Vitamin B6 is primarily absorbed in its dephosphorylated forms after enzymatic action in the digestive tract.
The Absorption Process Explained
For Vitamin B6 compounds that are phosphorylated (like pyridoxal 5'-phosphate, pyridoxine 5'-phosphate, and pyridoxamine 5'-phosphate), a crucial step occurs before they can enter the body.
- Dephosphorylation: The phosphate forms of Vitamin B6 require removal of their phosphate group to be absorbed. This process is catalyzed by the enzyme alkaline phosphatase.
- Absorption: Once dephosphorylated, these simpler forms (pyridoxine, pyridoxal, pyridoxamine) are absorbed, mainly in the jejunum of the small intestine.
- Transport to the Liver: After absorption, most of the vitamin is taken up by the liver.
- Reconversion and Metabolism: In the liver, the dephosphorylated vitamins are converted back to their phosphorylated forms, specifically pyridoxal 5'-phosphate (PLP), pyridoxine 5'-phosphate (PNP), and pyridoxamine 5'-phosphate (PMP). The PNP and PMP forms are then further converted into the primary biologically active form, PLP.
This process ensures that the vitamin is in the correct state for absorption and subsequent utilization in the body, largely centered around the liver's metabolic functions.
Key Forms and Absorption Steps
| Form of B6 | State in Food | Before Absorption | Enzymatic Action | Absorbed Form | Primary Location After Absorption |
|---|---|---|---|---|---|
| Pyridoxal 5'-phosphate (PLP) | Phosphorylated | Requires Dephosphorylation | Alkaline phosphatase | Pyridoxal | Liver |
| Pyridoxine 5'-phosphate (PNP) | Phosphorylated | Requires Dephosphorylation | Alkaline phosphatase | Pyridoxine | Liver |
| Pyridoxamine 5'-phosphate (PMP) | Phosphorylated | Requires Dephosphorylation | Alkaline phosphatase | Pyridoxamine | Liver |
| Pyridoxine, Pyridoxal, Pyridoxamine | Non-phosphorylated | Ready for Absorption | N/A (Already in absorbable form) | Pyridoxine, Pyridoxal, Pyridoxamine | Liver |
Reference Information: Absorption of the phosphate forms involves their dephosphorylation catalyzed by the enzyme alkaline phosphatase. Most of the vitamin is taken up by the liver. There, the dephosphorylated vitamins are converted to the phosphorylated PLP, PNP and PMP, with the two latter converted to PLP.
Why Dephosphorylation Matters
Alkaline phosphatase, found in the lining of the intestines, plays a critical role. By removing the phosphate group, it makes the B6 molecule smaller and less charged, facilitating its transport across the intestinal cell membrane. Without this step, absorption of the phosphorylated forms would be significantly impaired.
This hepatic conversion is essential because PLP is the coenzyme form of B6, vital for numerous metabolic reactions in the body, particularly those involving amino acids.
