Figure 1.

Uptake, transport, and metabolism of pyridoxine and its derivatives. (A) Pyridoxine (PN) (or its closely related form pyridoxamine), is transported from the gut to the liver, where it is metabolized successively to pyridoxine phosphate (PNP) and pyridoxal phosphate (PLP). PLP is given off to the circulation, but cannot cross the blood-brain barrier and enter the brain unless it is converted to pyridoxal (PL). Once inside brain cells, PL is re-phosphorylated to PLP. (B) Molecular structures of pyridoxine and its metabolites. PN is phosphorylated to PNP by pyridoxine kinase (a) in the liver. PNP is oxidized to PLP by PNP oxidase (b). In brain capillaries, PLP is dephosphorylated to PL by tissue non-specific alkaline phosphatase (c). In brain cells, PL is re-phosphorylated to PLP by PL kinase (d). The circle shows the OH group that gives rise to the aldehyde function in PLP and PL.