Does vitamin B6 support the cystathionine-to-cysteine pathway?

Vitamin B6 supports transsulfuration through pyridoxal 5′-phosphate (PLP), its active coenzyme form. The central biochemical claim is well established; the clinical consequence of modest insufficiency is more nuanced.

Mechanistic evidence

• Cystathionine β-synthase (CBS) uses PLP to condense serine and homocysteine into cystathionine. In human CBS, PLP is covalently linked to catalytic Lys119 and stabilizes intermediates essential to turnover; removing bound PLP abolishes activity.
• Cystathionine γ-lyase (CGL/CSE) then performs PLP-dependent γ-elimination of cystathionine to yield cysteine, ammonia, and α-ketobutyrate. PLP forms an internal aldimine with Lys212, organizes the active site, and stabilizes catalytic carbanion intermediates.
• Thus, B6 is not merely associated with these enzymes: PLP is a direct catalytic requirement at both steps.

Human functional evidence

• Controlled dietary B6 restriction lowering plasma PLP from roughly 49–55 to 19–23 nmol/L increased plasma cystathionine by approximately 50–124%. This pattern supports impaired clearance of cystathionine at the PLP-dependent CGL step.
• However, moderate depletion has not consistently lowered measured whole-body cysteine production or net transsulfuration flux. Accumulation of cystathionine may compensate for reduced enzyme efficiency by increasing substrate availability.
• Functional impairment may therefore be most evident biochemically—particularly elevated cystathionine—and under greater deficiency or methionine/substrate challenge, rather than as predictably low circulating cysteine.

Bottom line

• Vitamin B6, specifically PLP, is required for CBS and CGL, and functional B6 insufficiency can constrain conversion of cystathionine toward cysteine. Elevated cystathionine is the better-supported functional marker; moderate insufficiency does not necessarily cause a measurable fall in net cysteine production.