local BRS Mechanism Relationships​

  • BRS2-FM2-PM6 — Glutathione Synthesis — Cysteine released when cystathionine is cleaved is the sulfur amino acid consumed when glutathione is assembled. Assembly of glutathione, and the redox reactions that spend it, sit outside this conversion.

7. Phenome Connections​

These mappings are translational relationships, not single-mechanism outcome claims. Phenomes are emergent functional patterns supported by multiple interacting PMs across the BRAIN Framework.

Stress Reactivity — modulatesOpen Page →
Recovery Capacity — indirectOpen Page →
Stage 2A/2B dietary-target assessment — 23 October 2026

ENRICHED. CBS/CTH routing from homocysteine through cystathionine to cysteine.

Serine and B6/PLP are already admitted. Add a human serine-tracer Finding and composition-supported provisional serine foods; retain single-person, intravenous and compartment-inference limits. No new substrate admission or extra-serine benefit is claimed.

New implementation: 0 dietary-input admissions (0 completed Unset candidates), 0 practices, 0 biochemical participants, 1 unscored Findings. Existing accepted inputs, source assessments, mission, KCs, Phenome relationships and scores are retained.

Source-to-claim boundary: Assessment source. Stop after adjudicating the stated material candidate and tracing supported delivery. Keep uncertain specificity, formulation, anatomical or measurement links explicit; no negative evidence is inferred from incomplete retrieval.

8. References​