Transsulfuration (homocysteine → cysteine)
The sulfur-metabolism branch that turns toxic homocysteine into cysteine, glutathione and protective H₂S
Definition
Transsulfuration is the metabolic pathway that irreversibly channels homocysteine toward cysteine synthesis, draining a vasculotoxic amino acid while fueling the production of glutathione, taurine and hydrogen sulfide (H₂S). It comprises two enzymatic steps that both depend on pyridoxal-5-phosphate (the active form of vitamin B6): cystathionine β-synthase (CBS) condenses homocysteine and serine into cystathionine, and cystathionine γ-lyase (CSE/CGL) cleaves it into cysteine. It is the counterpart to the remethylation branch of the methionine cycle and a key node of antioxidant defense and healthy aging.
Detailed explanation
Whereas remethylation returns homocysteine to methionine (requiring folate and B12), transsulfuration disposes of it permanently as cysteine — the rate-limiting amino acid for synthesizing glutathione, the main intracellular antioxidant. Both pathway enzymes, CBS and CSE, are PLP-dependent, so vitamin B6 status governs their function: in B6 deficiency CSE activity is lost before CBS, causing cystathionine to accumulate (a sensitive biomarker of B6 insufficiency).
Beyond producing cysteine, CBS and CSE generate hydrogen sulfide (H₂S), a gasotransmitter with vasodilatory, anti-inflammatory and cytoprotective effects. H₂S production via this pathway mediates much of the benefit of dietary restriction: studies in Nature Communications show that caloric restriction expands the tissue "persulfidome" in a CSE-dependent manner, modifying proteins across metabolic and longevity pathways.
Clinically, when transsulfuration or remethylation falters, homocysteine accumulates (hyperhomocysteinemia), linked to endothelial damage, stroke and brain atrophy. The OXFORD VITACOG trial showed that lowering homocysteine with B vitamins slows brain atrophy in mild cognitive impairment. Canonical nutritional support pairs active B6 (P5P) for transsulfuration with folate and B12 for remethylation.
Scientific sources
- PubMed — Vitamin B6 status and PLP availability govern the transsulfuration pathway and H2S production (PNAS)
- PubMed — Vitamin B-6 deficiency suppresses the hepatic transsulfuration pathway but increases glutathione (J Nutr)
- Nature — Dietary restriction transforms the mammalian persulfidome in a cystathionine γ-lyase-dependent manner (Nat Commun)
- PubMed — Homocysteine-lowering by B vitamins slows the rate of accelerated brain atrophy (VITACOG)
Related terms
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