Vitamin B6 is composed of three compounds: pyridoxine, pyridoxal, and pyridoxamine. Pyridoxine contains a methylhydroxyl group (-CH3OH), pyridoxal an aldehyde (-CHO), and pyridoxamine an aminomethyl group (-CH3NH2), as shown below.
Figure 10.71 Structure of pyridoxine1
Figure 10.72 Structure of pyridoxal2
Figure 10.73 Structure of pyridoxamine3
All three forms can be activated by being phosphorylated. The phosphorylated forms can be interconverted to the active, or the cofactor form of vitamin B6, pyridoxal phosphate (PLP). This active form has a phosphate group added in place of a hydroxyl group. The enzyme that catalyzes this reaction requires FMN (riboflavin cofactor), as shown below.
Figure 10.74 Vitamin B6 activation1,4
In animal products, vitamin B6 is found in its cofactor forms, PLP and pyridoxamine phosphate (PMP). The latter cofactor is less common than PLP. In plants, vitamin B6 is primarily found as pyridoxine, with up to 75% being pyridoxine glucoside, which is believed to be the plant storage form6. Pyridoxine glucoside has a glucose added to pyridoxine as shown below.
Figure 10.75 Structure of pyridoxine glucoside5
Vitamin B6 is well absorbed from foods (~75%) through passive diffusion. PLP and PMP are dephosphorylated before uptake into the enterocyte. Some of the pyridoxamine glucoside is cleaved to form free pyridoxine, but some pyridoxine glucoside is absorbed intact. Pyridoxine glucoside absorption is lower (~50%) than pyridoxine alone. The primary circulating forms of vitamin B6 are pyridoxal and PLP. Vitamin B6 is primarily excreted in the urine, and like many other B vitamins, vitamin B6 is destroyed during cooking or heating6.
References & Links
- Shils ME, Shike M, Ross AC, Caballero B, Cousins RJ, editors. (2006) Modern nutrition in health and disease. Baltimore, MD: Lippincott Williams & Wilkins.