Introduction to Human Nutrition

The Vitamins 163

descriptor, with “pyridoxol” as the specifi c name for
the alcohol. The vitamers are metabolically intercon-
vertible and, as far as is known, they have equal bio-
logical activity; they are all converted in the body to
the metabolically active form, pyridoxal phosphate.
4-Pyridoxic acid is a biologically inactive end-product
of vitamin B 6 metabolism.

Absorption and metabolism

The phosphorylated vitamers are dephosphorylated
by membrane-bound alkaline phosphatase in the
intestinal mucosa; pyridoxal, pyridoxamine, and pyri-
doxine are all absorbed rapidly by passive diffusion.
Intestinal mucosal cells have pyridoxine kinase and
pyridoxine phosphate oxidase (Figure 8.11), so that
there is net accumulation of pyridoxal phosphate by

metabolic trapping. Much of the ingested pyridoxine

is released into the portal circulation as pyridoxal,

after dephosphorylation at the serosal surface. Unlike

other B vitamins, there seems to be no limit on the

amount of vitamin B 6 that is absorbed.

Most of the absorbed vitamin is taken up by the

liver by passive diffusion, followed by metabolic trap-

ping as phosphate esters, which do not cross cell

membranes, then oxidation to pyridoxal phosphate.

The liver exports both pyridoxal phosphate (bound

to albumin) and pyridoxal (which binds to both

albumin and hemoglobin). Free pyridoxal remaining

in the liver is rapidly oxidized to 4-pyridoxic acid,

which is the main excretory product.

Extrahepatic tissues take up pyridoxal and pyri-

doxal phosphate from the plasma. The phosphate is

N

H 2

C

HC O

HO

N

H 2

C

CH 2 OH

HO

N

H 2

C

H 2 C NH 2

HO

N

H 2

C

CH 2 OH

P O

O-

O-

O

N

H 2

C

HC

P O

O-

O-

O

N

H 2

C

CH 2 NH 2

P O

O-

O-

O

O

OH

CH 3

OH

CH 3

OH

CH 3

OH

CH 3

OH

CH 3

OH

CH 3

N

H 2

C

COO–

HO OH

CH 3

Kinase

Phosphatase

Kinase

Phosphatase

Kinase

Phosphatase

Oxidase

Oxidase

Transaminases Oxidase

Pyridoxine Pyridoxine phosphate

4-Pyridoxic acid Pyridoxal Pyridoxal phosphate

Pyridoxamine Pyridoxamine phosphate
Figure 8.11 Interconversion of the vitamin B 6 vitamers. Pyridoxal kinase (EC 2.7.1.38), pyridoxine phosphate oxidase (EC 1.1.1.65), pyridoxamine
phosphate oxidase (EC 1.4.3.5).