Tuesday, July 3, 2007

Vitamin B6

In 1934 Paul György observed the appearance of a
scaly dermatitis (acrodynia) in rats fed on diets free
from the whole vitamin B complex and supplemented
with thiamin and ribofl avin. This observation led to
the establishment of a ‘rat acrodynia-preventative factor’
and its designation as vitamin B6. The isolation of the pure crystalline vitamin was first reported by Lepkovsky in 1938, and the synthesis of pyridoxine
was accomplished by Harris and Folkers in the following
year. Discovery of the existence of pyridoxal
and pyridoxamine and the recognition of their phosphorylated
forms as coenzymes is largely credited to
Esmond E. Snell during 1944–1948.

Vitamin B6 is the generic descriptor for all 3-hydroxy-
2-methylpyridine derivatives which exhibit qualitatively
in rats the biological activity of pyridoxine.
Six B6 vitamers are known, namely pyridoxine or
pyridoxol (PN), pyridoxal (PL) and pyridoxamine
(PM), which possess, respectively, alcohol, aldehyde
and amine group in the 4-position; their respective
5´-phosphate esters are designated as PNP, PLP and
PMP (Fig. 14.1).
In its role as a coenzyme, PLP is attached to the
apoenzyme by a Schiff base (aldimine) linkage
(–N=CH–) formed through condensation of the 4-
carbonyl group with the ε-amino group of specifi c
lysine residues A ubiquitous bound form of PN that occurs in
plant tissues is a glucoside conjugate, 5´-O-(β-Dglucopyranosyl)
pyridoxine (Fig. 14.3), designated
in this text as PN-glucoside. A more complex derivative
of PN-glucoside containing cellobiose and
5-hydroxydioxindole-3-acetic acid moieties has been
identifi ed as a major form of vitamin B6 in rice bran
and a minor form in wheat bran and legumes (Tadera
& Orite, 1991).
All of the six B6 vitamers are considered to have approximately
equivalent biological activity in humans
as a result of their ultimate conversion to coenzymes.

No comments: