Does vitamin B6 support homocysteine transsulfuration, yet high blood B6 not ensure cellular disposal?

Biochemical mechanisms of transsulfuration

• Vitamin B6, in its active pyridoxal 5′-phosphate (PLP) form, serves as an obligatory cofactor for the sequential enzymes cystathionine $\beta$-synthase (CBS) and cystathionine $\gamma$-lyase (CSE), which govern the irreversible disposal of homocysteine.
• Within CBS, PLP forms a covalent internal aldimine with a conserved lysine residue (Lys119), acting as an electrophilic electron sink to stabilize reaction intermediates and catalyze the condensation of L-homocysteine and L-serine into cystathionine. CSE subsequently utilizes PLP to cleave cystathionine into cysteine.

Cellular uptake and the intracellular disconnect

• High circulating levels of PLP do not guarantee efficient intracellular homocysteine clearance. Because of its highly charged phosphate group, extracellular PLP is membrane-impermeable and cannot passively cross cell membranes.
• Cellular entry requires dephosphorylation of extracellular PLP into membrane-permeable pyridoxal (PL) by the cell-surface ectoenzyme tissue-nonspecific alkaline phosphatase (TNSALP). Once inside the cytosol, PL is re-phosphorylated back into active PLP.
• When TNSALP activity is impaired (such as in hypophosphatasia), extracellular PLP accumulates in the blood, resulting in elevated circulating vitamin B6 levels. However, because PL cannot enter the cells, a functional intracellular B6 deficiency occurs, halting CBS and CSE activity and impairing cellular homocysteine disposal.

Bottom line

• While vitamin B6 is essential for CBS- and CSE-mediated transsulfuration, high circulating B6 levels can paradoxically coexist with cellular homocysteine accumulation if TNSALP-mediated dephosphorylation is compromised, preventing the cofactor from entering target cells.