Pyridoxine (Vitamin B6)

Vitamin B6, which plays a critical role in protein metabolism, occurs in three forms in the body (pyridoxine, pyridoxal, and pyridoxamine). All three forms are relatively stable in an acidic medium but are not heat stable under alkaline conditions. The active coenzyme forms are pyridoxal 5’ phosphate (PLP) and pyridoxamine 5-phosphate (PMP)
All forms of vitamin B6 are absorbed in the upper part of the small intestine. They are phosphorylated within the mucosal cells to form PLP and PMP. PLP can be oxidized further to form other metabolites that are excreted in the urine. Vitamin B6 is stored in muscle tissues. PLP plays an important role in amino acid metabolism. It has the ability to transfer amino groups from compounds by removing an amino acid from one component and adding to another. This allows the body to synthesize nonessential amino acids when amino groups become available. Pyridoxal 5 phosphate’s ability to add and remove amino groups makes it invaluable for protein and urea metabolism.

Vitamin B6 is transferred in the blood, in both plasma and red blood cells. PLP and PMP can both be bound to albumin, with PLP binding more tightly, or to hemoglobin in the red blood cell. The liver is the primary organ that is responsible for the metabolism of vitamin B6 metabolites. As a result, the liver supplies the active form PLP to the blood as well as to other tissues. The three non-phosphorylated forms of vitamin B6 are converted to their respective phosphorylated forms by pyridoxine kinase, with zinc and ATP as cofactors. Pyridoxamine 5’ phosphate and pyridoxine 5’ phosphate can then be converted to PLP by flavin mononucleotide (FMN) oxidase.