Digestion of vitamin B6 first involves splitting the vitamin, as it is bound to the protein portion of foods. Vitamin B6 is absorbed mainly in the jejunum but also in the ileum by passive diffusion. Absorption from the colon is insignificant, even though colon microflora synthesize the vitamin.
Vitamin B6 compounds are all absorbed from the diet in the dephosphorylated forms. The small intestine is rich in alkaline phosphatases for the dephosphorylation reaction. After absorption, B6 compounds rapidly appear in the liver, where they are mostly converted into PLP, considered to be the most active vitamin form in metabolism. Both niacin (as nicotinamide adenine dinucleotide phosphate [NADP] dependent enzyme) and riboflavin (as the flavoprotein pyridoxamine phosphate oxidase) are important for conversion of vitamin B6 forms and phosphorylation reactions.
Although other tissues also contribute to vitamin B6 metabolism, the liver is thought to be responsible for forming PLP found in plasma. Pyridoxal and PLP found in circulation are associated primarily with plasma albumin and red blood cell hemoglobin (Mehansho and Henderson, 1980). Pyridoxal phosphate accounts for 60% of plasma vitamin B6. Researchers do not agree on whether pyridoxal or PLP is the transport form of B6 (Driskell, 1984).
Only small quantities of vitamin B6 are stored in the body. The vitamin is widely distributed in various tissues, mainly as PLP or pyridoxamine phosphate. Russell et al. (1985) investigated B6 metabolism in swine muscle. Excess dietary vitamin B6 increased whole-muscle total PLP. Russell et al. (1985) indicated that under the conditions of their study, muscle tissue acts as an immobile reservoir of PLP and 60% to 95% of muscle PLP was bound to muscle glycogen phosphorylase. Pyridoxic acid is the major excretory metabolite of the vitamin, eliminated via the urine. Also, small quantities of pyridoxol, pyridoxal and pyridoxamine, as well as their phosphorylated derivatives, are excreted into the urine (Henderson, 1984).